Derivation Web

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by researka:v2 · 2026-06-24 03:00:49.921176+04:00

# Hypothesis-Generating Brief: Spermidine supplementation — full paper

## Abstract

This paper synthesizes evidence on Spermidine supplementation across 42 accepted source papers and 1250 high-confidence extracted claims.

The evidence profile contains 5 direct clinical sources, 33 adjacent clinical sources, and 4 mechanistic or model-system sources, with 206 cross-study disagreements across the evidence base.

Positive study-level signals are summarized in the immune and inflammation outcome class, null signals in the contextual adjacent evidence, mechanism, dosing and pharmacokinetics outcome classes, and negative signals in the contextual adjacent evidence outcome class. The paper therefore interprets the corpus as a tiered evidence profile rather than as a single pooled effect.

The conclusion is that Spermidine supplementation remains a bounded geroscience case: the retained clinical and mechanistic evidence profile defines the scope for targeted testing, while mixed and null findings limit any unqualified anti-aging claim.

For that reason, the manuscript does not collapse every source into a single recommendation. It presents the intervention as a set of linked claims whose strength depends on the evidence tier and the match between mechanism, population, and endpoint.

The research value of the synthesis lies in making these boundaries explicit. It identifies which evidence streams are already aligned, which ones remain discordant, and which future studies would most directly test the unresolved bridge.

## Introduction

Population aging has shifted the central question of clinical medicine from single-disease management toward extending the years spent in good health. Healthspan—the period of life free of chronic disease and functional loss—appears to compress for many adults, while the social and economic costs of multimorbidity, frailty, and dementia continue to grow. The hypothesis that targeting the biology of aging itself, rather than its downstream manifestations, could simultaneously delay multiple age-related conditions has therefore attracted considerable investment, even as the regulatory and methodological paths for such claims remain unsettled. Spermidine, a naturally occurring polyamine present in common foods and produced endogenously, has been positioned at the intersection of this debate because of its hypothesized links to autophagy, immune regulation, and longevity pathways. Whether supplementation with spermidine can produce clinically meaningful gains in older adults remains the question that frames the present synthesis, and the urgency of the demographic transition makes the stakes tangible rather than abstract.

Spermidine belongs to the polyamine family of small aliphatic cations and is generated endogenously via ornithine decarboxylase-dependent pathways or absorbed from dietary sources, a fact that allows it to be marketed and consumed as a nutritional supplement rather than a prescription drug. The compound's regulatory status has practical consequences: it has been widely available to consumers for years, has appeared in functional foods and rice-germ extracts at daily doses typically in the low-milligram range, and has been evaluated in safety studies in older adults with subjective cognitive decline. Several human pharmacokinetic and supplementation studies have examined whether oral spermidine reliably elevates circulating or salivary levels, with mixed results that may reflect dose, formulation, or baseline status. As an intervention, spermidine therefore enjoys unusually low barriers to access and a sizeable consumer-experience base, even as the clinical evidence base remains comparatively young.

Even where signals appear, several unresolved questions complicate interpretation. First, the mechanistic literature linking spermidine to autophagy induction is dense, but whether short-term oral supplementation in older humans reproduces the cellular flux changes seen in model systems has not been definitively established. Second, observational studies have linked both higher and lower polyamine concentrations to adverse outcomes, including stroke-related cognitive impairment, suggesting that the dose-response relationship may be non-linear and population-specific. Third, duration and dose vary widely across protocols, with no clear consensus on a target plasma concentration, and trials of high-dose supplementation in healthy adults have failed to show the expected rise in circulating spermidine. Fourth, the populations studied—older adults with subjective cognitive decline, patients with coronary artery disease, individuals with vaccine-relevant immune senescence—differ in ways that make cross-trial inference hazardous, and the appropriate comparator for a dietary constituent is itself contested.

## Background

In animal/preclinical evidence, geroscience positions aging not as a single disease but as a shared upstream driver of multiple chronic conditions, and the hallmarks-of-aging framework (subsequent extensions in the broader geroscience literature) has become the dominant organizing schema for evaluating candidate geroprotectors. Within this frame, spermidine sits at an unusual intersection: it is an endogenous polyamine that declines with age in several tissues, is obtainable from dietary sources such as wheat germ, aged cheese, legumes, and fermented soy products (Munoz-Esparza 2019), and has been proposed to act on multiple hallmarks simultaneously rather than on any one pathway alone. The geroprotection thesis holds that if a compound engages several aging hallmarks with tolerable safety, it could in principle delay the onset or compress the morbidity of age-related syndromes rather than treating each disease downstream in isolation. The regulatory implications are considerable, because demonstrating a geroprotective indication would require either validated surrogate endpoints or very long follow-up windows, a problem already highlighted in the metformin TAME trial design discussions (Ioannidis 2005, surrogate endpoint caution). The field has therefore leaned on proxy endpoints, biomarker changes, and intermediate functional outcomes while waiting for definitive hard-outcome data, and this methodological concession is part of the backdrop against which any specific spermidine RCT must be read. As the synthesis that follows will make explicit, the corpus of available evidence on spermidine in humans is heterogeneous in design, dose, duration, and population, which compounds the usual regulatory uncertainty.

Human evidence on spermidine is sparse but growing, and it spans observational cohorts, mechanistic biomarker studies, and a small set of pilot or full RCTs. Source documents were screened for quantitative outcome statements, and 1250 extracted quantitative findings were retained for synthesis after role, unit, and citation checks. Corpus construction used the topic query terms with aging, longevity, healthspan, frailty, cardiometabolic, immune, safety, and function terms across bibliographic, trial, and project-curated source indexes when available. The output is therefore framed as a structured evidence synthesis rather than a claim of exhaustive systematic-review coverage.

### Evidence selection and synthesis

Claims were retained only when their numeric value, endpoint, and study label could be reconciled with the source record. Evidence was grouped by outcome class, study design, direction of effect, and clinical directness. Cross-paper tensions were summarized when two retained findings addressed related outcomes but differed in direction, directness, population, comparator, or follow-up. Records that lacked a traceable endpoint, citation, or study identity were excluded from main-text inference and kept in the supplementary audit trail when available.

### Manuscript controls

Public prose was constrained to the retained evidence set. Numeric statements were checked against the extracted claim table, and rows with unresolved endpoint, unit, study-label, or citation problems were kept out of the journal main text.

### Interpretation rules

Clinical, observational, review, and mechanistic findings were interpreted according to their design limits. Direct human trials were weighted most heavily for clinical endpoints, whereas cellular, animal, or tissue-level findings were used to clarify plausible mechanisms and boundary conditions rather than to establish clinical benefit. Directional agreement was treated as stronger when findings shared population, comparator, endpoint, and follow-up context. Disagreement was retained when it reflected different outcome classes, exposure windows, disease states, or measurement methods, because those differences define where the synthesis should remain conditional.

### Quantitative handling

Effect estimates, confidence intervals, p-values, sample sizes, and threshold comparisons were used only when the surrounding source context identified the same endpoint and study arm. Measures with incompatible units were not pooled narratively as if they measured the same construct. When a finding came from indirect evidence, the manuscript used cautious language and separated mechanism from clinical inference. Topic-level conclusions were therefore bounded by the strongest matched human evidence. This approach keeps the Methods section focused on reproducible evidence handling rather than implementation metadata.

## Results
| Evidence domain | Corpus slice | Strongest signal | Directness | Main limitation |
|---|---|---|---|---|
| Contextual Adjacent Evidence | n=25; claims=671 | no extracted directional signal in 22/25 sources | 2 direct; 14 indirect; 1 protocol; 8 review | limited corpus depth in this outcome class |
| Immune and Inflammation | n=3; claims=240 | positive signal in 2/3 sources | 1 direct; 2 indirect | limited corpus depth in this outcome class |
| Mechanism | n=3; claims=29 | no extracted directional signal in 3/3 sources | 3 mechanistic | limited corpus depth in this outcome class |
| Dosing and Pharmacokinetics | n=2; claims=77 | no extracted directional signal in 2/2 sources | 2 review | limited corpus depth in this outcome class |
| Longevity | n=2; claims=37 | no extracted directional signal in 2/2 sources | 1 indirect; 1 review | limited corpus depth in this outcome class |
| Safety and Comorbidity | n=2; claims=77 | unclear signal in 1/2 sources | 1 indirect; 1 mechanistic | limited corpus depth in this outcome class |
| Cardiometabolic | n=1; claims=4 | no extracted directional signal in 1/1 sources | 1 direct | single-source slice; hypothesis-generating |
| Deficiency Prevalence | n=1; claims=24 | no extracted directional signal in 1/1 sources | 1 indirect | single-source slice; hypothesis-generating |
| Frailty | n=1; claims=75 | mixed signal in 1/1 sources | 1 indirect | single-source slice; hypothesis-generating |
| Muscle Function | n=1; claims=11 | no extracted directional signal in 1/1 sources | 1 review | single-source slice; hypothesis-generating |
| Skeletal, Fracture, and Bone | n=1; claims=5 | no extracted directional signal in 1/1 sources | 1 direct | single-source slice; hypothesis-generating |

**Outcome-class note:** Contextual Adjacent Evidence denotes background, boundary-condition, or adjacent-outcome sources. It is not pooled with direct outcome evidence; these sources bound scope, safety, methods, and translation rather than serving as equal-weight support for the main efficacy claim.



### Results Summary

- Contextual Adjacent Evidence: n=25; claims=671; no extracted directional signal in 22/25 sources | directness: 2 direct; 14 indirect; 8 review; 1 protocol; main limitation: directionally heterogeneous.
- Immune and Inflammation: n=3; claims=240; benefit signal in 2/3 sources | directness: 1 direct; 2 indirect; main limitation: directionally heterogeneous.
- Mechanism: n=3; claims=29; no extracted directional signal in 3/3 sources | directness: 3 mechanistic; main limitation: no direct clinical anchor.
- Dosing and Pharmacokinetics: n=2; claims=77; no extracted directional signal in 2/2 sources | directness: 2 review; main limitation: no direct clinical anchor.
- Longevity: n=2; claims=37; no extracted directional signal in 2/2 sources | directness: 1 indirect; 1 review; main limitation: no direct clinical anchor.
- Safety and Comorbidity: n=2; claims=77; mixed signal in 1/2 sources | directness: 1 indirect; 1 mechanistic; main limitation: no direct clinical anchor.

### Cardiometabolic Outcomes


One double-blind, within-subject, placebo-controlled trial evaluated oral spermidine at 40 mg/day in healthy older men and tracked cardiometabolically relevant endpoints including circulating polyamines [Keohane 2024]. The trial enrolled 37 men aged 50-70 years and used a within-subject crossover design with placebo control. The intervention duration and primary endpoint specification followed a standard supplementation-withdrawal paradigm. Per the source, this represents the principal clinical RCT evidence within the cardiometabolic outcome class in the curated corpus.

Quantitatively, the source records that supplementation at 40 mg/day had minimal effects on circulating polyamines, with no p-values reported in the available excerpt [Keohane 2024]. The within-subject comparison against placebo did not demonstrate a meaningful shift in the principal cardiometabolic biomarker measured, and the source's effect direction field is null rather than positive or negative. The cardiometabolic signal in the corpus is therefore best characterised as null rather than as either favourable or adverse.

Mechanistically, the absence of a measurable change in circulating polyamines after 40 mg/day oral dosing in healthy older adults does not by itself adjudicate cardiometabolic function, because polyamine flux and tissue-level autophagy engagement are not necessarily reflected in steady-state plasma pools [Keohane 2024]. Preclinical data and mechanistic human studies have separately linked spermidine to autophagic flux and to lipid-handling pathways relevant to cardiometabolic risk, but this clinical RCT did not include functional vascular or glycaemic endpoints in the reported excerpt. The mechanistic substrate underlying potential cardiometabolic benefit therefore remains plausible without yet being demonstrated at the clinical-RCT layer represented here.

Within the cardiometabolic outcome class, no second clinical RCT source is available to test the agreement or disagreement of the [Keohane 2024] null finding. The cross-study disagreement map supplied with this synthesis contains no non-orthogonal pairs for cardiometabolic outcomes, which means the only adjudicated discussion point is the internal characterisation of this single trial as exploratory and null. Until a confirmatory RCT with functional cardiometabolic endpoints (vascular reactivity, blood pressure, lipid panel, glycaemic markers) is added to the corpus, the cardiometabolic class reads as hypothesis-generating only, consistent with the broader integrating thesis that the spermidine anti-aging case is currently incomplete.

### Contextual Adjacent Evidence Outcomes


Across the curated corpus, the contextual evidence base for spermidine spans human clinical trials, observational cohorts, mechanistic ex vivo work, plant-biology models, and dietary-intake reviews. Improving Vaccination in Older 2023 and Metabolic Responses to Spermidine 2023 are uniformly designated as protocol or review-stage material without reported p-values.

Quantitative findings from the contextual literature are dominated by null effects with two notable exceptions. He 2025 reported a negative contextual signal: in a multicenter prospective stroke cohort, the adjusted odds ratio for poststroke cognitive impairment in the highest versus lowest quartile of spermidine was 1.81.

Mechanistically, the contextual findings are coherent with the corpus-wide autophagy-induction and anti-inflammatory framework. Clinical RCT data from Wirth 2018 and Wirth 2019, paired with the biomarker-RCT report Schwarz 2022, attempt to translate the autophagy-flux biology documented by Wet 2021 (P < 0.05 for rapamycin-induced LC3-II shifts) into cognitive endpoints, but the human RCT layer is thin. The analysis tracked serum spermidine concentrations and modeled MCI associations across both study frames, with effect estimates reported as odds ratios and 95% confidence intervals alongside probability values for the cross-sectional and longitudinal contrasts. The endpoint was MCI incidence and prevalence, and the exposure window captured habitual serum spermidine rather than a supplemented dose. Within the corpus this is the only study mapped to the deficiency-prevalence class, so it carries the entire direct evidence load for that outcome.

Mechanistically, a non-linear serum-spermidine/MCI association is consistent with the corpus-wide framing in which spermidine exposure sits within an aging-relevant biology of autophagy, cellular polyamine homeostasis, and cognitive reserve, and any population-level signal is conditional on where a given cohort sits on the deficiency-sufficiency continuum. Because the outcome class is anchored by a single observational cohort, the mechanistic substrate cannot be triangulated against a clinical RCT in this subsection; the discussion here is restricted to human observational epidemiology and its biological plausibility. The null effect direction flag cautions readers against treating either tail of the spermidine distribution as uniformly protective or harmful in cognitively at-risk adults.

### Dosing and Pharmacokinetics Outcomes


Two cohort-style investigations inform the pharmacokinetic profile of oral spermidine in adult populations, one framed as a randomized placebo-controlled pharmacokinetic study and the other as a pilot dose-escalation study. The first, Senekowitsch 2023, evaluated high-dose spermidine supplementation in healthy adults, with the thesis statement indicating that the work "investigated the pharmacokinetics of oral s[permidine]" and explicitly concluded that the high-dose regimen "[does] not increase spermidine levels in blood plasma and saliva." The second, Rhodes 2024, examined a fasting-mimetic combination containing spermidine, nicotinamide, palmitoylethanolamide (PEA), and oleoylethanolamide (OEA) in healthy young adult men, measuring plasma levels of 1-methylnicotinamide, PEA, and OEA, among other analytes. Together these two sources provide the human pharmacokinetic scaffolding against which downstream efficacy signals must be interpreted.

Quantitative findings diverge sharply between the two studies. Rhodes 2024, by contrast, observed significant increases in the combination-product constituents downstream of spermidine, with plasma analytes rising at P < 0.05 across multiple time points of the dose-escalation design. The contrast is not in the sign of an effect but in the analyte of interest: parent spermidine was unchanged in Senekowitsch 2023, whereas co-administered nicotinamide-family and endocannabinoid-family species were elevated in Rhodes 2024.

Mechanistically, the two studies are not directly comparable because they interrogate different molecular layers of the same intervention. Senekowitsch 2023 focuses on parent-spermidine bioavailability and finds the systemic exposure window flat, an observation that constrains any downstream mechanistic claim predicated on elevated circulating spermidine. Rhodes 2024, as a pilot dose-escalation study, documents absorption and downstream anti-inflammatory and cardioprotective signals of the fasting-mimetic combination, with the source's thesis noting "Absorption, anti-inflammatory, antioxidant, and cardioprotective impacts" of the four-component product. The mechanistic substrate underlying these pharmacokinetic observations, cellular autophagy flux and polyamine-pool replenishment, is therefore invoked in Rhodes 2024 through surrogate analytes, whereas in Senekowitsch 2023 it is tested directly and found absent at the parent-compound level.

Within-corpus tension on this outcome class is centered on the apparent disagreement between Senekowitsch 2023, which reports no rise in plasma or saliva spermidine after high-dose oral supplementation, and Rhodes 2024, which reports P < 0.05 elevations in combination-product analytes. The disagreement is best read as non-contradictory once one notes that the two studies measured different molecules, used different carrier matrices (monotherapy spermidine versus a fasting-mimetic blend), and enrolled different demographic windows (general adults versus healthy young adult men). This within-corpus tension therefore sets a boundary condition for the synthesis: claims about systemic spermidine exposure from oral dosing should be treated as formulation-dependent and population-dependent until direct head-to-head pharmacokinetic work is performed.

### Frailty Outcomes


The frailty-relevant evidence in the curated corpus derives from a single observational cohort study of older adults that examined the whole-blood spermine/spermidine ratio as a candidate indicator of sarcopenia status. The design was cross-sectional/observational rather than a clinical RCT, and the endpoint of interest was the biochemical ratio's discriminating value between sarcopenia and non-sarcopenia participants, rather than a randomized spermidine supplementation effect. The canonical trial slot is not populated for this study, and dose, follow-up duration, and randomization are therefore not applicable in the usual sense. Per the source, the population consisted of community-dwelling older adults stratified post-hoc into sarcopenia and non-sarcopenia groups. The source surfaces 15 distinct p-values, the majority of which pertain to between-group biochemical and clinical comparisons.

Quantitative findings from this cohort directly inform the frailty outcome class. Spermidine concentrations were higher in the sarcopenia group than in the non-sarcopenia group (P = 0.002), and the spermine/spermidine ratio was lower in the sarcopenia group (P < 0.001). as a sarcopenia-status indicator. The source also enumerates additional p-values for related comparisons: P = 0.046, P = 0.070, P = 0.339, P = 0.701, P = 0.802, P = 0.979, P = 0.106, P = 0.495, P = 0.010, P = 0.772, P = 0.915, and P = 0.087, with at least one further contrast meeting a generic P < 0.05 threshold. Per the evidence synthesis, the headline discriminators (P = 0.002 for spermidine, P < 0.001 for the ratio) carry the analytic weight, while the remainder describe secondary biochemical and clinical features with mostly non-significant differences.

Mechanistically, the Sanayama 2023 observation that elevated spermidine and a reduced spermine/spermidine ratio track with sarcopenia status sits in tension with the broader mechanistic literature in which spermidine is implicated in autophagy and protein-turnover pathways that would, a priori, be expected to protect against muscle loss. Because this study is observational and biomarker-focused rather than interventional, the directionality of the biochemical association cannot be interpreted as causal — i.e., whether elevated spermidine reflects compensatory upregulation in at-risk muscle, altered polyamine metabolism secondary to sarcopenic pathophysiology, or an epiphenomenon of comorbidity burden in older adults remains unspecified by the source. The mechanistic substrate underlying this functional finding thus requires human RCT confirmation before the ratio can be treated as more than a candidate indicator.

Within-corpus tensions for the frailty outcome class are constrained by the fact that only one source (Sanayama 2023) populates this class, so there are no direct between-source disagreements to surface. The closest interpretive tension is internal to the source itself: the sarcopenia group simultaneously shows higher spermidine (P = 0.002) and a lower spermine/spermidine ratio (P < 0.001), which is directionally consistent but underscores that the ratio — not spermidine alone — is the proposed indicator. The selected integrating thesis notes that positive signals cluster in immune/inflammation endpoints and that null findings dominate mechanistic and contextual outcomes; the frailty evidence here is better characterized as a biomarker-association signal than as a positive supplementation outcome, and the boundary conditions of that signal remain to be established in randomized work.

### Immune and Inflammation Outcomes


Three sources in the curated corpus address immune and inflammatory endpoints of spermidine exposure, spanning a randomized controlled human trial, a pilot vaccine-response study, and a translational Gulf War Illness cohort. Felix 2024 is a randomized controlled trial in adults evaluating a blend containing AM3 (150 mg), spermidine (0.6 mg), and hesperidin, with immune function, biological age, and oxidative-inflammatory state as the biomarker panel.

Mechanistically, the human RCT evidence (Felix 2024) supports an immune-enhancing and oxidative-inflammatory lowering action for a low-milligram spermidine dose when delivered alongside AM3 and hesperidin, while the indirect human cohort evidence (Trivedi 2026) localizes benefit to gut-brain axis modulation and intestinal homeostasis in a neuroimmune context. Across these three sources, the mechanistic substrate converges on oxidative-inflammatory tone and immune-cell competence, but the magnitude of the human signal is dose-, combination-, and population-dependent.

Within the immune-inflammation corpus, a substantive disagreement separates Trivedi 2026 (positive on immune inflammation) from Alsaleh 2026 (null on immune inflammation), with the pilot's small sample, vaccine-restored baseline, and older-adult population plausibly accounting for the divergence. A second, structural disagreement is the directness gap between Felix 2024, which uses a randomized controlled trial design with a mechanistic/biomarker endpoint, and the indirect observational designs of Trivedi 2026 and Alsaleh 2026, where the spermidine exposure is embedded in a multi-component or naturalistic context. These direct-versus-indirect and positive-versus-null contrasts define the boundary conditions of the current evidence base: direct human RCT support is limited to a single combination-product trial, while observational and pilot data are split between encouraging cohort signals and a null vaccine-response read-out, and the field will require a dedicated, monotherapy RCT in a defined inflammatory phenotype to settle the question.

### Longevity Outcomes


Two curated sources address the longevity outcome class, and each reaches a different verdict depending on the biological context in which spermidine or its metabolites were measured. Nayak 2020 is an observational cohort study in adults that quantified N8-acetylspermidine (N8 AS) as a polyamine biomarker in ischemic cardiomyopathy with reduced ejection fraction; the source reports that N8 AS was elevated in ICM cases compared with patients who had coronary disease without ICM, and it lists eight p-values (P < 0.001, P = 0.001, P = 0.01, P = 0.14, P = 0.03, P = 0.02, P = 0.10, P = 0.72) covering comparative analyses across groups and subgroups. The longevity-relevant signal is that N8 AS levels — a modified spermidine species — differ significantly between ICM cases and comparator coronary-disease controls (P < 0.001), indicating that the polyamine pool is perturbed in a disease of accelerated cardiac aging, even though the study does not directly test whether spermidine supplementation extends human lifespan. The source's effect direction is null, which means the cohort analysis does not position N8 AS as a unidirectional longevity-promoting biomarker, only as a differentially abundant metabolite. Accordingly, the human longevity claim must be qualified: the source supports an associative link between a polyamine species and an age-related cardiomyopathy phenotype, not a causal lifespan-extending claim in humans.

The second curated longevity source, SPERMIDINE TOXICITY in MITOCHONDRIAL 2022, is a review-tier observational cohort entry that aggregates yeast data on spermidine pharmacology and reports a directly opposing effect in mitochondrial DNA-deficient Saccharomyces cerevisiae: in this genetic background spermidine significantly shortened lifespan, decreasing the median survival of cells that lack functional mitochondrial DNA. The source is categorized as a review (directness: review) rather than a primary report, so it can be interpreted as a synthesis-level statement rather than a single experimental record, and its effect direction is null because the qualitative claim is balanced against broader literature that frames spermidine as beneficial. Even so, the directional language in the excerpt — "spermidine significantly shortened lifespan" — places this source at the negative end of the longevity effect spectrum for the specific biological context of mitochondrial deficiency. Mechanistically, the finding implies that spermidine's longevity-relevant activity depends on intact mitochondrial function, and that loss of mitochondrial DNA converts a typically beneficial intervention into a toxic one. This conditional toxicity is the single most important nuance the curated longevity corpus contributes: the same molecule is protective in cells with competent mitochondria and harmful in cells without.

Mechanistically, the two longevity sources point in opposite directions but converge on a common substrate: mitochondrial integrity as the gatekeeper of spermidine's longevity-relevant activity. The human cohort study (Nayak 2020) places elevated N8-acetylspermidine alongside ischemic cardiomyopathy with reduced ejection fraction, a clinical state in which mitochondrial bioenergetics is broadly compromised, yet it does not specify whether mitochondrial failure is upstream or downstream of the polyamine change. By contrast, the yeast review (SPERMIDINE TOXICITY in MITOCHONDRIAL 2022) explicitly isolates mitochondrial DNA deficiency as the experimental condition that flips spermidine's effect from beneficial to detrimental. Read together, the curated corpus supports a model in which the longevity-relevant activity of spermidine is not invariant but is contingent on the energetic and genomic state of the mitochondrion, and the human observational finding can be re-interpreted as consistent with that model because ICM with reduced ejection fraction is itself a state of mitochondrial stress. This mechanistic reading is not contradicted by either source, but neither source directly tests it; the conclusion is an integrative inference drawn from associative human data (Nayak 2020) and conditional yeast data (SPERMIDINE TOXICITY in MITOCHONDRIAL 2022) and should be cited as such.

Within-corpus tensions in the longevity outcome class are explicit and should be surfaced rather than smoothed. SPERMIDINE TOXICITY in MITOCHONDRIAL 2022, in turn, reports shortened lifespan in mitochondrial DNA-deficient yeast, which is the opposite direction from the broader mechanistic literature on spermidine but applies only to a narrow genetic background. The disagreement is therefore not between two competing pro-longevity findings; it is between an associative human biomarker signal that is partly null (Nayak 2020) and a mechanistic yeast signal that is negative but conditional (SPERMIDINE TOXICITY in MITOCHONDRIAL 2022). The cross-study disagreement map contains no same-outcome non-orthogonal pairs for longevity, which means the curated corpus does not supply a head-to-head reconciliation, and readers should treat the longevity claim as context-dependent rather than as a settled positive or negative effect.

### Mechanism Outcomes


Choi 2026 reports a preclinical biochemical characterization of the spermidine synthase CauSpe3 from Candidozyma auris, in which the enzyme catalyzed efficient conversion of putrescine to spermidine in the presence of dc-SAM. These values define the lower-bound substrate affinities required for productive polyamine flux through this enzyme system. Because the work is preclinical and organism-distant, the substrate constants can be interpreted as proof-of-principle biochemistry rather than as a direct human metabolic parameter.

Yuan 2021 examined the autophagic response of female germline stem cells (FGSCs) to spermidine in vitro and in an oxidative-stress aging model, with western blot readouts for LC3B, p62, and β-tubulin loading control. The directionality indicates a cytoprotective autophagic phenotype under oxidative challenge, an effect plausibly linking polyamine availability to selective autophagy in stem-cell compartments.

Saiyed 2026 approached mechanism from a clinical-metabolite angle in postoperative delirium (POD), profiling polyamine and amino-acid metabolites in patient samples. The authors reported significant correlations in POD patients involving citrulline, ornithine, and glutamine, and the modeling work highlighted glutamine, glutamic acid, and putrescine as discriminator metabolites (Saiyed 2026). Although Saiyed 2026 does not directly assay spermidine as an intervention, the putrescine node connects this clinical-metabolite picture to the biochemical pathway characterized in Choi 2026. Mechanistically, the convergent signal is that polyamine-pathway intermediates — not spermidine alone — track with adverse postoperative cognitive trajectories.

Within the mechanistic outcome class, the three sources are mutually compatible rather than contradictory, but their evidentiary weights differ. Choi 2026 and Yuan 2021 are direct spermidine-pathway experiments with explicit biochemical or autophagic endpoints, while Saiyed 2026 is a correlative human metabolomics study in which spermidine is one node in a broader polyamine-and-amino-acid network. The within-corpus tension is therefore one of granularity: a tightly defined enzyme-kinetics result (Choi 2026), a defined cellular-autophagy result (Yuan 2021), and a diffuse clinical-metabolite result (Saiyed 2026). No two studies here offer discordant findings on a shared endpoint, so the surface-level coherence should be interpreted as absence of conflict rather than as convergence on a single mechanism.

### Muscle Function Outcomes


The available direct human evidence on spermidine and muscle function in this corpus is anchored by a single cross-sectional observational study, Thorup 2026, which enrolled elderly patients with coronary artery disease and examined dietary intake alongside plasma and skeletal muscle concentrations. The reported median dietary spermidine intake was 11.5 mg/day with an interquartile range of 9.5–14.3 (n = 184), and median spermidine concentrations were 21.0 ng in the relevant biospecimen, framing the exposure range that any downstream muscle-function signal must be interpreted against. The endpoint structure is observational and review-oriented, with no randomized allocation or controlled dose escalation, which constrains the strength of causal inference despite the biological plausibility of polyamine-muscle cross-talk.

Within Thorup 2026, several muscle- and performance-related comparisons are reported with exact p-values that must be read against the cross-sectional design. The study documents P = 0.02 for at least one primary association, with additional analyses reported as P > 0.05, P < 0.01, and P < 0.05 across related contrasts, indicating that not all muscle-function endpoints moved in the same direction. Per-Study Endpoint Evidence is consolidated in the evidence synthesis, which carries the full study × p-value tuple list so that this paragraph can reference rather than restate each individual contrast. The pattern is consistent with a mixed quantitative profile: at least one signal crosses conventional significance thresholds while other endpoints remain null or only trend-level within the same cohort.

Mechanistically, a clinical cross-sectional finding linking dietary and tissue polyamine status to muscle function in an elderly cardiovascular population is biologically coherent with the broader spermidine literature on autophagy, mitochondrial quality control, and skeletal muscle proteostasis, although the present corpus does not contain a paired mechanistic human substudy to substantiate that link. The mechanistic substrate underlying this functional finding — polyamine-driven induction of autophagy and improved mitochondrial respiration in aging muscle — is therefore inferred from the wider literature rather than demonstrated within Thorup 2026 itself. Preclinical data on spermidine analog supplementation in aged rodent muscle have repeatedly shown improved grip strength and preserved fiber cross-sectional area, but those rodent endpoints are not represented as sources in the present corpus, so this mechanistic bridge is offered as context, not as evidentiary support.

Within-corpus tension in the muscle-function outcome class is limited because only one curated source addresses this endpoint directly; however, the internal disagreement is still informative. By contrast with the broader thesis that null findings dominate the spermidine evidence base, the muscle-function class shows a partially positive but heterogeneous pattern, and the absence of an independent replication source in the corpus means that the directionality of the significant associations cannot yet be confirmed. This boundary condition — single-cohort, cross-sectional, mixed endpoint direction — should be carried into any downstream inference about spermidine as a muscle-function modulator in older adults.

### Safety and Comorbidity Outcomes


The safety and comorbidity evidence base for spermidine in the curated corpus is anchored by two complementary investigations that span a Phase II human trial in older adults with subjective cognitive decline and a large prospective cohort in adults hospitalized with acute ischemic stroke. The human Phase II study randomized participants aged 60 to 80 years with subjective cognitive decline to either spermidine or placebo for three months, providing the only direct tolerability readout in an older clinical population within the corpus. The observational cohort study enrolled 3570 patients presenting with acute ischemic stroke and measured admission plasma polyamine levels, with clinical outcomes tracked to 3 months post-event. Together these two designs supply one direct clinical-RCT anchor and one large indirect epidemiologic anchor for the safety-comorbidity outcome class, and the trial durations of 3 months each permit short-horizon comparisons without long-term exposure inference.

Additional corpus sources included animal/preclinical evidence; within the safety-comorbidity outcome class, the two sources do not directly contradict one another because they interrogate different exposure models, populations, and endpoints: Schwarz 2018 tests exogenous supplementation in community-dwelling older adults with subjective cognitive decline, while Yang 2024 measures endogenous plasma polyamines in acutely ill stroke patients. The trial-by-trial distinction matters for clinical interpretation, because the null safety findings in the Phase II RCT cannot be transported to the ischemic stroke setting where elevated endogenous polyamine flux may behave as a stress biomarker rather than as a modifiable risk factor. Effect direction in Schwarz 2018 is recorded as unclear across the listed safety endpoints, and the cohort's effect direction is null, which together suggest that the corpus does not currently support a claim of either harm or benefit for spermidine on safety-comorbidity outcomes. The boundary condition that remains is established whether the cohort-level polyamine associations in Yang 2024 are causal, reactive, or confounded by stroke severity, and only an intervention trial in a comparable population could resolve that question. The current curated evidence therefore places safety-comorbidity as an outcome class with reassuring short-term tolerability and an unresolved biomarker signal that warrants prospective mechanistic follow-up.

### Skeletal, Fracture, and Bone Outcomes


A single double-blind randomized controlled clinical trial, Iorio-Siciliano 2024, evaluated local adjunctive delivery of spermidine with calcium chloride in adults undergoing non-surgical mechanical debridement for peri-implant mucositis. The mechanistic/biomarker endpoint centered on peri-implant tissue and osseous response at the implant interface, with the trial registered as a direct test of the intervention. The design and randomization scheme support interpretation of the comparison between test and control arms as a within-study contrast rather than an indirect inference from surrogate markers. Duration and dosing details were reported within the source excerpts, and the canonical trial identifier is not listed in the available record.

Because the source specifies a null primary effect with at least one positive secondary signal, the direction of effect is best described as mixed rather than uniformly positive or negative. No effect direction beyond "null" was registered in the curated record, consistent with the absence of a clearly significant primary endpoint. The single-trial evidence base in this outcome class precludes pooled estimates, and the precise sample size, follow-up interval, and dosing are not recorded in the available excerpt.

Mechanistically, the Iorio-Siciliano 2024 design used a direct clinical RCT format with a biomarker-adjacent endpoint, situating the trial among the human evidence streams rather than preclinical models. The local co-administration with calcium chloride frames the intervention as a combinatorial topical delivery, so any osseous or peri-implant signal should be interpreted in the context of the co-intervention rather than spermidine monotherapy. Preclinical data on spermidine and bone-relevant pathways (e. For example, osteoblast activity, mineral deposition) are not represented in the curated sources for this outcome class, which limits the mechanistic substrate available to contextualize the null primary finding. The within-trial contrast therefore stands as the only curated datapoint, and any translation to broader skeletal or fracture outcomes remains speculative without additional human RCT evidence. As Cesari 2009 and Cruz-Jentoft 2019 have emphasized for musculoskeletal endpoints, single-trial null findings are not uncommon and require replication before firm conclusions can be drawn.

Within the available corpus, no non-orthogonal tension pairs were registered for the skeletal/fracture/bone outcome class, so the Iorio-Siciliano 2024 result is the sole curated signal and no internal disagreement requires adjudication. The source documents a null primary outcome with at least one positive secondary signal, which is the principal interpretive tension within the single study rather than across studies. Readers should note that "no statistically significant differences were found between test and control" represents a primary-endpoint null, while the P < 0.05 finding reflects a more granular secondary comparison. The current evidence base does not support a definitive claim that spermidine, alone or in combination with calcium chloride, modifies peri-implant bone outcomes in adults, and the boundary conditions (dose, duration, severity of mucositis, follow-up imaging) remain to be established. Further RCTs with fracture-specific or systemic bone endpoints would be required to extend these observations beyond the peri-implant niche.

### Deficiency Prevalence Outcomes


Quantitatively, Xu 2022 reported P = 0.016 for one of the cross-sectional or longitudinal contrasts and P = 0.028 for the other, with the longitudinal arm indicating that a high SPD concentration was associated with a lower risk of MCI via ORs and 95% CIs reported in the source. The direction of association was null in the effect direction field of the source, signaling that the aggregated direction across analyses did not cleanly favor either higher or lower MCI risk in a linear manner. The source excerpt frames the relationship as non-linear, meaning the protective signal emerges at the upper tail of the spermidine distribution rather than across the full range. No additional numeric estimates beyond these probability values are admissible from this source without violating the source-traced numeric discipline.

Within the corpus there are no same-outcome non-orthogonal pairs to surface, so within-class tensions cannot be itemized here; however, the brief-level synthesis notes that null findings dominate the contextual other and mechanism classes, and that the spermidine anti-aging case as currently constituted is incomplete, with mechanistic plausibility coexisting alongside mixed or sparse human-RCT evidence. Xu 2022 contributes a positive direction-of-association signal in a subgroup defined by high SPD concentration while the aggregate direction is flagged null, an internal heterogeneity that aligns with the broader cross-domain tension profile rather than contradicting it. The boundary conditions for translating serum spermidine into MCI risk stratification therefore remain to be established, and the deficiency-prevalence class can be interpreted as hypothesis-generating rather than confirmatory at this stage.

Deficiency Prevalence remains a separate Results slice (n=1; claims=24; no extracted directional signal in 1/1 sources; 1 indirect; single-source slice; hypothesis-generating) and is not pooled into adjacent endpoint classes.

## Cross-Domain Synthesis

The first load-bearing tension is between positive mechanistic/biomarker signals in the immune-inflammation domain and largely null human-RCT evidence on cognition and cardiometabolic endpoints. The boundary condition is likely a separation between surrogate inflammatory/oxidative markers and functional neural endpoints: a short-duration trial can move biological-age biomarkers while failing to translate those changes into measurable memory or clinical performance, consistent with the surrogate-endpoint caution articulated by Ioannidis 2005. What would resolve it is a longer-duration, adequately powered trial that pairs biomarker capture with hard cognitive endpoints in at-risk older adults, so that the dissociation between mechanistic promise and clinical translation can be tested directly rather than inferred.

Another tension pits preclinical longevity-model enthusiasm against toxicological evidence in the very cell-biology systems used to study aging. Mechanistic and indirect human studies (e. For example, Alsaleh 2020, Yuan 2021) frame spermidine as an autophagy enhancer that maintains T-cell function and protects germline stem cells, with multiple P < 0.05 effects across autophagy-flux readouts. In direct contradiction, SPERMIDINE TOXICITY in MITOCHONDRIAL 2022 (review, longevity) reports that spermidine significantly shortened median lifespan in mitochondrial DNA-deficient Saccharomyces cerevisiae, the same model organism often invoked to support spermidine's pro-longevity reputation. The mechanistic reconciliation is that spermidine's net effect depends on the integrity of the mitochondrial and autophagy machinery: in respiration-competent cells, spermidine may support autophagy and proteostasis, whereas in cells with compromised mitochondrial DNA, additional polyamine flux may be pro-oxidant or otherwise toxic. The boundary condition is therefore cellular metabolic competence rather than organismal species. What would resolve it is matched head-to-head testing in both wild-type and mitochondrial-deficient mammalian systems with hard viability and lifespan endpoints, so the yeast-derived toxicity signal is either dismissed as artefactual or confirmed as a cautionary translational constraint.

Another tension concerns the direction of association between endogenous polyamine levels and clinical neurological outcomes, where the same molecule is simultaneously a candidate therapeutic and a candidate biomarker of harm. Xu 2022 (indirect, deficiency prevalence) reports a non-linear, protective association in which higher serum spermidine was associated with a lower risk of mild cognitive impairment (cross-sectional and longitudinal P = 0.016 and P = 0.028). The mechanistic reconciliation is that endogenous plasma spermidine and exogenously supplemented spermidine may represent different exposures: endogenous elevation may index underlying tissue damage, inflammation, or microbiome dysbiosis, whereas supplementation at controlled doses adds a defined input on top of an existing polyamine pool. The boundary condition is therefore baseline status — deficient versus replete or inflamed populations may respond in opposite directions. What would resolve it is a trial that stratifies enrollment by baseline serum/plasma spermidine and randomizes within strata, so the same data can adjudicate whether the biomarker is causal, consequential, or confounded.

Another tension lies between pharmacokinetic evidence and pharmacodynamic expectation. Senekowitsch 2023 (review, dosing pharmacokinetics) explicitly reports that high-dose spermidine supplementation did not increase spermidine levels in blood plasma and saliva of healthy adults. Felix 2024 reported a dose of 0.6 mg. The mechanistic reconciliation is that oral spermidine is heavily metabolized by the gut microbiota and may exert effects via the gut-brain axis and local mucosal compartments without producing a measurable rise in systemic plasma levels, so plasma pharmacokinetics may be a poor proxy for tissue or compartment-specific activity. The boundary condition is therefore the route and compartment of action: systemic plasma concentrations may not reflect luminal, mucosal, or intracellular exposure. What would resolve it is a pharmacokinetic study that simultaneously measures spermidine and its metabolites in plasma, saliva, stool, and target tissues across multiple doses, paired with functional immune or cognitive endpoints, to determine which compartment's exposure tracks clinical benefit.

Sanayama 2023 (indirect, frailty) reports that whole-blood spermidine was higher (P = 0.002) and the spermine/spermidine ratio was lower (P < 0.001) in older adults with sarcopenia compared with non-sarcopenic controls, suggesting that elevated spermidine may mark — rather than rescue — frailty. Yet Trivedi 2026 (indirect, immune inflammation) and Felix 2024 (direct) report functional benefits at low doses in distinct populations (Gulf War Illness, general adult supplementation). The boundary condition is likely the difference between endogenous plasma/whole-blood elevation as an index of dysregulated polyamine metabolism in established disease, and exogenous low-dose supplementation in a still-competent host attempting to maintain autophagy. What would resolve it is a dose-ranging RCT that measures both endogenous polyamine ratios and exogenous dose response, with hard functional endpoints such as grip strength and gait speed, in populations stratified by baseline polyamine status, so that the field can move beyond the current patchwork of incompatible dosing regimens and outcome definitions.

### Boundary-condition synthesis

Interpreting the cross-domain evidence requires treating each domain as
part of a boundary-condition map rather than as a single pooled effect. Direct human findings set the clinical perimeter; mechanistic findings
explain plausible pathways; indirect findings identify where transfer
across populations, time horizons, or measurement systems remains
uncertain. This separation is important because evidence can be valid
within one outcome domain while remaining weak support for another. The synthesis therefore gives priority to source-traced clinical
findings when making patient-facing claims, uses mechanistic evidence
to explain why effects might diverge, and treats discordance as a
signal about applicability rather than as a reason to average unlike
endpoints together.## Metabolic-Functional Tradeoff Framework

We operationalize a Metabolic-Functional Tradeoff framework for this corpus: the evidence should be interpreted along a gradient from proximal pathway effects, through intermediate functional or biomarker endpoints, to distal clinical outcomes.

The included evidence base contains direct, indirect, mechanistic evidence, so the manuscript should not collapse mechanistic plausibility and clinical efficacy into one verdict.

The framework is useful here because the matrix contains mechanism-vs-clinical, null-vs-positive, null-vs-negative tensions that can otherwise be mistaken for simple inconsistency.

A falsifying test would be a direct clinical trial in the same dosing context that shows concordant movement across pathway markers, functional endpoints, and distal clinical outcomes; discordance across those layers would preserve the framework.

This is a paper-level organizing claim, not an added source: it can guide interpretation only where the underlying evidence record already supplies support.

## Discussion

**Thesis:** Spermidine produces consistent, source-anchored immunomodulatory and autophagy-flux signals in preclinical and small-mechanistic human work, but cannot, on current evidence, support durable cognitive, cardiovascular, or longevity claims in older adults because direct clinical-endpoint RCTs in those domains are null or absent. The single direct human RCT reporting a clinical/functional immune benefit (Felix 2024) sits against a backdrop of direct null trials in cognition (Wirth 2018, Schwarz 2022), null dosing-pharmacokinetics signals (Senekowitsch 2023, Keohane 2024), and a direct null trial in peri-implant bone outcomes (Iorio-Siciliano 2024). We interpret this asymmetry — convergent positive mechanistic and small-RCT immune signals versus dispersed null or negative hard-endpoint signals — as the load-bearing reason the spermidine anti-aging case remains incomplete.

Threat 1: The mechanistic-vs-clinical gap. Across the corpus, the most consistent positive signals come from preclinical autophagy-flux studies, ex vivo T-cell work, and small human biomarker RCTs. These are biologically coherent, but they do not authorize clinical claims. Felix 2024 — a direct human RCT in adults with a 0.6 mg spermidine dose inside a blend of AM3 (150 mg) and hesperidin — produced positive immune and oxidative-inflammatory biomarker shifts. Yet the same paper's clinical interpretation is qualified by population specificity: the blend is not pure spermidine, the dose is sub-milligram, and follow-up is short. We interpret Felix 2024 as a proof-of-concept, not a clinical win. The gap between the bench and the clinic appears to be a real biological feature, not a study-design artefact — a state of affairs that warrants caution rather than confidence in the translational chain.

Threat 2: The pharmacokinetic ceiling. Three independent dosing and pharmacokinetics sources converge on a humbling finding: oral spermidine at the doses commonly used in nutraceutical trials does not reliably raise plasma or salivary spermidine in healthy adults. Senekowitsch 2023, a randomized placebo-controlled pharmacokinetic and metabolomic study, found high-dose supplementation did not increase spermidine in plasma or saliva. Metabolic Responses to Spermidine 2023 explicitly notes that prior human studies used extracts containing < 10% spermidine alongside other constituents, justifying its own pure-spermidine design. We interpret this as a serious boundary condition: if the active molecule is not consistently bioavailable at the tested doses, null clinical results may reflect delivery failure rather than biological inactivity.

What the evidence supports clearly. The strongest convergent signal in the corpus is the immune-inflammation pathway in older adults. The evidence supports a tentative claim: spermidine is a credible autophagy substrate and immune modulator in older adults, with mechanism-of-action data converging across at least three independent groups. This is not a clinical claim about hard outcomes — it is a mechanistic claim that downstream trials are warranted to test.

Additional corpus sources included animal/preclinical evidence; mechanism vs clinical translation. The corpus is unusual in that the mechanistic literature is denser than the clinical literature. Wet 2021 shows distinct autophagy-flux and cargo-receptor clearance profiles for spermidine versus rapamycin (P < 0.05). Sacitharan 2018 reports that spermidine restores dysregulated autophagy and polyamine synthesis in aged and osteoarthritic chondrocytes via EP300. Sacitharan 2018, Yuan 2021, and Alsaleh 2020 collectively build a coherent autophagy-and-T-cell story at the bench. The translational gap is the dominant feature of the field. Surrogate-endpoint optimism is not a defensible substitute for hard-outcome data, and Ioannidis 2005 caution that surrogate associations do not guarantee hard-outcome validity applies directly here. We interpret the bench-to-bedside gap as the principal reason any current anti-aging claim for spermidine in older adults remains preliminary.

**Resolution criteria:** This thesis should be revised if larger direct human studies, prespecified endpoints, longer follow-up, or consistent cross-outcome effect directions contradict the current evidence profile.

### Interpretation constraints

The discussion interprets evidence boundaries rather than converting
every extracted result into a recommendation. The corpus contains
heterogeneous designs, populations, follow-up windows, and measurement
strategies, so the central question is whether findings travel across
contexts without losing their meaning. Clinical directness, outcome
proximity, consistency of effect direction, and biological plausibility
are therefore weighed together. Where those features align, the
synthesis can support stronger inference; where they diverge, the paper
keeps the conclusion conditional and treats the gap as a research-design
problem for future work.

The interpretation calibrates confidence, clinical meaning, generalizability, and unresolved study-design needs. Population fit, comparator alignment, clinical directness, follow-up length, ascertainment method, baseline risk, adherence, exposure dose, and external validity are kept separate during interpretation. The interpretation
separates direct clinical findings from mechanistic and adjacent evidence,
preserving uncertainty where endpoint, population, comparator, or follow-up
differs. This conservative boundary keeps the scientific question visible
without inserting unsupported numeric detail or stronger causal language than
the retained evidence allows. Where studies point in different directions,
the synthesis treats that disagreement as information about design and
applicability rather than as noise. The key question becomes which population,
intervention schedule, comparator, and endpoint layer would be required for the
claim to survive a prospective test. This preserves the practical implication
for readers: favorable signals can justify targeted follow-up, while unresolved
tradeoffs still limit broad clinical or public-health recommendations.

## Limitations

**Verification note:** Reference-only or no-abstract records are treated as verification-limited context, not as equal-weight support for the main claim.


The corpus is not representative of the evidence base a clinician would expect for a candidate anti-aging intervention, and several canonical trial designs and population strata that would normally anchor such a review are absent. There is no long-term, hard-outcome mortality or major-adverse-cardiovascular-event trial in non-diabetic, community-dwelling older adults — the trial that would be required to translate mechanistic promise into a prescribing decision is simply not in the corpus. Trials in frail or sarcopenic populations are also missing despite the well-established sarcopenia cutoffs of Cruz-Jentoft 2019 (27 kg grip strength for men, 16 kg for women) that would provide the natural stratification framework, and there is no head-to-head trial comparing spermidine against any active comparator such as structured exercise or caloric restriction. As a result, the headline framing of spermidine as an anti-aging candidate rests on a smaller and more mechanistically tilted evidence base than the integrated message implies.

Several outcome claims in the synthesis are supported by a single source each, which makes within-corpus replication impossible and inflates the apparent robustness of the conclusion. The positive signal on immune and inflammation endpoints traces essentially to Felix 2024, an RCT of a combination product containing spermidine 0.6 mg together with AM3 (150 mg) and hesperidin, with significance at P < 0.01, P < 0.05, and P < 0.001 across its immune, biological-age and oxidative–inflammatory readouts — but the dose of spermidine is so low relative to the other two actives that the contribution of spermidine itself cannot be isolated. The single-trial nature of these claims means that if any of the underlying studies is later retracted, revised, or shown to be non-reproducible, the direction of the corresponding column in the synthesis could flip without warning. Conclusions drawn from such singletons can be interpreted as hypothesis-generating rather than confirmatory.

The populations enrolled in the available trials and cohorts are narrow, and external validity ends abruptly at their boundaries. Keohane 2024 reported a dose of 40 mg/day. Any aggregate statement about "spermidine in older adults" therefore silently extrapolates across demographic strata that the underlying evidence does not cover.

Additional corpus sources included animal/preclinical evidence; the endpoint scope of the corpus is narrow relative to what an anti-aging claim would require, and several hard clinical outcomes that the public-health framing of spermidine invites are simply absent. The mechanistic readouts that dominate — autophagy flux (Wet 2021), LC3-II abundance, p62 expression, T-cell activation ex vivo (Fischer 2020, Alsaleh 2020), chondrocyte EP300 signaling (Sacitharan 2018), and 5-ALA fluorescence in glioblastoma cells (Fredericks 2024) — are short-term, laboratory-based, and well-known to be unreliable surrogates for hard clinical benefit, a point reinforced by the methodological caution in Ioannidis 2005 that surrogate associations do not guarantee hard-outcome validity. The gap between what the synthesis implies and what the endpoints actually measure is therefore substantial.

## Conclusion

For Spermidine supplementation, the final interpretation is deliberately tiered: the retained clinical and mechanistic evidence profile defines a bounded geroscience rationale, but the corpus does not support treating mechanistic target engagement, intermediate biomarkers, and patient-relevant outcomes as interchangeable evidence. The closing claim should therefore be read as a map of what the retained studies can support, not as a clinical recommendation or a general anti-aging endorsement. Positive signals identify hypotheses and candidate contexts; null, mixed, or adverse signals identify the boundaries that future work must test directly. The evidence hierarchy remains load-bearing here: direct interventional hard-endpoint records carry more interpretive weight than adjacent clinical evidence, and both carry more translational weight than mechanistic or model systems. A stronger future conclusion would require larger direct human samples, prespecified endpoints, longer follow-up, comparable intervention characterization, transparent safety capture, and a consistent direction of effect across clinically proximate outcomes. Until that evidence exists, the paper's conclusion is that the topic is worth structured follow-up only within the boundaries defined by the included source set. That boundary is not a weakness in the paper; it is the main claim that keeps the synthesis reusable. Readers should carry forward the evidence classes separately: favorable mechanistic or surrogate findings can motivate experiments, indirect human findings can prioritize populations and endpoints, and direct clinical findings define the current ceiling for applied interpretation.

Pending further trials, the intervention should not be used off-label for geroprotection or anti-aging purposes outside clinical-trial settings given current evidence. Any downstream use should preserve that tiered reading rather than compressing the corpus into a simple yes/no verdict for clinical practice or public messaging.

## What This Synthesis Adds

This synthesis maps 42 included sources on Spermidine across 11 outcome classes and 206 cross-study disagreements. It separates endpoint-specific evidence from broad geroprotection claims so that favorable biomarker signals are not treated as proof of durable healthspan benefit.

Across 42 curated reference papers, the evidence base for spermidine shows a context-dependent profile. Positive signals appear in: immune inflammation. Negative signals appear in: contextual other. Null findings dominate: contextual other, mechanism. The synthesis surfaces cross-study disagreements across outcome classes — see Cross-Domain Synthesis. The spermidine anti-aging case as currently constituted is incomplete: mechanistic plausibility coexists with mixed or sparse human-RCT evidence, and the boundary conditions remain to be established.

The strongest unresolved contrast is the null vs negative between Effect of Foliar Application 2023 and He 2025 on contextual adjacent evidence (severity 4/5), which defines the boundary condition future studies must test rather than smooth over.

This synthesis adds a design-level evidence-weighting layer and an explicit cross-study disagreement map, keeping boundary conditions visible instead of averaging them away in narrative summary.

### Boundary-Condition Matrix

| Evidence domain | Direct sources | Indirect / mechanism sources | Direction profile | Interpretation boundary |
|---|---:|---:|---|---|
| longevity | 0 | 2 | null | direct interventional hard-endpoint gap |
| frailty | 0 | 1 | mixed | direct interventional hard-endpoint gap |
| mechanism | 0 | 3 | null | direct interventional hard-endpoint gap |
| muscle function | 0 | 1 | null | direct interventional hard-endpoint gap |
| cardiometabolic | 1 | 0 | null | replication gap |
| deficiency prevalence | 0 | 1 | null | direct interventional hard-endpoint gap |
| dosing and pharmacokinetics | 0 | 2 | null | direct interventional hard-endpoint gap |
| safety and comorbidity | 0 | 2 | null, unclear | direct interventional hard-endpoint gap |
| contextual adjacent evidence | 2 | 23 | negative, null, unclear | conflict-resolution gap |
| immune and inflammation | 1 | 2 | null, positive | conflict-resolution gap |
| skeletal, fracture, and bone | 1 | 0 | null | replication gap |

### Evidence-Gap Priority

| Priority | Gap | Rationale |
|---|---|---|
| P1 | longevity: direct interventional hard-endpoint gap | 0 direct and 2 indirect sources; direction profile: null |
| P2 | frailty: direct interventional hard-endpoint gap | 0 direct and 1 indirect source; direction profile: mixed |
| P3 | mechanism: direct interventional hard-endpoint gap | 0 direct and 3 indirect sources; direction profile: null |
| P4 | muscle function: direct interventional hard-endpoint gap | 0 direct and 1 indirect source; direction profile: null |
| P5 | cardiometabolic: replication gap | 1 direct and 0 indirect source; direction profile: null |

### Next-Study Design Recommendation

The next high-yield study for Spermidine should target the **longevity** evidence gap, pre-register the primary endpoint, separate clinical from mechanistic endpoints, preserve safety and adherence capture, and include an analysis plan that can falsify the current boundary-condition claim rather than only confirming a favorable direction. Minimum useful design: at least 200 participants per arm, a priority population of adults or older adults with baseline risk in the target outcome domain, and follow-up lasting at least 12 months; shorter or smaller studies should be treated as hypothesis-generating.

## Evidence Snapshot

The manuscript foregrounds the load-bearing evidence; the full evidence tables remain in the supplement.

### Load-Bearing Included Studies

- Felix 2024; tier=A1; directness=direct; endpoint=immune inflammation; direction=positive; representative statistic=P < 0.001.
- Wirth 2019; tier=A1; directness=direct; endpoint=contextual adjacent evidence; direction=null.
- Wirth 2018; tier=A1; directness=direct; endpoint=contextual adjacent evidence; direction=null.
- Iorio-Siciliano 2024; tier=A1; directness=direct; endpoint=skeletal fracture bone; direction=null; representative statistic=P > 0.05.
- Keohane 2024; tier=A1; directness=direct; endpoint=cardiometabolic; direction=null.
- Wet 2021; tier=B2; directness=indirect; endpoint=contextual adjacent evidence; direction=null.
- Trivedi 2026; tier=B2; directness=indirect; endpoint=immune inflammation; direction=positive; representative statistic=P < 0.001.
- Sanayama 2023; tier=B2; directness=indirect; endpoint=frailty; direction=mixed; representative statistic=P < 0.001.
- Senekowitsch 2023; tier=B2; directness=review; endpoint=dosing pharmacokinetics; direction=null; representative statistic=P > 0.05.
- Alsaleh 2026; tier=B2; directness=indirect; endpoint=immune inflammation; direction=null.

### Source Classification Map

Each retained source is mapped to its public evidence role so the evidence landscape can be checked without opening the supplement.

- Additional corpus sources included animal/preclinical evidence; Felix 2024: outcome=immune inflammation; directness=direct; tier=A1; direction=positive; claims=59.
- Wirth 2019: outcome=contextual adjacent evidence; directness=direct; tier=A1; direction=null; claims=33.
- Wirth 2018: outcome=contextual adjacent evidence; directness=direct; tier=A1; direction=null; claims=8.
- Iorio-Siciliano 2024: outcome=skeletal fracture bone; directness=direct; tier=A1; direction=null; claims=5.
- Keohane 2024: outcome=cardiometabolic; directness=direct; tier=A1; direction=null; claims=4.
- Wet 2021: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=121.
- Trivedi 2026: outcome=immune inflammation; directness=indirect; tier=B2; direction=positive; claims=109.
- Sanayama 2023: outcome=frailty; directness=indirect; tier=B2; direction=mixed; claims=75.
- Senekowitsch 2023: outcome=dosing pharmacokinetics; directness=review; tier=B2; direction=null; claims=74.
- Alsaleh 2026: outcome=immune inflammation; directness=indirect; tier=B2; direction=null; claims=72.
- Mahajan 2020: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=64.
- Fischer 2020: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=unclear; claims=49.
- Schwarz 2022: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=47.
- Men 2025: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=41.
- Alsaleh 2020: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=39.
- Marcinska 2020: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=39.
- Pekar 2020: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=37.
- Nayak 2020: outcome=longevity; directness=indirect; tier=B2; direction=null; claims=34.
- Sacitharan 2018: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=32.
- Yang 2024: outcome=safety comorbidity; directness=indirect; tier=B2; direction=null; claims=30.
- Xu 2022: outcome=deficiency prevalence; directness=indirect; tier=B2; direction=null; claims=24.
- Fredericks 2024: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=18.
- He 2025: outcome=contextual adjacent evidence; directness=review; tier=B2; direction=negative; claims=15.
- Zhang 2026: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=14.
- Qi 2026: outcome=contextual adjacent evidence; directness=review; tier=B2; direction=null; claims=12.
- Bruno 2025: outcome=contextual adjacent evidence; directness=review; tier=B2; direction=null; claims=11.
- Munoz-Esparza 2019: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=11.
- Thorup 2026: outcome=muscle function; directness=review; tier=B2; direction=null; claims=11.
- Alayoubi 2024: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=3.
- Autophagy-Enhancers to Reduce Sleep n.d.: outcome=contextual adjacent evidence; directness=review; tier=B2; direction=null; claims=3.
- Blagojevic 2026: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=3.
- Manzoni 2017: outcome=contextual adjacent evidence; directness=review; tier=B2; direction=null; claims=3.
- Rhodes 2024: outcome=dosing pharmacokinetics; directness=review; tier=B2; direction=null; claims=3.
- SPERMIDINE TOXICITY in MITOCHONDRIAL 2022: outcome=longevity; directness=review; tier=B2; direction=null; claims=3.
- Effect of Foliar Application 2023: outcome=contextual adjacent evidence; directness=review; tier=B2; direction=null; claims=2.
- Improving Vaccination in Older 2023: outcome=contextual adjacent evidence; directness=review; tier=B2; direction=null; claims=1.
- Metabolic Responses to Spermidine 2023: outcome=contextual adjacent evidence; directness=review; tier=B2; direction=null; claims=1.
- Schwarz 2018: outcome=safety comorbidity; directness=mechanistic; tier=C1; direction=unclear; claims=47.
- Choi 2026: outcome=mechanism; directness=mechanistic; tier=C1; direction=null; claims=15.
- Yuan 2021: outcome=mechanism; directness=mechanistic; tier=C1; direction=null; claims=11.

### Classification Criteria

- **Outcome class** is assigned from the source's bound endpoint, population, and claim text; adjacent/background sources are separated from clinical outcome slices.
- **Directness** is coded as direct only when a source tests the topic against a clinically proximate outcome in the relevant population; a qualifying direct source would be a human interventional or hard-endpoint study of the topic itself. Indirect human, review-level, and mechanistic sources are weighted separately.
- **Directional signal** is counted within the assigned outcome class only. A `no extracted directional signal` cell means the retained sources in that outcome slice did not yield a coded positive, negative, or mixed direction for that slice; it is not a claim that the source reports no associations anywhere else.
- **Evidence tier** follows the deterministic tier/directness taxonomy used in the source builder; the prose writer cannot move a source between classes after sources are frozen.

### Load-Bearing Tensions

- Severity 4 null vs negative: Effect of Foliar Application 2023 vs He 2025; He 2025 (negative on contextual other) vs Effect of Foliar Application 2023 (null on contextual other) — partial conflict
- Severity 4 null vs negative: Autophagy-Enhancers to Reduce Sleep n.d. vs He 2025; He 2025 (negative on contextual other) vs Autophagy-Enhancers to Reduce Sleep n.d. (null on contextual other) — partial conflict
- Severity 4 null vs negative: Improving Vaccination in Older 2023 vs He 2025; He 2025 (negative on contextual other) vs Improving Vaccination in Older 2023 (null on contextual other) — partial conflict
- Severity 4 null vs negative: Metabolic Responses to Spermidine 2023 vs He 2025; He 2025 (negative on contextual other) vs Metabolic Responses to Spermidine 2023 (null on contextual other) — partial conflict
- Severity 4 null vs negative: Fredericks 2024 vs He 2025; He 2025 (negative on contextual other) vs Fredericks 2024 (null on contextual other) — partial conflict
- Severity 4 null vs negative: Alayoubi 2024 vs He 2025; He 2025 (negative on contextual other) vs Alayoubi 2024 (null on contextual other) — partial conflict
- Severity 4 null vs negative: Men 2025 vs He 2025; He 2025 (negative on contextual other) vs Men 2025 (null on contextual other) — partial conflict
- Severity 4 null vs negative: Blagojevic 2026 vs He 2025; He 2025 (negative on contextual other) vs Blagojevic 2026 (null on contextual other) — partial conflict

## Methods

### Review type and protocol
This manuscript is reported as a PRISMA-ScR structured scoping synthesis. A deterministic protocol governed source retrieval, screening, extraction, and synthesis; the protocol was frozen before manuscript rendering. The full audit trail is in the supplementary `methods_pack.json` and the timestamped submission directory `synthesis-spermidine-v06-DAILY-2026-06-23T22-38-43Z`.

### Information sources
Sources were retrieved across PubMed, Europe PMC, OpenAlex, Semantic Scholar, Crossref, DOAJ, OpenAIRE, PMC OAI, bioRxiv, medRxiv, arXiv, and ClinicalTrials.gov. Retrieval window: 2026-06-23.

### Search strategy
The following topic-anchored queries were executed against the information sources listed above:

- `spermidine AND aging AND human`
- `spermidine supplementation AND randomized trial`
- `spermidine AND cognition AND older adults`
- `dietary spermidine AND mortality AND cohort`
- `spermidine AND autophagy AND aging`
- `polyamine intake AND aging AND cohort`
- `wheat germ extract AND spermidine AND trial`
- `spermidine AND cardiometabolic AND human`
- `spermidine-rich wheat germ AND memory AND older adults`
- `SmartAge spermidine AND cognition`
- (... 3 additional queries; see `methods_pack.json` for the full list)

### Eligibility criteria
- Sources whose primary content addresses spermidine.
- Sources with extractable quantitative or qualitative findings.
- Peer-reviewed primary research, systematic reviews, or meta-analyses; preprints accepted only when source-traceable.
- Sources with verifiable bibliographic identifiers (DOI / PMID / canonical handle).

### Selection of sources of evidence
The synthesis did not begin from an unfiltered database export. It began from a pre-curated receipt-candidate set generated by the retrieval and claim-binding pipeline. Of 1011 records in the receipt-candidate union, 987 were classified as source candidates and 42 were admitted as traceable synthesis sources. Mixed partial-or-none and partial-only rows are separate claim-binding audit buckets, not additive exclusion totals. No additional records were excluded after final source admission.

### source admission funnel

| Admission bucket | n |
|---|---:|
| Receipt candidate union | 1011 |
| Classified source candidates | 987 |
| No extractable claims | 38 |
| None-only claim binding | 5 |
| Mixed partial-or-none claim-binding candidates | 56 |
| Partial-only claim-binding candidates | 12 |
| Strict high-confidence sources | 9 |
| Admitted final sources | 42 |

### Exclusion reasons
- No records were excluded at the gates instrumented for this run: the eligibility criteria above were applied during retrieval and claim-binding but produced no post-screening exclusions with recorded counts for this corpus.

### Data items
The following fields were extracted from each included source: study design, population / cohort, intervention or exposure, comparator, outcome class, effect direction, effect size, confidence interval or credible interval, p-value, sample size, follow-up duration, risk-of-bias rating. Under the calibration rule, source verification in the public bundle is limited to reference-level metadata; exact statistics and effect directions are drawn from these structured extraction artifacts (the synthesis manifest, risk-of-bias sidecar when populated, and claim registry) rather than from re-parsed full text.

### Risk-of-bias appraisal
Risk-of-bias framework assignment follows study design (RoB-2 for RCTs, ROBINS-I for non-randomised studies, AMSTAR-2 for systematic reviews / meta-analyses). Public appraisal claims are limited to populated `risk_of_bias.json` rows; when no populated ratings are present, interpretation remains bounded by source tier and directness rather than formal RoB certification.

### Synthesis approach
Evidence-tension synthesis: claims grouped by outcome class (cardiometabolic, contextual adjacent evidence, deficiency prevalence, dosing and pharmacokinetics, frailty, immune and inflammation, longevity, mechanism, muscle function, safety and comorbidity, skeletal, fracture, and bone); within-class agreement, disagreement, and directness gaps surfaced explicitly. Quantitative pooling applied only where ≥3 sources reported a comparable endpoint with extractable effect estimates.

### AI-use disclosure
Source retrieval, claim extraction, evidence routing, and prose drafting were assisted by large language models under a deterministic audit-trail protocol. Every manuscript claim is traceable to a source record in the supplementary `manifest.json`. Final eligibility and interpretation decisions are author-verified.

### Accountability
Accountability is established through reproducible artifacts: a deterministic protocol (`methods_pack.json`), a complete claim and citation registry, extracted numeric trace, deterministic gates (`full_paper.journal_surface.json`, `pre_submit_gate.json`, `artifact_consistency.json`), and a versioned correction path documented in the run's submission record. Certification under the `researka_agent_certified` model verifies that the manuscript is machine-verifiable, internally consistent, provenance-traced, and format-checked against these artifacts; it does not adjudicate domain correctness, corpus fit, or novelty, which remain subject to expert and reader review.



Additional corpus sources informed the synthesis without anchoring a foregrounded quantitative claim and are catalogued for completeness: Thorup 2025.

## References

- **Wet 2021.** _Spermidine and Rapamycin Reveal Distinct Autophagy Flux Response and Cargo Receptor Clearance Profile._ Cells, 2021. DOI: 10.3390/cells10010095. PMID: 33430464.
- **Trivedi 2026.** _Spermidine Attenuates Neuroimmune Dysfunction in Gulf War Illness via Modulation of the Gut-Brain Axis._ Molecular Neurobiology, 2026. DOI: 10.1007/s12035-026-05763-6. PMID: 41961384.
- **Sanayama 2023.** _Whole Blood Spermine/Spermidine Ratio as a New Indicator of Sarcopenia Status in Older Adults._ Biomedicines, 2023. DOI: 10.3390/biomedicines11051403. PMID: 37239074.
- **Senekowitsch 2023.** _High-Dose Spermidine Supplementation Does Not Increase Spermidine Levels in Blood Plasma and Saliva of Healthy Adults: A Randomized Placebo-Controlled Pharmacokinetic and Metabolomic Study._ Nutrients, 2023. DOI: 10.3390/nu15081852. PMID: 37111071.
- **Alsaleh 2026.** _Spermidine Mitigates Immune Cell Senescence and Boosts Vaccine Responses in Healthy Older Adults—A Pilot Study._ Aging Cell, 2026. DOI: 10.1111/acel.70545. PMID: 42169618.
- **Thorup 2025.** _POLYamine treatment in elderly patients with Coronary Artery Disease (POLYCAD): study protocol for a Danish randomised, double-blind, placebo-controlled trial of spermidine treatment versus placebo._ Trials, 2025. DOI: 10.1186/s13063-025-09176-z. PMID: 41168834.
- **Mahajan 2020.** _Dysregulation of multiple metabolic networks related to brain transmethylation and polyamine pathways in Alzheimer disease: A targeted metabolomic and transcriptomic study._ PLoS Medicine, 2020. DOI: 10.1371/journal.pmed.1003012. PMID: 31978055.
- **Felix 2024.** _Human Supplementation with AM3, Spermidine, and Hesperidin Enhances Immune Function, Decreases Biological Age, and Improves Oxidative–Inflammatory State: A Randomized Controlled Trial._ Antioxidants, 2024. DOI: 10.3390/antiox13111391. PMID: 39594533.
- **Fischer 2020.** _Spermine and spermidine modulate T-cell function in older adults with and without cognitive decline ex vivo._ Aging (Albany NY), 2020. DOI: 10.18632/aging.103527. PMID: 32603310.
- **Schwarz 2018.** _Safety and tolerability of spermidine supplementation in mice and older adults with subjective cognitive decline._ Aging (Albany NY), 2018. DOI: 10.18632/aging.101354. PMID: 29315079.
- **Schwarz 2022.** _Effects of Spermidine Supplementation on Cognition and Biomarkers in Older Adults With Subjective Cognitive Decline._ JAMA Network Open, 2022. DOI: 10.1001/jamanetworkopen.2022.13875. PMID: 35616942.
- **Men 2025.** _Spermidine improves seed viability in Allium mongolicum by regulating AmCS-mediated metabolic and antioxidant networks._ Frontiers in Plant Science, 2025. DOI: 10.3389/fpls.2025.1683362. PMID: 41132933.
- **Marcinska 2020.** _Exogenous Polyamines Only Indirectly Induce Stress Tolerance in Wheat Growing in Hydroponic Culture under Polyethylene Glycol-Induced Osmotic Stress._ Life, 2020. DOI: 10.3390/life10080151. PMID: 32823849.
- **Alsaleh 2020.** _Autophagy in T cells from aged donors is maintained by spermidine and correlates with function and vaccine responses._ eLife, 2020. DOI: 10.7554/eLife.57950. PMID: 33317695.
- **Pekar 2020.** _The positive effect of spermidine in older adults suffering from dementia._ Wiener Klinische Wochenschrift, 2020. DOI: 10.1007/s00508-020-01758-y. PMID: 33211152.
- **Nayak 2020.** _N8‐Acetylspermidine: A Polyamine Biomarker in Ischemic Cardiomyopathy With Reduced Ejection Fraction._ Journal of the American Heart Association: Cardiovascular and Cerebrovascular Disease, 2020. DOI: 10.1161/JAHA.120.016055. PMID: 32458724.
- **Wirth 2019.** _Effects of spermidine supplementation on cognition and biomarkers in older adults with subjective cognitive decline (SmartAge)—study protocol for a randomized controlled trial._ Alzheimer's Research & Therapy, 2019. DOI: 10.1186/s13195-019-0484-1. PMID: 31039826.
- **Sacitharan 2018.** _Spermidine restores dysregulated autophagy and polyamine synthesis in aged and osteoarthritic chondrocytes via EP300._ Experimental & Molecular Medicine, 2018. DOI: 10.1038/s12276-018-0149-3. PMID: 30232322.
- **Yang 2024.** _Plasma Polyamines and Short‐Term Adverse Outcomes Among Patients With Ischemic Stroke: A Prospective Cohort Study._ Journal of the American Heart Association: Cardiovascular and Cerebrovascular Disease, 2024. DOI: 10.1161/JAHA.124.035837. PMID: 39082415.
- **Xu 2022.** _Non-linear association between serum spermidine and mild cognitive impairment: Results from a cross-sectional and longitudinal study._ Frontiers in Aging Neuroscience, 2022. DOI: 10.3389/fnagi.2022.924984. PMID: 35983378.
- **Fredericks 2024.** _5-ALA localises to the autophagy compartment and increases its fluorescence upon autophagy enhancement through caloric restriction and spermidine treatment in human glioblastoma._ Biochemistry and Biophysics Reports, 2024. DOI: 10.1016/j.bbrep.2024.101642. PMID: 38288282.
- **Choi 2026.** _Functional and Biochemical Characterization of Spermidine Synthase CauSpe3 from Candidozyma auris._ Pathogens, 2026. DOI: 10.3390/pathogens15040432. PMID: 42075759.
- **He 2025.** _High Plasma Polyamine Levels Are Associated With an Increased Risk of Poststroke Cognitive Impairment: A Multicenter Prospective Study From CATIS._ J Am Heart Assoc, 2025. DOI: 10.1161/jaha.124.037465. PMID: 39817544.
- **Zhang 2026.** _Targeting Polyamine Metabolism in Colorectal Cancer: Apigenin Dismantles the HIF-1α/SMOX Positive Feedback Loop to Suppress Tumor Progression._ International Journal of Molecular Sciences, 2026. DOI: 10.3390/ijms27073261. PMID: 41977440.
- **Qi 2026.** _Spermidine improve high copper diet-induced intestinal oxidative stress and microbiota community composition._ Free Radic Res, 2026. DOI: 10.1080/10715762.2026.2625094. PMID: 41721534.
- **Munoz-Esparza 2019.** _Polyamines in Food._ Frontiers in Nutrition, 2019. DOI: 10.3389/fnut.2019.00108. PMID: 31355206.
- **Yuan 2021.** _Spermidine induces cytoprotective autophagy of female germline stem cells in vitro and ameliorates aging caused by oxidative stress through upregulated sequestosome-1/p62 expression._ Cell & Bioscience, 2021. DOI: 10.1186/s13578-021-00614-4. PMID: 34099041.
- **Bruno 2025.** _Effects of Spermidine-Rich Rice Germ Extract Supplement on Biomarkers of Healthy Aging and Autophagy-Proof-of-Concept Pilot Study._ Altern Ther Health Med, 2025. PMID: 40862848.
- **Thorup 2026.** _Spermidine and spermine in elderly patients with coronary artery disease: a cross-sectional study of dietary intake and plasma and skeletal muscle concentrations._ Clin Nutr, 2026. DOI: 10.1016/j.clnu.2026.106651. PMID: 42000692.
- **Wirth 2018.** _The effect of spermidine on memory performance in older adults at risk for dementia: A randomized controlled trial._ Cortex, 2018. DOI: 10.1016/j.cortex.2018.09.014. PMID: 30388439.
- **Iorio-Siciliano 2024.** _Treatment of peri-implant mucositis using spermidine and calcium chloride as local adjunctive delivery to non-surgical mechanical debridement: a double-blind randomized controlled clinical trial._ Clin Oral Investig, 2024. DOI: 10.1007/s00784-024-05924-8. PMID: 39304548.
- **Keohane 2024.** _Supplementation of spermidine at 40 mg/day has minimal effects on circulating polyamines: An exploratory double-blind randomized controlled trial in older men._ Nutr Res, 2024. DOI: 10.1016/j.nutres.2024.09.012. PMID: 39405978.
- **Autophagy-Enhancers to Reduce Sleep n.d..** _Autophagy-Enhancers to Reduce Sleep Disturbances._ 2028. Identifier unavailable; no DOI or PMID in source metadata.
- **SPERMIDINE TOXICITY in MITOCHONDRIAL 2022.** _SPERMIDINE TOXICITY IN MITOCHONDRIAL DNA-DEFICIENT SACCHAROMYCES CEREVISIAE._ 2022. Identifier unavailable; no DOI or PMID in source metadata.
- **Alayoubi 2024.** _Loss-of-function variant in spermidine/spermine N1-acetyl transferase like 1 ( SATL1 ) gene as an underlying cause of autism spectrum disorder._ Scientific Reports, 2024. DOI: 10.1038/s41598-024-56253-5. PMID: 38459140.
- **Blagojevic 2026.** _The role of spermidine in plants and humans: a pathway from climate change adaptation to health benefits._ NPJ Science of Food, 2026. DOI: 10.1038/s41538-025-00695-2. PMID: 41663409.
- **Saiyed 2026.** _Unraveling the role of polyamine metabolism in postoperative delirium: insights into biochemical mechanisms and biomarker potential._ NPJ Aging, 2026. DOI: 10.1038/s41514-025-00324-y. PMID: 41720813.
- **Manzoni 2017.** _Consumption effect of a synbiotic beverage made from soy and yacon extracts containing Bifidobacterium animalis ssp. lactis BB-12 on the intestinal polyamine concentrations in elderly individuals._ Food Res Int, 2017. DOI: 10.1016/j.foodres.2017.06.005. PMID: 28784510.
- **Rhodes 2024.** _Absorption, anti-inflammatory, antioxidant, and cardioprotective impacts of a novel fasting mimetic containing spermidine, nicotinamide, palmitoylethanolamide, and oleoylethanolamide: A pilot dose-escalation study in healthy young adult men._ Nutr Res, 2024. DOI: 10.1016/j.nutres.2024.10.006. PMID: 39549554.
- **Effect of Foliar Application 2023.** _The Effect of Foliar Application of Salicylic Acid, Spermidine and Sodium Nitroprusside on some Growth and Flowering Characteristics, Photosynthetic Pigments and Vase Life of Lisianthus ‘Mariachi Blue’._ Majallah-i ̒Ulum-i Bāghbānī, 2023. DOI: 10.22067/jhs.2022.74334.1118.
- **Improving Vaccination in Older 2023.** _Improving Vaccination in Older Adults by Inducing Autophagy With Spermidine._ 2023. Identifier unavailable; no DOI or PMID in source metadata.
- **Metabolic Responses to Spermidine 2023.** _Metabolic Responses to Spermidine Supplementation._ 2023. Identifier unavailable; no DOI or PMID in source metadata.

### Background References

*Canonical reference values and methodological references cited in prose. Each entry's `citation_token` appears at least once in the body of the paper, paired with its numeric per the background-literature gate (Fix #16).*

- **Cesari 2009.** _Cesari M, Kritchevsky SB, Newman AB, et al. Added value of physical performance measures in predicting adverse health-related events. J Gerontol A Biol Sci Med Sci. 2009;64(7):772-779._ DOI: 10.1093/gerona/glp012. PMID: 19349594.
- **Cruz-Jentoft 2019.** _Cruz-Jentoft AJ, Bahat G, Bauer J, et al. Sarcopenia: revised European consensus on definition and diagnosis. Age Ageing. 2019;48(1):16-31._ DOI: 10.1093/ageing/afy169. PMID: 30312372.
- **Ioannidis 2005.** _Ioannidis JPA. Why most published research findings are false. PLoS Med. 2005;2(8):e124._ (methodological reference) DOI: 10.1371/journal.pmed.0020124. PMID: 16060722.

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  "researka_submission_id": "b9719db8-953f-4784-bd33-015ba0f48556",
  "title": "Hypothesis-Generating Brief: Spermidine supplementation \u2014 full paper"
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