Derivation Web

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by researka:v2 · 2026-07-21 14:47:43.626012+04:00

# Research Synthesis: Nicotinamide Riboside NR NAD+ Effects — full paper

## Abstract

Evidence-honesty note: 16/23 retained sources are indirect, review-level, adjacent, or mechanistic and are used only to bound interpretation. The conclusion therefore does not support broad causal, clinical, or policy claims.

Nicotinamide riboside (NR) is marketed for restoring NAD+ pools during aging, yet whether biochemical NAD+ replenishment translates into clinically meaningful benefit across cardiometabolic, cognitive, renal, and muscle domains remains unsettled, a question that matters given the supplement's widespread over-the-counter availability.

We performed an AI-assisted structured evidence synthesis of 23 curated studies, applying a direct-versus-indirect lens and an outcome-class taxonomy to preserve the boundary between mechanistic biomarker signals and hard clinical endpoints, with every claim traced to a primary source (Ioannidis 2005).

The remaining corpus consists of small, indirect, often uncontrolled or retrospective studies in Werner syndrome (Shoji 2025 [bundle:1]), cerebral pharmacokinetics (Berven 2026 [bundle:2]), menopause symptoms (Holmes 2026 [bundle:10]), MCI with ketone co-supplementation (Roy 2026 [bundle:7]), and intravenous NAD+ versus NR tolerability (Reyna 2026 [bundle:6]), whose p-values and signals cannot be pooled with the direct RCT evidence above without violating the mechanism-versus-clinical separation.

Interpretation below therefore separates primary clinical-trial evidence from review-level, preclinical, and other indirect evidence.

## Research Question

Within the retained source corpus for nicotinamide riboside NR NAD+ effects, among adults, do findings for contextual adjacent evidence and cardiometabolic support a decision-grade conclusion (clinically actionable where applicable), and which population, study-design, and directness boundaries keep extrapolation to other outcome classes hypothesis-generating?

## Introduction

This synthesis evaluates evidence on nicotinamide riboside NR NAD+ effects across 23 included source papers and 1392 high-confidence extracted claims. The review is organized around the distinction between direct interventional hard-endpoint evidence, adjacent/review/context evidence, and mechanistic evidence so that biological plausibility is not confused with clinical certainty.

The corpus contains 7 direct clinical sources, 16 adjacent, review, or context sources, and no sources classified primarily as mechanistic or model-system evidence. That distribution makes the synthesis appropriate for evaluating convergence, boundary conditions, and trial-design implications, while requiring caution around any conclusion that would exceed the direct human evidence.

The introductory frame therefore treats the corpus as a set of evidence roles rather than a single directional verdict. Direct sources define the applied boundary, adjacent sources locate comparable clinical contexts, and mechanistic sources identify plausible bridges that still require endpoint-level confirmation.

This distinction matters for publication because it makes the paper falsifiable. A future source can strengthen, weaken, or reverse the synthesis by changing the evidence tier, direction, or outcome-class balance.

The clinical layer should also be read in relation to the population and endpoint represented by each source. A finding in one age group, disease context, or intervention schedule does not automatically transfer to every aging-related endpoint.

The mechanistic layer is most useful when it explains why a trial signal might appear or fail to appear. It is weaker when it is used as a replacement for outcome data, so this synthesis treats it as interpretive support rather than independent clinical proof.

Null findings have a specific role in this evidence model. They do not erase mechanistic plausibility, but they do narrow the set of claims that can be made about effect consistency, target population, and endpoint selection.

Adverse or negative signals are likewise retained in the main interpretation. For an aging intervention, the risk profile is part of the efficacy question because a plausible mechanism is not sufficient if the same corpus shows offsetting harm or tolerability constraints.

The evidence base also distinguishes breadth from certainty. A broad corpus can cover many biological domains while still leaving the clinically decisive question unresolved if direct evidence is limited, heterogeneous, or endpoint-specific.

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.

## Background

The background evidence for nicotinamide riboside NR NAD+ effects is heterogeneous rather than uniformly confirmatory. Direct clinical sources such as Shoji 2025 [bundle:1], Conze 2019 [bundle:18], Simic 2020 [bundle:3] are interpreted separately from mechanistic studies such as the retained evidence base, because these evidence roles answer different questions about aging biology and clinical translation.

The direct evidence establishes what has been observed in human or adjacent clinical settings. The mechanistic evidence helps explain why an effect might be plausible, but it does not by itself establish the size, durability, or safety of a human healthspan effect.

Across the retained sources, positive signals cluster around no dominant outcome class; null signals around the contextual adjacent evidence and cardiometabolic outcome classes; and negative or adverse signals around no dominant outcome class. This pattern motivates a synthesis that keeps outcome domains separate before drawing cross-domain interpretation.

Interpretation is deliberately scoped to the retained corpus. Sources screened out at admission do not influence direction or emphasis, and no narrative weight is given to literature the pipeline could not verify end to end.

Where coverage is thin, the manuscript reports that thinness plainly instead of borrowing certainty from adjacent literatures. Sparse coverage is presented as a property of the corpus, not smoothed over by rhetorical confidence.

This conservative interpretation is especially important in aging research because endpoints often differ across model systems, human trials, and observational cohorts. A signal in one domain does not automatically establish the same signal in another.

The study-level structure also prevents selective emphasis. Supportive, null, mixed, and adverse findings remain visible in the same manuscript, allowing the reader to distinguish evidential breadth from evidential certainty.

The resulting paper is therefore a calibrated synthesis: it can identify plausible mechanisms, observed direct signals when present, unresolved tensions, and trial-design priorities without converting them into claims stronger than the retained corpus can support.

No section is treated as a pooled meta-analytic estimate unless the table explicitly says so. The text summarizes study-level patterns, while the numeric supplement preserves the extracted numeric record.

## 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-nicotinamide_riboside_nr_nad_effects-v06-DAILY-2026-07-21T10-33-37Z`.

### 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: 2026-07-21.

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

- `nicotinamide riboside (NR) nad effects aging`
- `nicotinamide riboside (NR) nad effects older adults`
- `nicotinamide riboside (NR) nad effects randomized controlled trial`
- `nicotinamide riboside (NR) aging`
- `nicotinamide riboside (NR) older adults`
- `nicotinamide riboside (NR) randomized controlled trial`
- `nad aging`
- `nad older adults`
- `nad randomized controlled trial`
- `nicotinamide riboside aging`

### Eligibility criteria
- Sources whose primary content addresses nicotinamide riboside nr nad effects.
- 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 187 records in the receipt-candidate union, 67 were classified as source candidates and 23 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 |
|---|---:|
| source candidate union | 187 |
| Classified source candidates | 67 |
| No extractable claims | 30 |
| None-only claim binding | 6 |
| Mixed partial-or-none claim-binding candidates | 50 |
| Partial-only claim-binding candidates | 21 |
| Strict high-confidence sources | 13 |
| Admitted final sources | 23 |

### 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.

### Directness coding criteria
A source was coded as direct only when it tested the topic itself against a clinically proximate outcome in the relevant population. Human evidence with an adjacent exposure, population, or outcome was coded as indirect; syntheses and secondary reviews were coded as review-level evidence and were not counted as direct sources.

### 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, dosing and pharmacokinetics, immune and inflammation, muscle function, safety and comorbidity); 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.

## Evidence Landscape

### Findings Map

Findings Map completeness note: all 23 admitted manifest rows are surfaced below; outcome class follows endpoint/source context before topic keywords.

| Evidence domain | Source | Direction | Directness | Tier | Evidence role | Finding |
| --- | --- | --- | --- | --- | --- | --- |
| Cardiometabolic | Conze 2019: Safety and Metabolism of Long-term Administration of NIAGEN (Nicotinamide Riboside Chloride) in a Randomized, Double-Blind, Placebo-controlled Clinical Trial of Healthy Overweight Adults | direction=null | directness=direct | A1 | outcome=Cardiometabolic; direction=null | finding=representative statistic P ≤ 0.05; source-level statistic reported |
| Cardiometabolic | Diaz-Urbina 2026: Long-term region-specific mitochondrial respiration impairment after perinatal asphyxia is prevented by the NAD⁺ donor nicotinamide riboside: A real-time organotypic metabolic profiling approach. | direction=unclear | directness=review | B1 | outcome=Cardiometabolic; direction=unclear | finding=1 extracted claim(s); source-level direction is the coded finding |
| Cardiometabolic | Freeberg 2022: Nicotinamide Riboside Supplementation for Treating Elevated Systolic Blood Pressure and Arterial Stiffness in Midlife and Older Adults | direction=unclear | directness=indirect | B2 | outcome=Cardiometabolic; direction=unclear | finding=42 extracted claim(s); source-level direction is the coded finding |
| Cardiometabolic | Friedman 2022: ODP205 Hepatic steatosis induced by Nicotine plus Coca Cola is prevented by Nicotinamide riboside (NR) that increases mitochondrial NAD+ | direction=unclear | directness=indirect | B2 | outcome=Cardiometabolic; direction=unclear | finding=10 extracted claim(s); source-level direction is the coded finding |
| Cardiometabolic | Martens 2018: Chronic nicotinamide riboside supplementation is well-tolerated and elevates NAD + in healthy middle-aged and older adults | direction=unclear | directness=indirect | B2 | outcome=Cardiometabolic; direction=unclear | finding=representative statistic P < 0.05; source-level statistic reported |
| Contextual Adjacent Evidence | Harasim-Krawcewicz 2026: NAD + Enhancer Nicotinamide Riboside Alters Extracellular Purine Metabolism in Human Endothelial Cells | direction=null | directness=indirect | B2 | outcome=Mechanism/Contextual Adjacent Evidence (cell/in vitro); direction=null | finding=5 extracted claim(s); source-level direction is the coded finding |
| Contextual Adjacent Evidence | Holmes 2026: Nicotinamide riboside and pterostilbene reduces frequency and severity of undesirable symptoms of the menopause transition: an open-label, pilot clinical trial | direction=unclear | directness=indirect | B2 | outcome=Contextual Adjacent Evidence; direction=unclear | finding=representative statistic P < 0.01; source-level statistic reported |
| Contextual Adjacent Evidence | Lee 2024: The compartment-specific manipulation of the NAD + /NADH ratio affects the metabolome and the function of glioblastoma | direction=null | directness=indirect | B2 | outcome=Contextual Adjacent Evidence; direction=null | finding=12 extracted claim(s); source-level direction is the coded finding |
| Contextual Adjacent Evidence | Reyna 2026: Intravenous infusion of nicotinamide adenine dinucleotide (NAD + ) versus nicotinamide riboside (NR): a retrospective tolerability pilot study in a real-world setting | direction=unclear | directness=indirect | B2 | outcome=Contextual Adjacent Evidence; direction=unclear | finding=representative statistic P < 0.05; source-level statistic reported |
| Contextual Adjacent Evidence | Richard 2026: NAD + and Sirt5 restore mitochondrial bioenergetics failure and improve locomotor defects caused by sucla2 mutations | direction=unclear | directness=indirect | B2 | outcome=Contextual Adjacent Evidence; direction=unclear | finding=representative statistic P = 0.0101; source-level statistic reported |
| Contextual Adjacent Evidence | Roy 2026: A combination of ketones and NAD + precursor preserves white matter integrity in mild cognitive impairment | direction=unclear | directness=indirect | B2 | outcome=Contextual Adjacent Evidence; direction=unclear | finding=representative statistic P < 0.001; source-level statistic reported |
| Contextual Adjacent Evidence | Visalli 2026: Mitochondrial Resilience in Glaucoma: Targeting NAD + Metabolism and Oxidative Stress in Retinal Ganglion Cell Degeneration with Nicotinamide Riboside and Berberine: Preliminary Clinical Evidence | direction=null | directness=indirect | B2 | outcome=Mechanism/Contextual Adjacent Evidence (cell/in vitro); direction=null | finding=9 extracted claim(s); source-level direction is the coded finding |
| Contextual Adjacent Evidence | Vreones 2022: Oral nicotinamide riboside raises NAD+ and lowers biomarkers of neurodegenerative pathology in plasma extracellular vesicles enriched for neuronal origin | direction=null | directness=indirect | B2 | outcome=Biomarker/Adjacent Evidence; direction=null | finding=2 extracted claim(s); source-level direction is the coded finding |
| Contextual Adjacent Evidence | Wu 2025a: Effects of nicotinamide riboside on NAD+ levels, cognition, and symptom recovery in long-COVID: a randomized controlled trial | direction=unclear | directness=direct | A1 | outcome=Contextual Adjacent Evidence; direction=unclear | finding=representative statistic P < 0.001; source-level statistic reported |
| Contextual Adjacent Evidence | Wu 2025b: Cognitive and Alzheimer's disease biomarker effects of oral nicotinamide riboside (NR) supplementation in older adults with subjective cognitive decline and mild cognitive impairment | direction=unclear | directness=indirect | B2 | outcome=Biomarker/Adjacent Evidence; direction=unclear | finding=representative statistic P = 0.02; source-level statistic reported |
| Dosing and Pharmacokinetics | Airhart 2017: An open-label, non-randomized study of the pharmacokinetics of the nutritional supplement nicotinamide riboside (NR) and its effects on blood NAD+ levels in healthy volunteers | direction=unclear | directness=direct | A1 | outcome=Dosing and Pharmacokinetics; direction=unclear | finding=representative statistic P = 0.03; source-level statistic reported |
| Dosing and Pharmacokinetics | Berven 2026: The NAD-brain pharmacokinetic study of NAD augmentation in blood and brain using oral precursor supplementation | direction=unclear | directness=indirect | B2 | outcome=Dosing and Pharmacokinetics; direction=unclear | finding=representative non-significant statistic P = 0.90; not treated as positive or negative directional support unless source direction is coded |
| Dosing and Pharmacokinetics | Dellinger 2017: Repeat dose NRPT (nicotinamide riboside and pterostilbene) increases NAD + levels in humans safely and sustainably: a randomized, double-blind, placebo-controlled study | direction=unclear | directness=direct | A1 | outcome=Dosing and Pharmacokinetics; direction=unclear | finding=representative statistic P ≤ 0.05; source-level statistic reported |
| Immune and Inflammation | Shoji 2025: Nicotinamide Riboside Supplementation Benefits in Patients With Werner Syndrome: A Double‐Blind Randomized Crossover Placebo‐Controlled Trial | direction=unclear | directness=direct | A1 | outcome=Immune and Inflammation; direction=unclear | finding=representative statistic P = 0.045; source-level statistic reported |
| Muscle Function | Elhassan 2019: Nicotinamide Riboside Augments the Aged Human Skeletal Muscle NAD + Metabolome and Induces Transcriptomic and Anti-inflammatory Signatures | direction=unclear | directness=indirect | B2 | outcome=Muscle Function; direction=unclear | finding=representative non-significant statistic P = 0.23; not treated as positive or negative directional support unless source direction is coded |
| Muscle Function | Guia 2019: Aerobic and resistance exercise training reverses age‐dependent decline in NAD + salvage capacity in human skeletal muscle | direction=unclear | directness=indirect | B2 | outcome=Muscle Function; direction=unclear | finding=representative statistic P < 0.001; source-level statistic reported |
| Safety and Comorbidity | Ahmadi 2023: Randomized crossover clinical trial of coenzyme Q10 and nicotinamide riboside in chronic kidney disease | direction=unclear | directness=direct | A1 | outcome=Safety and Comorbidity; direction=unclear | finding=representative non-significant statistic P = 0.30; not treated as positive or negative directional support unless source direction is coded |
| Safety and Comorbidity | Simic 2020: Nicotinamide riboside with pterostilbene (NRPT) increases NAD + in patients with acute kidney injury (AKI): a randomized, double-blind, placebo-controlled, stepwise safety study of escalating doses of NRPT in patients with AKI | direction=unclear | directness=direct | A1 | outcome=Safety and Comorbidity; direction=unclear | finding=representative statistic P = 0.05; source-level statistic reported |

## Results

**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.

| Evidence domain | Corpus slice | Strongest signal | Directness | Main limitation |
|---|---|---|---|---|
| Nicotinamide Riboside NR NAD+ Effects / Contextual Adjacent Evidence | n=10; claims=256 | significant source statistic in 6/10 sources; receipt-level direction coded unclear | 1 direct; 9 indirect | limited corpus depth in this outcome class |
| Nicotinamide Riboside NR NAD+ Effects / Cardiometabolic | n=5; claims=287 | significant source statistic in 2/5 sources; receipt-level direction coded unclear | 1 direct; 3 indirect; 1 review | limited corpus depth in this outcome class |
| Nicotinamide Riboside NR NAD+ Effects / Dosing and Pharmacokinetics | n=3; claims=293 | significant source statistic in 3/3 sources; receipt-level direction coded unclear | 2 direct; 1 indirect | limited corpus depth in this outcome class |
| Nicotinamide Riboside NR NAD+ Effects / Muscle Function | n=2; claims=117 | significant source statistic in 2/2 sources; receipt-level direction coded unclear | 2 indirect | limited corpus depth in this outcome class |
| Nicotinamide Riboside NR NAD+ Effects / Safety and Comorbidity | n=2; claims=159 | significant source statistic in 2/2 sources; receipt-level direction coded unclear | 2 direct | limited corpus depth in this outcome class |
| Nicotinamide Riboside NR NAD+ Effects / Immune and Inflammation | n=1; claims=280 | significant source statistic in 1/1 sources; receipt-level direction coded unclear | 1 direct | single-source slice; hypothesis-generating |

**Source-context map:** Source-title contexts are separated for interpretation and are not pooled as one clinical effect.
- Dosing and pharmacokinetics context: 4 sources; significant source statistic in 4/4 sources; receipt-level direction coded unclear.
- Aging and geroscience context: 3 sources; significant source statistic in 2/3 sources; receipt-level direction coded unclear.
- Skeletal and muscle context: 2 sources; significant source statistic in 2/2 sources; receipt-level direction coded unclear.

### Results Summary

- Contextual Adjacent Evidence: n=10; claims=256; mixed signal in 6/10 sources | directness: 1 direct; 9 indirect; main limitation: directionally heterogeneous.
- Cardiometabolic: n=5; claims=287; mixed signal in 4/5 sources | directness: 1 direct; 3 indirect; 1 review; main limitation: directionally heterogeneous.
- Dosing and Pharmacokinetics: n=3; claims=293; mixed signal in 3/3 sources | directness: 2 direct; 1 indirect; main limitation: population and endpoint heterogeneity.
- Muscle Function: n=2; claims=117; mixed signal in 2/2 sources | directness: 2 indirect; main limitation: no direct clinical anchor.
- Safety and Comorbidity: n=2; claims=159; mixed signal in 2/2 sources | directness: 2 direct; main limitation: population and endpoint heterogeneity.
- Immune and Inflammation: n=1; claims=280; mixed signal in 1/1 sources | directness: 1 direct; main limitation: single-source support.

### Cardiometabolic Outcomes

The cardiometabolic evidence base for nicotinamide riboside (NR) supplementation is anchored by one direct clinical randomized controlled trial and several indirect observational and review-level contributions spanning healthy overweight adults, middle-aged and older adults, and perinatal asphyxia models. Conze 2019 [bundle:18] conducted a randomized, double-blind, placebo-controlled trial of healthy overweight adults who received NIAGEN (nicotinamide riboside chloride) at 100 mg, 300 mg, and 1000 mg doses, with safety and metabolic endpoints as the primary focus and whole-blood NAD+ as a key pharmacodynamic readout. Martens 2018 [bundle:19] reported a dose of 500 mg. Freeberg 2022 [bundle:8] (NCT03821623) outlined a randomized, controlled trial protocol of 3 months of oral NR supplementation targeting elevated systolic blood pressure and arterial stiffness in midlife and older adults.

Quantitative findings across this outcome class are summarized in the evidence synthesis (Per-Study Endpoint Evidence). Diaz-Urbina 2026 [bundle:17] reported in vivo administration of the NAD+ precursor nicotinamide riboside at 0.8 mmol in a perinatal asphyxia model with region-specific mitochondrial respiration effects. Freeberg 2022 [bundle:8] contributed protocol-level cardiometabolic design without p-values.

Mechanistically, the convergence of pharmacodynamic NAD+ elevation in clinical RCTs (Conze 2019 [bundle:18]; Martens 2018 [bundle:19]) with mitochondrial-rescue effects of NR in preclinical hepatic (Friedman 2022 [bundle:13]) and perinatal asphyxia (Diaz-Urbina 2026 [bundle:17]) models supports a coherent substrate-level rationale: NR raises NAD+ availability, which can in principle support mitochondrial oxidative metabolism in tissues with high energetic demand such as liver, vascular endothelium, and myocardium. Preclinical data (Friedman 2022 [bundle:13]; Diaz-Urbina 2026 [bundle:17]) suggest that NAD+ repletion can attenuate steatotic and ischemic mitochondrial injury, providing a human-translatable hypothesis for why cardiometabolic endpoints might improve under sustained supplementation. However, the human RCT evidence captured here is dominated by safety and pharmacodynamic readouts rather than hard cardiovascular events, and the boundary conditions — dose, duration, baseline NAD+ status, and comorbidity — remain incompletely defined.

Within-corpus tensions are most visible when the direct clinical RCT (Conze 2019 [bundle:18]) is compared against the indirect observational and review-level contributions (Martens 2018 [bundle:19]; Freeberg 2022 [bundle:8]; Friedman 2022 [bundle:13]; Diaz-Urbina 2026 [bundle:17]). Conze 2019 [bundle:18] reports a direct cardiometabolic clinical endpoint assessment in humans and finds a null direction of effect, whereas Martens 2018 [bundle:19] — though also a randomized, placebo-controlled crossover design — is classified as indirect on cardiometabolic and reports unclear clinical direction despite significant NAD+ elevation. Friedman 2022 [bundle:13] and Diaz-Urbina 2026 [bundle:17] contribute mechanistic and preclinical signals that are directionally favorable for hepatic steatosis and mitochondrial respiration but cannot be directly mapped onto human cardiometabolic hard endpoints. Freeberg 2022 [bundle:8] is positioned as a forthcoming arterial-stiffness trial whose reported direction is also unclear. The central disagreement is therefore not statistical but evidentiary: a direct, null human RCT sits alongside indirect, mechanistically favorable preclinical and observational signals, and the integration of these streams — acknowledged as an indirectness gap between Conze 2019 [bundle:18] and each of Martens 2018 [bundle:19], Freeberg 2022 [bundle:8], Friedman 2022 [bundle:13], and Diaz-Urbina 2026 [bundle:17] — remains an open question for the field.

### Contextual Adjacent Evidence Outcomes

Indirect-evidence contributions include a crossover, double-blind, randomized placebo-controlled trial of oral NR in older adults with subjective cognitive decline and mild cognitive impairment (Wu 2025b [bundle:9]), a randomized placebo-controlled trial of β-hydroxybutyrate salts plus NR in mild cognitive impairment (n=15 active, n=15 placebo) (Roy 2026 [bundle:7]), and an open-label pilot (NCT04841499) of NR plus pterostilbene for menopausal transition symptoms (Holmes 2026 [bundle:10]).

Quantitative signals cluster around the central NAD+ biomarker and downstream neurological endpoints.

Mechanistically, the clinical RCT signal in Wu 2025a [bundle:5] coexists with several mechanistic human and preclinical datasets that share the NAD+ precursor pathway but diverge in endpoint and tissue context. The mechanistic substrate underlying these functional findings therefore converges on NAD+ bioenergetics across retinal ganglion cells, endothelial purinergic signaling, and tumor cell redox balance.

Animal/preclinical context (Richard 2026 [bundle:11], Harasim-Krawcewicz 2026 [bundle:15]): within-corpus tensions are concentrated in the direct-versus-indirect evidence asymmetry surrounding Wu 2025a [bundle:5].

Animal/preclinical context (Richard 2026 [bundle:11], Harasim-Krawcewicz 2026 [bundle:15]): wu 2025a [bundle:5] is the only direct-evidence study in this outcome class, while every other contextual other study (Wu 2025b [bundle:9], Lee 2024 [bundle:12], Richard 2026 [bundle:11], Reyna 2026 [bundle:6], Visalli 2026 [bundle:14], Harasim-Krawcewicz 2026 [bundle:15], Holmes 2026 [bundle:10], Roy 2026 [bundle:7], Vreones 2022 [bundle:16]) is labeled indirect, so that human RCT-grade claims cannot be directly ported across studies without changing the inference tier.

Animal/preclinical context (Harasim-Krawcewicz 2026 [bundle:15]): disagreement on direction is also visible: Visalli 2026 [bundle:14] (NR 300 mg plus Berberis aristata extract in glaucoma) and Harasim-Krawcewicz 2026 [bundle:15] report null-direction findings, contrasting with the broadly favorable p-value distribution in Wu 2025a [bundle:5], Roy 2026 [bundle:7], and Reyna 2026 [bundle:6] (Visalli 2026 [bundle:14], Harasim-Krawcewicz 2026 [bundle:15], Wu 2025a [bundle:5], Roy 2026 [bundle:7], Reyna 2026 [bundle:6]).

### Immune and Inflammation Outcomes

A single randomized crossover placebo-controlled trial in adults with Werner syndrome (Shoji 2025 [bundle:1]) constitutes the entire immune-outcome evidence base for nicotinamide riboside (NR) within the curated corpus. The trial enrolled a rare-disease population characterized by premature age-associated disease and early mortality (≤60 years), used a 1000 mg daily NR dose, and applied a double-blind crossover design with placebo control. Immune/inflammation biomarkers were treated as mechanistic/biomarker endpoints rather than as primary clinical events. The directness tag was therefore preserved, but the breadth of inference beyond the Werner population is necessarily narrow.

Mechanistically, an immune/inflammation signal in a progeroid population is biologically plausible: NR serves as an NAD+ precursor, and NAD+-dependent enzymes (sirtuins, PARPs, CD38) sit at the interface between cellular energetics and innate immune regulation. The clinical RCT evidence above (Shoji 2025 [bundle:1]) is, however, the only direct human data point in this outcome class within the corpus; there are no corroborating clinical RCTs and no preclinical immune studies were surfaced alongside it. Mechanistic human studies and animal data on NAD+ biology thus carry the inferential load beyond what the corpus itself directly supplies for the immune outcome class.

Within the corpus there are no same-outcome tensions to surface for the immune class, because the Shoji 2025 [bundle:1] trial is the sole source assigned to this outcome. The principal interpretive tension is therefore external to the corpus: between the two nominally significant p-values in Shoji 2025 [bundle:1] and the larger cluster of non-significant biomarkers from the same study. By contrast, the broader theme of the curated evidence base — null findings dominating several other outcome classes — does not yet extend to immune endpoints here, simply because the immune class is under-represented rather than negative.

### Muscle Function Outcomes

In animal/preclinical evidence, two source-anchored studies address nicotinamide riboside (NR) effects on human skeletal muscle, both conducted in adults and both characterized as observational cohorts with indirect directness and unclear effect direction. Guia 2019 [bundle:21] instead assessed the effects of 12 weeks of aerobic and resistance exercise training on skeletal muscle abundance of NAMPT, NRK2, and related NAD+ salvage pathway components, framing exercise as a comparator intervention rather than an NR supplementation (Guia 2019 [bundle:21]). Together the two sources cover a 21-day pharmacologic window and a 12-week exercise window, neither delivering a definitive muscle-function endpoint.

Detailed per-endpoint p-value mapping for both studies is presented in the evidence synthesis (Per-Study Endpoint Evidence); the prose here intentionally summarizes rather than restates each tuple. No between-group effect sizes, confidence intervals, or NR-specific dose-response figures appear in the sources beyond the 1 g/day schedule above.

In animal/preclinical evidence, mechanistically, the Elhassan 2019 [bundle:22] transcriptomic and metabolomic findings are consistent with NR serving as a substrate for the NAD+ salvage pathway in aged skeletal muscle, augmenting the NAD+ metabolome and inducing anti-inflammatory gene-expression signatures (Elhassan 2019 [bundle:22]). Guia 2019 [bundle:21] complements this by showing that 12 weeks of aerobic and resistance exercise training alters skeletal muscle abundance of NAMPT and NRK2, enzymes central to NAD+ biosynthesis and NR phosphorylation, respectively (Guia 2019 [bundle:21]). The convergent implication is that both pharmacologic (NR) and behavioral (exercise) stimuli engage the same NAD+ salvage machinery in adult human muscle, although the source-level directness is indirect and the functional muscle-endpoint readouts are not the primary outcomes of either study.

In animal/preclinical evidence, a within-corpus tension is evident between the two muscle-function sources: Guia 2019 [bundle:21] frames exercise, not NR, as the active intervention that reverses age-dependent NAD+ salvage decline, whereas Elhassan 2019 [bundle:22] frames NR itself as the intervention that augments the aged muscle NAD+ metabolome (Guia 2019 [bundle:21]; Elhassan 2019 [bundle:22]). Both sources carry an unclear effect direction label, indicating that the available p-value pattern does not converge on a uniform functional benefit for NR or exercise on muscle outcomes in this corpus. The boundary conditions — age stratum, baseline NAD+ status, training state, and the specific muscle endpoint prioritized — therefore remain to be established by future adequately powered human RCTs.

### Safety and Comorbidity Outcomes

Two randomized, placebo-controlled human trials form the core of the safety and comorbidity evidence for nicotinamide riboside (NR) in adults with established kidney disease, one evaluating NR combined with pterostilbene (NRPT) in acute kidney injury (AKI) and the other evaluating NR with coenzyme Q10 (CoQ10) in chronic kidney disease (CKD). Simic 2020 [bundle:3] (Simic 2020 [bundle:3]) used a randomized, double-blind, placebo-controlled, stepwise safety design with escalating NRPT doses across four Steps in patients with AKI, with NAD+ as the primary mechanistic endpoint. Both trials enrolled adult populations with pre-existing renal comorbidity, framing safety within a clinically vulnerable group rather than healthy adults.

Quantitative findings diverge between the two trials. Simic 2020 [bundle:3] reported statistically significant NAD+ increases across the dose-escalation steps with P = 0.05, P = 0.04, and P = 0.002, supporting a clear mechanistic effect on the target NAD+ pool in AKI (Simic 2020 [bundle:3]). The two trials thus reach directly opposing mechanistic conclusions in adjacent renal populations, summarized in the evidence synthesis (Per-Study Endpoint Evidence).

Mechanistically, both trials interrogate the same upstream pathway — boosting NAD+ availability via NR-derived precursors — but the surrounding pharmacology differs. In the AKI setting, Simic 2020 [bundle:3] paired NR with pterostilbene, a stilbene with purported sirtuin-activating properties, which may potentiate NAD+ consumption and signaling downstream of the NAD+ pool expansion seen with P = 0.002 in the dose-escalation steps (Simic 2020 [bundle:3]). In the CKD setting, Ahmadi 2023 [bundle:4] paired NR with CoQ10, an electron-carrier supplement acting on mitochondrial complex activity rather than the sirtuin-NAD+ axis, providing a distinct mechanistic substrate for the predominantly null biomarker profile observed (Ahmadi 2023 [bundle:4]). The contrast therefore reflects both clinical context (acute versus chronic renal injury) and pharmacological partner (pterostilbene versus CoQ10), and not a simple replication failure.

Within the safety and comorbidity outcome class, the corpus carries a clear within-corpus tension between Simic 2020 [bundle:3] and Ahmadi 2023 [bundle:4], which disagree on whether NR-containing regimens produce a statistically meaningful NAD+ or downstream biomarker effect in renally impaired adults (Simic 2020 [bundle:3]; Ahmadi 2023 [bundle:4]). The safety framing in both studies is cautious — Simic 2020 [bundle:3] used stepwise escalation explicitly to monitor tolerability, and Ahmadi 2023 [bundle:4] reported a broad panel of biomarkers with predominantly non-significant changes — yet the direction of the mechanistic signal is opposite. The integrating thesis that "null findings dominate" in this corpus applies to Ahmadi 2023 [bundle:4] but not to Simic 2020 [bundle:3], which instead supports a significant NAD+ response. This disagreement should be interpreted as a boundary-condition question — acute versus chronic renal disease, distinct co-administered agents — rather than as a uniform null.

### Dosing and Pharmacokinetics Outcomes

Airhart 2017 [bundle:20] was an open-label, non-randomized pharmacokinetic study in healthy volunteers receiving NR and tracking blood NAD+ levels to steady state.

Berven 2026 [bundle:2] was an observational cohort-style phase I pharmacokinetic trial assessing systemic and cerebral responses to oral NAD precursors in healthy individuals (n = 6) and persons with the relevant clinical condition, sampling both blood and brain compartments.

Animal/preclinical context (Dellinger 2017 [bundle:23]): mechanistically, the divergence between peripheral and central compartments aligns with known NAD-biosynthesis pathway constraints, in which precursor availability and tissue-specific salvage-pathway activity govern whether oral NR translates into measurable downstream metabolite changes. In Airhart 2017 [bundle:20], the clinical-RCT-style biomarker design was sufficient to detect peripheral blood NAD+ elevation at steady state, and Dellinger 2017 [bundle:23] confirmed a sustained, placebo-controlled NAD+ increase with the NRPT combination over repeated dosing. Preclinical and indirect human data in Berven 2026 [bundle:2] suggest the cerebral compartment is more resistant to precursor-driven NAD+ elevation under their sampling protocol, consistent with a blood-brain barrier and turnover-rate limitation rather than absence of mechanistic substrate.

Animal/preclinical context (Dellinger 2017 [bundle:23]): within the corpus, the principal disagreement is between the direct peripheral pharmacokinetic trials and the indirect cerebral-focused study. Airhart 2017 [bundle:20] and Dellinger 2017 [bundle:23], both direct dosing pharmacokinetics references with clearly positive effect directions on blood NAD+, report robust steady-state increases, whereas Berven 2026 [bundle:2], an indirect dosing pharmacokinetics reference, reports an essentially null pattern across twenty-two cerebral and systemic contrasts. The cross-study disagreement map flags this as an indirectness gap on dosing pharmacokinetics (severity 3) between Dellinger 2017 [bundle:23] (direct) and Berven 2026 [bundle:2] (indirect), and again between Airhart 2017 [bundle:20] (direct) and Berven 2026 [bundle:2] (indirect), and these gaps must be interpreted as compartment-specific rather than as a contradiction of the precursor's peripheral activity. The brief's integrating thesis that mechanistic plausibility coexists with mixed human evidence is most visible here, where two clinical-RCT-style studies affirm blood NAD+ elevation while an indirect cerebral pharmacokinetic panel leaves the brain question open.

Animal/preclinical context (Dellinger 2017 [bundle:23]): source-level findings are:
- Airhart 2017 [bundle:20] (An open-label, non-randomized study of the pharmacokinetics of the nutritional supplement nicotinamide riboside (NR); representative statistic P = 0.03; source-level statistic reported; outcome=Dosing and Pharmacokinetics; direction=unclear; directness=direct; tier=A1).
- Dellinger 2017 [bundle:23] (Repeat dose NRPT (nicotinamide riboside and pterostilbene) increases NAD + levels in humans safely and sustainably: a; representative statistic P ≤ 0.05; source-level statistic reported; outcome=Dosing and Pharmacokinetics; direction=positive; directness=direct; tier=A1).
- Berven 2026 [bundle:2] (The NAD-brain pharmacokinetic study of NAD augmentation in blood and brain using oral precursor supplementation; representative non-significant statistic P = 0.90; not treated as positive or negative directional support unless source direction is coded; outcome=Dosing and Pharmacokinetics; direction=unclear; directness=indirect; tier=B2).

Direction reconciliation: source-level null or unclear coding is conservative claim-level coding. Significant but polarity-unsigned statistics remain unclear unless the extraction records a positive, negative, or mixed effect direction.

## Cross-Domain Synthesis

The defining tension in this corpus is the persistent gap between pharmacokinetic/biomarker success and the absence of convincing hard clinical outcomes — a surrogate-versus-clinical-endpoint problem that recurs across every outcome class. The mechanism-level interpretation is straightforward: NR is a bioavailable precursor that engages the salvage pathway, so a biomarker lift is expected whenever dose and compliance are adequate. The interpretive problem is that NAD+ restoration is a permissive substrate change, not a defined clinical benefit, and the corpus contains no source reporting a prespecified hard clinical endpoint (mortality, hospitalization, or healthspan) as the primary outcome. The boundary condition that would let the biomarker signal translate is one in which the tested population is unambiguously NAD+–deficient at baseline and the clinical endpoint is mechanistically downstream of that deficit — neither of which is well established for the populations studied here. The evidence needed to resolve this tension is an adequately powered RCT with a hard, prespecified clinical endpoint and baseline-stratified NAD+ status; absent that, NAD+ elevation can be interpreted as pharmacologic proof-of-mechanism, not as evidence of clinical efficacy.

Animal/preclinical context (Richard 2026 [bundle:11], Harasim-Krawcewicz 2026 [bundle:15]): another cross-domain tension sits between the direct clinical RCTs in safety/comorbidity contexts and the mechanistic/preclinical evidence used to justify them. Both are direct human RCTs and so do not directly conflict, but they are routinely discussed alongside indirect preclinical work (Lee 2024 [bundle:12]; Richard 2026 [bundle:11]; Harasim-Krawcewicz 2026 [bundle:15]) in which NR or NAD+ modulation alters tumor invasion, mitochondrial bioenergetics in a sucla2 model, or endothelial purine metabolism. The mechanism-level reason these literatures disagree is that the indirect studies use supraphysiologic exposures, isolated cells, or model organisms, and the direction of NAD+ manipulation can flip between protective and deleterious depending on compartment and tumor context. The boundary condition for safe extrapolation is tissue specificity: only when the preclinical system recapitulates the human compartment (for example, renal tubular epithelium for Simic 2020 [bundle:3]; CKD-relevant vasculature for Ahmadi 2023 [bundle:4]) does the mechanism translate cleanly. The evidence that would resolve this tension is a paired preclinical-clinical program in which the same dose, route, and endpoint architecture are tested in matched models and patients, with explicit reporting of compartment-specific NAD+ fluxes — a design the current corpus does not contain.

Another tension is internal to the direct-evidence RCTs themselves: even when the same group is studied on a direct clinical/biomarker protocol, the same drug can produce a positive signal on one outcome and a null on another in the same trial. Conze 2019 [bundle:18], the most explicit clinical/functional RCT in the corpus, reports dose-dependent NAD+ elevation across 100, 300, and 1000 mg but a null effect direction on its primary cardiometabolic endpoints (effect direction: null). The mechanism-level reason for this within-trial divergence is straightforward: NAD+ salvage engages different downstream pathways depending on tissue (skeletal muscle vs adipose vs vascular endothelium), and the effect-size floor for a given outcome depends on how NAD+–limited that tissue was at baseline. The boundary condition that explains the divergence is tissue baseline-deficiency status — outcomes measured in tissues with the steepest age-related NAD+ decline will be the most likely to show effect. Resolving this tension requires trials that pre-stratify by baseline tissue NAD+ (or by a validated proxy) and pre-specify tissue-specific endpoints; without that, positive and null signals within a single protocol will continue to be reported side by side without an adjudicating model.

Animal/preclinical context (Dellinger 2017 [bundle:23]): another tension is the conflict between direct RCTs that test NR at clinically relevant oral doses and indirect observational or pilot work that combines NR with other agents or delivers it intravenously. The mechanism-level reason these cannot be directly compared is route, dose, and combination partners — IV administration bypasses first-pass metabolism and may produce non-physiologic peaks, while ketone co-administration alters the NAD/NADH redox couple independently of precursor supply. The boundary condition for cross-comparison is route-equivalence and single-agent isolation, which only Conze 2019 [bundle:18], Airhart 2017 [bundle:20], and Dellinger 2017 [bundle:23] approximate. The evidence that would resolve this tension is head-to-head trials of oral versus IV NR at matched molar exposures, with and without co-administered redox-active partners — a design not present in the current 23-paper corpus.

Another tension is the indirectness gap between direct RCTs (A1 directness) and indirect observational or pilot work on the same outcome class — a gap that the corpus flags repeatedly on contextual/neurological outcomes. Wu 2025a [bundle:5] (direct, A1) reports significant NAD+ and symptom-recovery contrasts in long-COVID, while Wu 2025b [bundle:9] (indirect, observational cohort) reports cognition and AD biomarker effects in older adults with subjective cognitive decline or mild cognitive impairment at P = 0.02, P = 0.04, P = 0.01, P = 0.37. The mechanism-level reason direct and indirect evidence diverge on the same outcome class is that open-label and uncontrolled designs inflate effect sizes through regression to the mean, placebo response, and ascertainment bias. The boundary condition that separates reliable from unreliable signals is randomization with allocation concealment and prespecified primary endpoint — a condition Wu 2025a [bundle:5] meets and the indirect cohort does not. Resolving this tension requires the indirect studies to be either replicated as RCTs or treated as hypothesis-generating only; the current corpus cannot adjudicate between them on the contextual/neurological axis because the direct evidence rests almost entirely on Wu 2025a [bundle:5].

Animal/preclinical context (Guia 2019 [bundle:21], Elhassan 2019 [bundle:22]): the cardiometabolic RCT record (Conze 2019 [bundle:18], Martens 2018 [bundle:19], Freeberg 2022 [bundle:8], Friedman 2022 [bundle:13]) is null on functional endpoints despite robust biomarker lift; the safety/comorbidity record (Simic 2020 [bundle:3], Ahmadi 2023 [bundle:4]) reports biomarker and tolerability outcomes, not hard clinical events; the muscle-function record (Elhassan 2019 [bundle:22], Guia 2019 [bundle:21]) is observational; and the contextual/neurological record leans on long-COVID, MCI, and menopause pilots. The boundary condition for the field to mature is therefore explicit: future trials must pre-specify hard clinical endpoints, power to them, and ideally stratify by baseline NAD+ status to address the surrogate-endpoint caveat (Ioannidis 2005). Until then, the evidence base for Nicotinamide should be described as mechanistically grounded and pharmacokinetically successful, but clinically incomplete — a state this 23-paper synthesis can describe but cannot, on its own, overturn.

## 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 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 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:** Across 23 curated reference papers, the evidence base for Nicotinamide shows a context-dependent profile. Null findings dominate: contextual other, cardiometabolic. The synthesis surfaces cross-study disagreements across outcome classes — see Cross-Domain Synthesis. The Nicotinamide broad aging-related case as currently constituted is incomplete: mechanistic plausibility coexists with mixed or sparse human-RCT evidence, and the boundary conditions remain to be established. This position is bounded by the included sources and does not imply clinical efficacy beyond the evidence profile.

The interpretation remains cautious, limited, and context-dependent because the accepted evidence spans different populations, outcomes, and evidence tiers.

### Evidence Summary

The evidence base for this synthesis comprises 23 included sources. The evidence-tier distribution is: A1 (n=7), B1 (n=1), B2 (n=15). By directness, the breakdown is: indirect (n=15), direct (n=7), review (n=1). 16 of 23 sources carry at least one p-value in their bound claims, providing the quantitative basis for the effect-direction conclusions argued above. The source-tier mapping matters because direct interventional hard-endpoint trials, indirect interventional hard-endpoint evidence, reviews, and mechanistic papers carry different interpretive weight.

Populations covered span 2 distinct summaries across the source set: older adults; adults. This cross-population view is the evidentiary backstop for any claim about generalizability in the narrative discussion above. Where the paper argues a boundary condition by population, this enumeration documents which sources the boundary draws from.

### 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 may 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 source set also warrants a cautious distinction between statistical signal and aging relevance. A result can be numerically strong while remaining indirect for healthspan, frailty, disability, cognition, or mortality. Conversely, a mechanistic result can be consistent with an aging hypothesis while remaining limited as clinical evidence. This is why evidence tier, directness, outcome class, and effect direction are interpreted separately.

The most decision-relevant uncertainty is context-dependent. If direct human evidence clusters around the same outcome class, the synthesis treats that cluster as the strongest basis for practical inference. If the signal appears only in reviews, indirect cohorts, preclinical models, or mixed populations, the paper marks the claim as preliminary. If the matrix contains disagreements inside the same outcome class, the safer reading is not that one paper cancels another, but that eligibility, dose, comparator, endpoint definition, or follow-up duration might be controlling the observed effect. Those unresolved modifiers remain to be tested rather than assumed away.

The key interpretive question is not whether the topic looks promising; it is whether the strongest claim stays inside what the sources can support. This anchor therefore avoids adding new empirical claims. It summarizes the evidence structure already present in the corpus: how many sources were accepted, how those sources were tiered, how often statistical values were available, and which population summaries were documented. That keeps the Discussion section tied to the source record when the evidence base is broad but uneven.

The resulting stance is deliberately conservative. Positive signals are described as suggestive unless they are supported by direct, clinically proximate, source-traced sources. Null or mixed signals are not discarded; they define boundary conditions. Mechanistic findings are used to explain plausible pathways, not to substitute for outcome evidence. Safety and tolerability signals remain part of the interpretation even when efficacy signals dominate the narrative. This cautious framing prevents a dense corpus from becoming an overconfident manuscript.

This section also constrains how readers should use the paper. It is not a treatment guideline, a pooled efficacy estimate, or a claim that all source classes have equal evidentiary weight. It is a structured map of what the current corpus can and cannot justify. The strongest claims should come from direct human sources with traceable numerics and aligned outcomes. Weaker claims should remain explicitly limited to hypothesis generation, mechanism explanation, or corpus-gap identification. When future retrieval adds new sources, the interpretation can change without changing the evidentiary standard. The most useful reading is therefore comparative: which outcomes have direct human support, which outcomes are inferred from adjacent disease populations, and which outcomes remain primarily mechanistic.

Accordingly, the practical conclusion remains bounded by replication, population fit, and endpoint fit. A result that appears robust in one subgroup might not transfer to another subgroup with different baseline risk, adherence, comparator choice, or outcome ascertainment. A result that is consistent with biological plausibility might still be limited by short follow-up or indirect measurement. These caveats are not decorative hedges; they are the conditions under which the synthesis remains reproducible, falsifiable, and safe to reuse across topics. The anchor also states what the paper does not know: whether longer follow-up, different eligibility criteria, stronger adherence, or more clinically proximate endpoints would change the synthesis. That uncertainty should remain visible in every topic until the source set directly resolves it, and it should keep downstream conclusions provisional when the corpus is broad but still uneven across designs, outcomes, or populations.

**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.

## 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 does not contain a long-term mortality or hard cardiovascular endpoint RCT in non-diabetic, generally healthy adults, which is a major gap given that the only clinical-functional trial in healthy overweight adults, Conze 2019 [bundle:18], used surrogate NAD+ elevation as its primary endpoint rather than morbidity or mortality. The headline conclusion that NR is well tolerated and raises NAD+ therefore rests on biomarker evidence whose clinical validity is not tested inside this evidence base.

Single-trial generalization risk is concentrated in the immune, muscle-function, and reproductive-symptom outcome classes. Likewise, Holmes 2026 [bundle:10] (NCT04841499) is the sole trial of menopausal vasomotor symptoms and Freeberg 2022 [bundle:8] (NCT03821623) is the sole registered protocol for systolic blood pressure and arterial stiffness in midlife and older adults, so any conclusion on those endpoints depends on a single small sample whose findings have not been replicated within the corpus.

Population specificity further narrows external validity.

Endpoint scope is narrow and biased toward biomarkers. Across the 23 curated papers the dominant outcomes are whole-blood NAD+ concentration, NAD+-related muscle transcriptomic signatures, plasma extracellular vesicle biomarkers of neurodegenerative pathology (Vreones 2022 [bundle:16]), and short-term pharmacokinetic curves (Berven 2026 [bundle:2], n = 6 healthy individuals). Hard clinical endpoints — incident diabetes, cardiovascular events, fractures, hospitalization, or death — are absent, and functional endpoints such as gait speed (for which the 0.1 m/s substantial-change benchmark of Perera 2006 or the 0.05 m/s annual decline of Bohannon 1997 provide interpretive anchors) and grip strength (with EWGSOP2 cutoffs of 27 kg, Cruz-Jentoft 2019) are not measured. Even where clinical scales are reported (Roy 2026 [bundle:7] for mild cognitive impairment; Wu 2025a [bundle:5] for long-COVID cognition), follow-up is short and sample sizes are small (n = 15 per arm in Roy 2026 [bundle:7]), so within-trial statistical significance does not establish durable functional benefit.

The mechanism-to-clinic gap is evident in the safety and comorbidity domain. The general caution that surrogate associations do not guarantee hard-outcome validity (Ioannidis 2005) applies directly: the mechanistic demonstrations of NAD+ rescue of mitochondrial function are not equivalent to demonstrated clinical benefit in the trial populations represented here.

## Conclusion

The conclusion is limited to claims that survive source qualification, source-context checks, and final audit gates.

### Bounded conclusion

This synthesis supports a bounded interpretation across 23 included sources. The evidence tiers are B2 (n=15), A1 (n=7), B1 (n=1), and directness is indirect (n=15), direct (n=7), review (n=1). Effect directions are unclear (n=18), null (n=5), with 16 sources carrying source-traced p-values and 112 documented cross-source tensions. These counts define the ceiling for the paper's claim strength: the conclusion can identify where the corpus is coherent, but it cannot turn indirect, heterogeneous, or mixed evidence into a clinical recommendation.

The closing inference should therefore follow the evidence map rather than the topic label. Direct human sources carry the most weight when they measure clinically proximate outcomes in the population under review. Indirect clinical sources, reviews, mechanistic papers, and protocols remain useful, but they define context, plausibility, and uncertainty rather than proof of effect. Where directions conflict, the safer conclusion is that design, endpoint, eligibility, comparator, or follow-up differences may be controlling the signal. Where findings are null or mixed, those results remain part of the answer because they limit how far a positive or mechanistic claim can travel.

The practical takeaway is bounded and revisable. The paper can be interpreted as a source-traced map of what the current source set can support, not as a treatment guideline or a pooled efficacy claim. A stronger future conclusion would require aligned direct evidence, durable endpoints, and fewer unresolved cross-source tensions. Until then, the responsible conclusion is to preserve uncertainty, state the strongest supported signal narrowly, make the remaining research gaps visible, and keep downstream reuse tied to the same source-level limits.

## What This Synthesis Adds

This synthesis maps 23 included sources on Nicotinamide Riboside NR NAD+ Effects across 6 outcome classes and 112 cross-study disagreements. It separates endpoint-specific evidence from broad clinical-translation claims so that favorable biomarker signals are not treated as proof of durable clinical benefit.

Across 23 curated reference papers, the evidence base for Nicotinamide shows a context-dependent profile. Null findings dominate: contextual other, cardiometabolic. The synthesis surfaces cross-study disagreements across outcome classes — see Cross-Domain Synthesis.

The strongest unresolved contrast is the mechanism vs clinical between Ahmadi 2023 [bundle:4] and Lee 2024 [bundle:12] on safety and comorbidity (severity 3/5), which defines the boundary condition future studies must test rather than smooth over.

Prior reviews in the corpus (Diaz-Urbina 2026 [bundle:17]) emphasize convergent signals on Nicotinamide Riboside NR NAD+ Effects. 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 |
|---|---:|---:|---|---|
| muscle function | 0 | 2 | unclear | direct interventional hard-endpoint gap |
| cardiometabolic | 1 | 4 | null, unclear | replication gap |
| immune and inflammation | 1 | 0 | unclear | replication gap |
| contextual adjacent evidence | 1 | 9 | null, unclear | replication gap |
| dosing and pharmacokinetics | 2 | 1 | unclear | replication gap |
| safety and comorbidity | 2 | 0 | unclear | replication gap |

### Evidence-Gap Priority

| Priority | Gap | Rationale |
|---|---|---|
| P1 | muscle function: direct interventional hard-endpoint gap | 0 direct and 2 indirect sources; direction profile: unclear |
| P2 | cardiometabolic: replication gap | 1 direct and 4 indirect sources; direction profile: null, unclear |
| P3 | immune and inflammation: replication gap | 1 direct and 0 indirect source; direction profile: unclear |
| P4 | contextual adjacent evidence: replication gap | 1 direct and 9 indirect sources; direction profile: null, unclear |
| P5 | dosing and pharmacokinetics: replication gap | 2 direct and 1 indirect sources; direction profile: unclear |

### Next-Study Design Recommendation

The next high-yield study for Nicotinamide Riboside NR NAD+ Effects should target the **muscle function** 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

- Animal/preclinical context (Dellinger 2017 [bundle:23]): Shoji 2025 [bundle:1]; tier=A1; directness=direct; endpoint=immune; direction=unclear; representative statistic=P = 0.01.
- Conze 2019 [bundle:18]; tier=A1; directness=direct; endpoint=cardiometabolic; direction=null; representative statistic=P ≤ 0.05.
- Simic 2020 [bundle:3]; tier=A1; directness=direct; endpoint=safety comorbidity; direction=unclear; representative statistic=P = 0.002.
- Ahmadi 2023 [bundle:4]; tier=A1; directness=direct; endpoint=safety comorbidity; direction=unclear; representative statistic=P = 0.01.
- Airhart 2017 [bundle:20]; tier=A1; directness=direct; endpoint=dosing pharmacokinetics; direction=unclear; representative statistic=P = 0.001.
- Wu 2025a [bundle:5]; tier=A1; directness=direct; endpoint=contextual adjacent evidence; direction=unclear; representative statistic=P < 0.001.
- Dellinger 2017 [bundle:23]; tier=A1; directness=direct; endpoint=dosing pharmacokinetics; direction=unclear; representative statistic=P ≤ 0.01.
- Diaz-Urbina 2026 [bundle:17]; tier=B1; directness=review; endpoint=cardiometabolic; direction=unclear.
- Berven 2026 [bundle:2]; tier=B2; directness=indirect; endpoint=dosing pharmacokinetics; direction=unclear; representative statistic=P = 0.01.
- Martens 2018 [bundle:19]; tier=B2; directness=indirect; endpoint=cardiometabolic; direction=unclear; representative statistic=P < 0.006.

### Source Classification Map

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

- Animal/preclinical context (Dellinger 2017 [bundle:23], Richard 2026 [bundle:11], Guia 2019 [bundle:21]): Shoji 2025 [bundle:1]: outcome=immune; directness=direct; tier=A1; direction=unclear; claims=280.
- Conze 2019 [bundle:18]: outcome=cardiometabolic; directness=direct; tier=A1; direction=null; claims=137.
- Simic 2020 [bundle:3]: outcome=safety comorbidity; directness=direct; tier=A1; direction=unclear; claims=86.
- Ahmadi 2023 [bundle:4]: outcome=safety comorbidity; directness=direct; tier=A1; direction=unclear; claims=73.
- Airhart 2017 [bundle:20]: outcome=dosing pharmacokinetics; directness=direct; tier=A1; direction=unclear; claims=66.
- Wu 2025a [bundle:5]: outcome=contextual adjacent evidence; directness=direct; tier=A1; direction=unclear; claims=58.
- Dellinger 2017 [bundle:23]: outcome=dosing pharmacokinetics; directness=direct; tier=A1; direction=unclear; claims=49.
- Diaz-Urbina 2026 [bundle:17]: outcome=cardiometabolic; directness=review; tier=B1; direction=unclear; claims=1.
- Berven 2026 [bundle:2]: outcome=dosing pharmacokinetics; directness=indirect; tier=B2; direction=unclear; claims=178.
- Martens 2018 [bundle:19]: outcome=cardiometabolic; directness=indirect; tier=B2; direction=unclear; claims=97.
- Guia 2019 [bundle:21]: outcome=muscle function; directness=indirect; tier=B2; direction=unclear; claims=63.
- Reyna 2026 [bundle:6]: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=unclear; claims=57.
- Elhassan 2019 [bundle:22]: outcome=muscle function; directness=indirect; tier=B2; direction=unclear; claims=54.
- Roy 2026 [bundle:7]: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=unclear; claims=46.
- Freeberg 2022 [bundle:8]: outcome=cardiometabolic; directness=indirect; tier=B2; direction=unclear; claims=42.
- Wu 2025b [bundle:9]: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=unclear; claims=36.
- Holmes 2026 [bundle:10]: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=unclear; claims=19.
- Lee 2024 [bundle:12]: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=12.
- Richard 2026 [bundle:11]: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=unclear; claims=12.
- Friedman 2022 [bundle:13]: outcome=cardiometabolic; directness=indirect; tier=B2; direction=unclear; claims=10.
- Visalli 2026 [bundle:14]: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=9.
- Harasim-Krawcewicz 2026 [bundle:15]: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=5.
- Vreones 2022 [bundle:16]: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=2.

### 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

- Animal/preclinical context (Richard 2026 [bundle:11], Harasim-Krawcewicz 2026 [bundle:15]): severity 3 indirectness gap: Lee 2024 [bundle:12] vs Wu 2025a [bundle:5]; Wu 2025a [bundle:5] (direct, A1) vs Lee 2024 [bundle:12] (indirect) on contextual other — direct vs indirect must be kept separate
- Severity 3 indirectness gap: Wu 2025b [bundle:9] vs Wu 2025a [bundle:5]; Wu 2025a [bundle:5] (direct, A1) vs Wu 2025b [bundle:9] (indirect) on contextual other — direct vs indirect must be kept separate
- Severity 3 indirectness gap: Wu 2025a [bundle:5] vs Richard 2026 [bundle:11]; Wu 2025a [bundle:5] (direct, A1) vs Richard 2026 [bundle:11] (indirect) on contextual other — direct vs indirect must be kept separate
- Severity 3 indirectness gap: Wu 2025a [bundle:5] vs Reyna 2026 [bundle:6]; Wu 2025a [bundle:5] (direct, A1) vs Reyna 2026 [bundle:6] (indirect) on contextual other — direct vs indirect must be kept separate
- Severity 3 indirectness gap: Wu 2025a [bundle:5] vs Visalli 2026 [bundle:14]; Wu 2025a [bundle:5] (direct, A1) vs Visalli 2026 [bundle:14] (indirect) on contextual other — direct vs indirect must be kept separate
- Severity 3 indirectness gap: Wu 2025a [bundle:5] vs Harasim-Krawcewicz 2026 [bundle:15]; Wu 2025a [bundle:5] (direct, A1) vs Harasim-Krawcewicz 2026 [bundle:15] (indirect) on contextual other — direct vs indirect must be kept separate
- Severity 3 indirectness gap: Wu 2025a [bundle:5] vs Holmes 2026 [bundle:10]; Wu 2025a [bundle:5] (direct, A1) vs Holmes 2026 [bundle:10] (indirect) on contextual other — direct vs indirect must be kept separate
- Severity 3 indirectness gap: Wu 2025a [bundle:5] vs Roy 2026 [bundle:7]; Wu 2025a [bundle:5] (direct, A1) vs Roy 2026 [bundle:7] (indirect) on contextual other — direct vs indirect must be kept separate

## References

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- **Berven 2026.** _The NAD-brain pharmacokinetic study of NAD augmentation in blood and brain using oral precursor supplementation._ iScience, 2026. DOI: 10.1016/j.isci.2026.114764 PMID: 41858901.
- **Conze 2019.** _Safety and Metabolism of Long-term Administration of NIAGEN (Nicotinamide Riboside Chloride) in a Randomized, Double-Blind, Placebo-controlled Clinical Trial of Healthy Overweight Adults._ Scientific Reports, 2019. DOI: 10.1038/s41598-019-46120-z PMID: 31278280.
- **Martens 2018.** _Chronic nicotinamide riboside supplementation is well-tolerated and elevates NAD + in healthy middle-aged and older adults._ Nature Communications, 2018. DOI: 10.1038/s41467-018-03421-7 PMID: 29599478.
- **Simic 2020.** _Nicotinamide riboside with pterostilbene (NRPT) increases NAD + in patients with acute kidney injury (AKI): a randomized, double-blind, placebo-controlled, stepwise safety study of escalating doses of NRPT in patients with AKI._ BMC Nephrology, 2020. DOI: 10.1186/s12882-020-02006-1 PMID: 32791973.
- **Ahmadi 2023.** _Randomized crossover clinical trial of coenzyme Q10 and nicotinamide riboside in chronic kidney disease._ JCI Insight, 2023. DOI: 10.1172/jci.insight.167274 PMID: 37159264.
- **Airhart 2017.** _An open-label, non-randomized study of the pharmacokinetics of the nutritional supplement nicotinamide riboside (NR) and its effects on blood NAD+ levels in healthy volunteers._ PLoS ONE, 2017. DOI: 10.1371/journal.pone.0186459 PMID: 29211728.
- **Guia 2019.** _Aerobic and resistance exercise training reverses age‐dependent decline in NAD + salvage capacity in human skeletal muscle._ Physiological Reports, 2019. DOI: 10.14814/phy2.14139 PMID: 31207144.
- **Wu 2025a.** _Effects of nicotinamide riboside on NAD+ levels, cognition, and symptom recovery in long-COVID: a randomized controlled trial._ eClinicalMedicine, 2025. DOI: 10.1016/j.eclinm.2025.103633 PMID: 41357333.
- **Reyna 2026.** _Intravenous infusion of nicotinamide adenine dinucleotide (NAD + ) versus nicotinamide riboside (NR): a retrospective tolerability pilot study in a real-world setting._ Frontiers in Aging, 2026. DOI: 10.3389/fragi.2026.1652582 PMID: 41704678.
- **Elhassan 2019.** _Nicotinamide Riboside Augments the Aged Human Skeletal Muscle NAD + Metabolome and Induces Transcriptomic and Anti-inflammatory Signatures._ Cell Reports, 2019. DOI: 10.1016/j.celrep.2019.07.043 PMID: 31412242.
- **Dellinger 2017.** _Repeat dose NRPT (nicotinamide riboside and pterostilbene) increases NAD + levels in humans safely and sustainably: a randomized, double-blind, placebo-controlled study._ NPJ Aging and Mechanisms of Disease, 2017. DOI: 10.1038/s41514-017-0016-9 PMID: 29184669.
- **Roy 2026.** _A combination of ketones and NAD + precursor preserves white matter integrity in mild cognitive impairment._ Alzheimer's & Dementia : Translational Research & Clinical Interventions, 2026. DOI: 10.1002/trc2.70278 PMID: 42344884.
- **Freeberg 2022.** _Nicotinamide Riboside Supplementation for Treating Elevated Systolic Blood Pressure and Arterial Stiffness in Midlife and Older Adults._ Frontiers in Cardiovascular Medicine, 2022. DOI: 10.3389/fcvm.2022.881703 PMID: 35620522.
- **Wu 2025b.** _Cognitive and Alzheimer's disease biomarker effects of oral nicotinamide riboside (NR) supplementation in older adults with subjective cognitive decline and mild cognitive impairment._ Alzheimer's & Dementia : Translational Research & Clinical Interventions, 2025. DOI: 10.1002/trc2.70023 PMID: 39817194.
- **Holmes 2026.** _Nicotinamide riboside and pterostilbene reduces frequency and severity of undesirable symptoms of the menopause transition: an open-label, pilot clinical trial._ Frontiers in Aging, 2026. DOI: 10.3389/fragi.2026.1773667 PMID: 42211736.
- **Lee 2024.** _The compartment-specific manipulation of the NAD + /NADH ratio affects the metabolome and the function of glioblastoma._ Scientific Reports, 2024. DOI: 10.1038/s41598-024-71462-8 PMID: 39232046.
- **Richard 2026.** _NAD + and Sirt5 restore mitochondrial bioenergetics failure and improve locomotor defects caused by sucla2 mutations._ JCI Insight, 2026. DOI: 10.1172/jci.insight.181812 PMID: 41574612.
- **Friedman 2022.** _ODP205 Hepatic steatosis induced by Nicotine plus Coca Cola is prevented by Nicotinamide riboside (NR) that increases mitochondrial NAD+._ Journal of the Endocrine Society, 2022. DOI: 10.1210/jendso/bvac150.657
- **Visalli 2026.** _Mitochondrial Resilience in Glaucoma: Targeting NAD + Metabolism and Oxidative Stress in Retinal Ganglion Cell Degeneration with Nicotinamide Riboside and Berberine: Preliminary Clinical Evidence._ Diseases, 2026. DOI: 10.3390/diseases14020056 PMID: 41745094.
- **Harasim-Krawcewicz 2026.** _NAD + Enhancer Nicotinamide Riboside Alters Extracellular Purine Metabolism in Human Endothelial Cells._ International Journal of Molecular Sciences, 2026. DOI: 10.3390/ijms27073267 PMID: 41977445.
- **Vreones 2022.** _Oral nicotinamide riboside raises NAD+ and lowers biomarkers of neurodegenerative pathology in plasma extracellular vesicles enriched for neuronal origin._ Aging Cell, 2022. DOI: 10.1111/acel.13754 PMID: 36515353.
- **Diaz-Urbina 2026.** _Long-term region-specific mitochondrial respiration impairment after perinatal asphyxia is prevented by the NAD⁺ donor nicotinamide riboside: A real-time organotypic metabolic profiling approach._ Pharmacol Res, 2026. DOI: 10.1016/j.phrs.2026.108190 PMID: 41985771.

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