Research Synthesis: NAD+ Biomarker Effects

Research Synthesis: NAD+ Biomarker Effects

Abstract

This paper synthesizes evidence on NAD+ biomarker effects across 24 accepted source papers and 1531 high-confidence extracted claims.

The evidence profile contains 2 direct clinical sources, 22 adjacent clinical sources, and no sources classified primarily as mechanistic or model-system evidence, with 61 cross-study disagreements across the evidence base.

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

The conclusion is that NAD+ biomarker effects remains a bounded geroscience case: the retained clinical and adjacent 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.

Introduction

Population aging has become one of the defining demographic challenges of the twenty-first century, and with it the question of how to extend healthspan — the portion of life spent in good health — has moved from a fringe concern to a central public-health priority. The geroscience hypothesis proposes that interventions targeting the biological hallmarks of aging could simultaneously delay or compress morbidity across multiple organ systems, rather than treating each chronic disease in isolation. This logic has catalyzed a wave of repurposing efforts, in which compounds developed for other indications are now being evaluated as candidate geroprotectors. Among these candidate geroprotectors, the NAD+ literature has attracted particular attention because of the central role of nicotinamide adenine dinucleotide in cellular energetics, DNA repair, and sirtuin signaling, and because NAD+ precursors are already widely available as nutritional supplements. The clinical stakes are considerable: even modest extensions of healthy years could reshape late-life disability trajectories and reduce the burden of multimorbidity.

Mechanistically, the rationale for NAD+ interventions rests on the well-documented decline of nicotinamide adenine dinucleotide pools with age in multiple tissues, a decline that has been proposed to contribute to mitochondrial dysfunction, impaired stress responses, and the accumulation of senescent cells. The NAD+ precursor family includes nicotinamide riboside, nicotinamide mononucleotide, nicotinamide, and niacin, each of which feeds into the salvage or Preiss–Handler pathways at different points and with different pharmacokinetic signatures. Because several of these molecules are sold as dietary supplements, NAD+ interventions have entered widespread consumer use ahead of definitive clinical evidence, a pattern that complicates the interpretation of observational data. Regulatory pathways have so far treated most NAD+ precursors as foods rather than drugs, and the threshold at which dosing, indication, or formulation should trigger pharmaceutical-grade evaluation appears to be evolving. The combination of mechanistic plausibility and easy over-the-counter access has made NAD+ a particularly important test case for the geroscience hypothesis.

Additional corpus sources included animal/preclinical evidence; a recurring issue in the NAD+ literature is that demonstrating a clean pharmacokinetic or biomarker effect does not automatically translate into measurable clinical benefit, and this is the central mechanistic-versus-clinical tension that the field is now grappling with. The NAD+ trial of Elhassan 2019, for example, reported that nicotinamide riboside augmented the skeletal-muscle NAD+ metabolome and induced anti-inflammatory transcriptional signatures in older men, whereas NAD+ supplementation in physically compromised older adults did not improve mitochondrial or muscle function (Connell 2021). Whether NAD+ effects on circulating NAD+ levels can be sustained over clinically meaningful durations, and whether they propagate into hard functional endpoints, remains uncertain. Population specificity, including baseline NAD+ status, age, sex, and comorbidity burden, may also modify the response, and the dose-response relationship for NAD+ has not been mapped in a way that supports evidence-based dosing recommendations. Until these questions are answered, the clinical translation of NAD+ biology will continue to lag behind the mechanistic narrative.

Background

The background evidence for NAD+ biomarker effects is heterogeneous rather than uniformly confirmatory. Direct clinical sources such as Yi 2022, Xue 2022 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 the contextual adjacent evidence outcome class; null signals around the contextual adjacent evidence, dosing and pharmacokinetics, cardiometabolic outcome classes; and negative or adverse signals around the longevity and contextual adjacent evidence outcome classes. 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-nad_biomarker_effects-v06-DAILY-2026-06-16T16-19-51Z.

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: 2026-06-16.

Search strategy

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

• nad biomarker effects aging
• nad biomarker effects older adults
• nad biomarker effects randomized controlled trial
• nad aging
• nad older adults
• nad randomized controlled trial
• biomarker aging
• biomarker older adults
• biomarker randomized controlled trial
• nicotinamide riboside aging

Eligibility criteria

• Sources whose primary content addresses nad biomarker 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 174 records in the receipt-candidate union, 54 were classified as source candidates and 24 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	174
Classified source candidates	54
No extractable claims	33
None-only claim binding	10
Mixed partial-or-none claim-binding candidates	48
Partial-only claim-binding candidates	17
Strict high-confidence sources	12
Admitted final sources	24

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 appraisal, and claim registry) rather than from re-parsed full text.

Risk-of-bias appraisal

Per-source risk-of-bias was rated using design-appropriate Cochrane RoB-2 (RCTs), ROBINS-I (non-randomised studies), and AMSTAR-2 (systematic reviews / meta-analyses).

Synthesis approach

Evidence-tension synthesis: claims grouped by outcome class (cardiometabolic, contextual adjacent evidence, dosing and pharmacokinetics, frailty, longevity, muscle function); 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.

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
Contextual Adjacent Evidence	n=13; claims=659	no extracted directional signal in 9/13 sources	1 direct; 10 indirect; 2 review	limited corpus depth in this outcome class
Dosing and Pharmacokinetics	n=3; claims=256	no extracted directional signal in 2/3 sources	1 direct; 1 indirect; 1 review	limited corpus depth in this outcome class
Muscle Function	n=3; claims=256	unclear signal in 3/3 sources	3 indirect	limited corpus depth in this outcome class
Cardiometabolic	n=2; claims=274	unclear signal in 1/2 sources	2 indirect	limited corpus depth in this outcome class
Longevity	n=2; claims=73	unclear signal in 1/2 sources	1 indirect; 1 review	limited corpus depth in this outcome class
Frailty	n=1; claims=13	no extracted directional signal in 1/1 sources	1 indirect	single-source slice; hypothesis-generating

Results Summary

• Contextual Adjacent Evidence: n=13; claims=659; no extracted directional signal in 9/13 sources | directness: 1 direct; 10 indirect; 2 review; main limitation: directionally heterogeneous.
• Dosing and Pharmacokinetics: n=3; claims=256; no extracted directional signal in 2/3 sources | directness: 1 direct; 1 indirect; 1 review; main limitation: directionally heterogeneous.
• Muscle Function: n=3; claims=256; mixed signal in 3/3 sources | directness: 3 indirect; main limitation: no direct clinical anchor.
• Cardiometabolic: n=2; claims=274; no extracted directional signal in 1/2 sources | directness: 2 indirect; main limitation: no direct clinical anchor.
• Longevity: n=2; claims=73; adverse or limiting signal in 1/2 sources | directness: 1 indirect; 1 review; main limitation: no direct clinical anchor.
• Frailty: n=1; claims=13; no extracted directional signal in 1/1 sources | directness: 1 indirect; main limitation: no direct clinical anchor.

Cardiometabolic Outcomes

Two clinical RCTs in the curated corpus provide the principal cardiometabolic evidence on NAD+ precursor supplementation. Martens 2018 was a randomized, placebo-controlled, crossover clinical trial of nicotinamide riboside (NR) supplementation at 500 mg twice daily in healthy middle-aged and older adults, with chronic tolerability and NAD+ elevation as the principal endpoints. Both studies are categorized in the curated corpus as indirect with respect to the broader anti-aging framing but direct with respect to cardiometabolic and biomarker physiology, providing the empirical anchor for this outcome class.

Quantitative findings cluster around biomarker change rather than hard clinical events. The complete per-study p-value inventory is presented in the evidence synthesis (Per-Study Endpoint Evidence) and is not restated in full here. No clinical event rates (myocardial infarction, stroke, cardiovascular mortality) appear in either source, so the cardiometabolic synthesis is restricted to physiologic and biomarker endpoints.

Mechanistically, the cardiometabolic signal aligns with the NAD+-dependent sirtuin and PARP substrate rationale that motivates precursor supplementation in vascular tissue. In a clinical RCT setting, Katayoshi 2023 linked NMN exposure to changes in arterial stiffness, a downstream physiologic readout sensitive to endothelial NAD+ availability. The mechanistic substrate underlying this functional finding is consistent with the broader human physiology literature, in which Martens 2018 demonstrated that chronic NR reliably elevates NAD+ in healthy middle-aged and older adults. The coherence between the two source-defined endpoints — biomarker elevation in Martens 2018 and vascular physiologic change in Katayoshi 2023 — provides the principal within-corpus mechanistic chain for the cardiometabolic outcome class.

Within-corpus tensions in the cardiometabolic class are limited but informative. Directness is rated indirect in both Katayoshi 2023 and Martens 2018 relative to a hard clinical-event anti-aging endpoint, which constrains inference: the p-values reported above describe physiologic and biomarker change, not event reduction. Effect direction in Martens 2018 is recorded as unclear in the curated matrix even though the contrast-level p-values are conventionally significant, reflecting the chronic-tolerability framing of the trial rather than a null primary endpoint. Read together, the two sources converge on tolerability and biomarker feasibility while leaving the clinical-event translation unresolved.

Contextual Adjacent Evidence Outcomes

Across the curated corpus, the contextual outcome class is the dominant reporting surface, with 13 of 13 sources contributing biomarker, metabolomic, or disease-adjacent measurements. The integrating observation is that endpoint heterogeneity — from plasma NAD+ metabolome (Xue 2022) to neuronal-enriched extracellular vesicle biomarkers (Vreones 2022) to ferroptosis-related mitochondrial NAD+ homeostasis (Gao 2026) — defines the contextual class rather than a single clinical outcome.

The mechanistic substrate underlying the contextual signals is consistent with a pathway in which NAD+ precursor availability gates sirtuin activity, mitochondrial redox turnover, and ferroptotic sensitivity.

In a clinical RCT, Xue 2022 documented measurable shifts in the NAD+ metabolome after 1520 mg RiaGev twice daily for 7 days, and in an open-label pilot (NCT04841499), Holmes 2026 reported an effect at P < 0.01 (Holmes 2026).

Mechanistically, the Gao 2026 experimental cohort established that SERPINE1 drives ferroptosis in acute respiratory distress syndrome by disrupting mitochondrial NAD+ homeostasis and suppressing Sirt3 activity, with a principal signal at P < 0.0001 (Gao 2026).

Frailty Outcomes

One observational cohort study addresses frailty-class outcomes in the NAD+ corpus. The study design is observational cohort with indirect directness relative to a head-to-head NAD+ supplementation trial, and no canonical trial identifier is recorded. Endpoint coverage is biomarker-centric rather than functional-performance-based, so frailty readouts are reported as mechanistic context rather than as primary clinical endpoints.

Quantitative outputs in Membrez 2024 do not include p-values in the source block, which constrains the inference to direction-of-effect interpretation only. The study positions trigonelline as an NAD+ precursor that improves muscle function during ageing and is reduced in human sarcopenia, framing the effect direction as favourable for the sarcopenic cohort relative to controls. Because no effect estimate or p-value is available in the source, this frailty-class signal is reported qualitatively and any clinical extrapolation should be read with that limitation in view.

Mechanistically, the Membrez 2024 cohort aligns with broader human NAD+-biology work showing that precursor availability tracks with muscle protein homeostasis and mitochondrial capacity in older adults. The mechanistic substrate underlying this clinical-observational signal implicates NAD+ salvage-pathway flux as a modifiable node in age-related muscle decline. Where the corpus later contrasts this with RCT-level frailty data, the indirectness of the Membrez 2024 design should be carried forward as a contextual qualifier, not as a refutation.

Within the frailty outcome class, the corpus carries no second source, so within-outcome tension cannot be assessed directly; however, the indirect directness label on Membrez 2024 stands in contrast to the direct-RCT posture of any future trial embedded in this synthesis. The endpoint structure spans further decompensation events at 12 months alongside a panel of biomarker-comparison contrasts. Because the design is observational rather than interventional, the directionality of the biomarker-mortality association cannot be assigned to a NAD-repletion protocol, and any longevity inference is necessarily indirect.

Several of the reported p-values (P = 0.003, P = 0.004, P < 0.001, P = 0.016, P = 0.032) are conventionally significant, while others (P = 0.120, P = 0.460, P = 0.046) sit at or above the conventional 0.05 threshold only at the margin. Because the source enumerates nine p-values without an accompanying effect-size column, the table carries the study × p-value tuples and the prose confines itself to a faithful count of significant versus non-significant contrasts rather than to derived summary statistics.

Because the review is a meta-analytic or systematic synthesis, the 0.39% / 0.74% / 0.37% figures represent upper-bound effect attenuations rather than per-study point estimates, and the absence of a significance column prevents any inference about whether the attenuations exceed chance. The result is best interpreted as a graded monotonic improvement signal without the inferential scaffolding needed to confirm it.

The synthesis therefore surfaces a definitional tension rather than a numerical one: longevity-class and contextual-other-class evidence both invoke NAD as a label but operationalise it in incompatible ways, and resolving the tension would require the corpus to disambiguate the acronym at intake.

Muscle Function Outcomes

The evidence base for NAD supplementation on muscle function in physically compromised older adults is anchored in three randomized, double-blind, placebo-controlled trials that collectively enrolled 212 participants across distinct dosing and outcome paradigms. These trials were designed to detect functional changes in muscle performance (e.g., strength, endurance) and mitochondrial respiration, with dosing strategies spanning acute short-term supplementation to chronic administration in clinically compromised populations.

In animal/preclinical evidence, quantitatively, the trials reported mixed findings with respect to muscle function endpoints, as summarized in the evidence synthesis. Elhassan 2019, however, identified statistically significant enhancements in NAD metabolome signatures and transcriptional anti-inflammatory responses (e.g., P < 0.001, P = 0.004), though these did not translate into measurable functional gains in muscle performance (P = 0.22, P = 0.31).

Mechanistically, the disconnect between NAD metabolome augmentation and functional muscle outcomes may reflect the temporal and dose-dependent nature of NAD pathway activation, as well as the heterogeneity of the enrolled populations. Elhassan et al. 2019 demonstrated that NR supplementation increased intramuscular NAD+ levels and upregulated genes associated with oxidative phosphorylation and anti-inflammatory pathways, suggesting a plausible substrate for functional improvement. However, the absence of concurrent gains in muscle performance in Connell 2021 and Yu 2025 implies that NAD augmentation alone may be insufficient to reverse sarcopenic or disease-related declines without additional anabolic or exercise stimuli. Preclinical models consistently show that NAD+ precursors enhance mitochondrial biogenesis and muscle endurance, but these findings have not been consistently replicated in human RCTs, highlighting a translational gap.

Additional corpus sources included animal/preclinical evidence; within the corpus, the most notable tension arises from Elhassan 2019’s mechanistic successes versus the null functional findings in Connell 2021 and Yu 2025. While Elhassan et al. 2019 reported robust increases in NAD metabolome signatures and transcriptional changes (e.g., P < 0.001), these biochemical effects did not translate to measurable improvements in muscle function metrics, contrasting with the study’s stated thesis of functional augmentation. Conversely, Connell 2021’s trial, which employed a multi-precursor approach over 12 weeks, reported uniformly non-significant p-values across mitochondrial and muscle function outcomes, further underscoring the lack of functional benefit despite theoretical mechanistic plausibility. These discrepancies suggest that the boundary conditions for NAD supplementation—including dose, duration, and baseline NAD status—remain poorly defined in human populations.

Dosing and Pharmacokinetics Outcomes

Yi 2022 was a randomized, multicenter, double-blind, placebo-controlled, parallel-group, dose-dependent clinical trial of β-nicotinamide mononucleotide (NMN) in healthy middle-aged adults.

Simic 2020 reported a randomized, double-blind, placebo-controlled, stepwise safety study of escalating doses of nicotinamide riboside with pterostilbene (NRPT) in patients with acute kidney injury (AKI).

Airhart 2017 was an open-label, non-randomized pharmacokinetic study of nicotinamide riboside (NR) and its effects on blood NAD+ levels in healthy volunteers.

Simic 2020 yielded three significant p-values: P = 0.05, P = 0.04, and P = 0.002 across the four escalating-dose steps in AKI patients. the evidence synthesis carries the per-endpoint mapping of each p-value to its study.

Mechanistically, the Yi 2022 NMN trial operates as the only direct clinical RCT within this outcome class, while Airhart 2017 supplies pharmacokinetic substrate from an open-label healthy-volunteer cohort and Simic 2020 frames precursor exposure within an injured-tissue (AKI) milieu. The dose-dependent NMN signal in Yi 2022 aligns conceptually with the escalating-dose paradigm of Simic 2020, and the steady-state blood NAD+ elevations reported by Airhart 2017 provide a substrate-level mechanism for the clinical biomarker movement detected in the Yi 2022 trial. The AKI context in Simic 2020, however, complicates extrapolation to healthy aging because injured tissue may alter NAD+ salvage kinetics relative to the uninjured substrate in Airhart 2017.

Within-corpus tensions arise from the directness asymmetry: Yi 2022 is direct, whereas Simic 2020 functions as a review-style synthesis and Airhart 2017 is indirect. The Yi 2022 and Simic 2020 pair produces a direct-versus-review directness gap, and the Yi 2022 and Airhart 2017 pair produces a direct-versus-indirect directness gap; these asymmetries mean that dose-response claims should rest primarily on Yi 2022, with the other two sources serving complementary pharmacokinetic and safety-evidence roles.

Dosing and Pharmacokinetics remains a separate Results slice (n=3; claims=256; no extracted directional signal in 2/3 sources; 1 direct; 1 indirect; 1 review; limited corpus depth in this outcome class) and is not pooled into adjacent endpoint classes.

Longevity Outcomes

Mechanistically, the Simonis 2025 cohort is best read as a human observational study in which the longevity signal is mediated through hepatic decompensation biology rather than through a primary NAD-repletion mechanism, which is consistent with the indirect directness coding in the source. The mechanistic substrate underlying the AD-versus-NAD divergence plausibly involves systemic inflammatory load and organ-reserve exhaustion, but the source does not supply effect sizes that would let a reviewer partition mortality risk into NAD-attributable versus comorbidity-attributable components. By contrast, the broader NAD+ literature would expect any longevity benefit to be channelled through cellular energetics and sirtuin-axis pathways, a frame that this cirrhosis cohort is not equipped to test.

The two sources therefore do not conflict directly: Simonis 2025 supplies patient-level decompensation data with a coded negative direction, while Han 2022 supplies an ecological longitudinal comparison of air-quality improvements with no reportable significance test. The longevity case is consequently underdetermined — a single cohort with nine p-values, an unclear-direction ecological companion, and no clinical RCT of NAD repletion in non-cirrhotic adults.

The contextual-other outcome class is populated by a single systematic review with longitudinal comparative design examining continuous air-quality improvements and cause-specific mortality in Beijing (Han 2022). The review's endpoint architecture partitions mortality into NAD, circulatory-disease (CD), and respiratory-disease (RD) strata, and the source reports no specific p-values, consistent with the empty p values field. The population is adult residents of Beijing followed across staged air-quality intervention periods, and the source's effect direction is coded as unclear, reflecting the absence of a discrete NAD-repletion intervention.

Mechanistically, the Han 2022 finding frames NAD mortality as a downstream consequence of cumulative air-pollution exposure, with circulatory and respiratory mortality acting as candidate mediators between NO2 and NAD-coded deaths. The source does not specify NAD+ or NADH biomarker assays, so any link to the NAD+ topic is purely thematic — the NAD acronym is reused for non-alcoholic disease or natural-cause-of-death categories rather than for the nicotinamide-adenine dinucleotide pool. Preclinical data on NAD+ depletion under oxidative challenge would be needed to bridge the air-pollution context to the nicotinamide-adenine-dinucleotide molecular axis, and the source does not provide that bridge.

Cross-Domain Synthesis

The most consequential cross-domain tension in this evidence base is the divergence between mechanistic/biomarker success and functional or clinical-organ benefit. The boundary condition is plausibly the gap between a population whose NAD+ axis is pharmacologically responsive (healthy middle-aged adults) and a population in which downstream muscle bioenergetics is constrained by frailty, comorbidity, or training status. Resolution would require a head-to-head RCT that simultaneously measures the NAD+ metabolome and a hard functional endpoint — for example, gait speed interpreted against the Studenski 2011 0.8 m/s frailty-relevant threshold or the Cruz-Jentoft 2019 grip-strength cutoffs of 27 kg (men) and 16 kg (women) — in the same enrolled cohort. Until that study is done, the responsible synthesis is that biomarker elevation has been demonstrated, while functional translation remains unproven.

Another tension is the disagreement pair between Curran 2025 and the broader set of contextual other human reports. Curran 2025 is a systematic review/meta-analysis of niacin and NAD-metabolite treatment in infectious-disease animal studies and reports a positive effect direction, with mixed significance indicators including P < 0.01, p ≤ 0.05, P = 0.18, P = 0.74, P = 0.82, P = 0.05, and P = 0.55 — i.e., a literature where individual studies oscillate between signal and null. The boundary condition is species and model: animal infectious-disease models can be standardized for pathogen, timing, and immune readout, whereas human "contextual other" outcomes are clinically heterogeneous and statistically underpowered. Resolving the tension requires (i) restricting Curran 2025-type claims to the model-organism frame — model-organism evidence suggests benefit but requires confirmation in clinically relevant models, in the authors' own framing — and (ii) treating any cross-species extrapolation as hypothesis-generating, not confirmatory. Hard-outcome adjudication in humans must wait for adequately powered trials using clinically meaningful endpoints rather than the Ioannidis 2005 surrogate-endpoint caution being applied to NAD+ itself.

The tension is not a clean positive-versus-null conflict but a measurement-class issue: these are pharmacokinetic and biomarker-adjacent readouts, not hard cardiovascular events. There is no mortality, hospitalization, or major-adverse-cardiovascular-event endpoint in the corpus. The boundary condition is endpoint tier: when the outcome is interpreted as a biomarker (NAD+ levels, arterial-stiffness surrogates), the evidence is suggestive; when it is interpreted as a hard cardiovascular outcome, the evidence is essentially absent. Until then, the synthesis must hold cardiometabolic claims at the surrogate level, with explicit acknowledgement that surrogate associations do not guarantee hard-outcome validity (Ioannidis 2005).

Another tension concerns the muscle function class, where the available signals are uniformly null or unclear rather than positive. The boundary condition here is that the muscle function evidence is in a population (older, physically compromised, or cardiomyopathic) in which the NAD+ axis may be necessary but not sufficient: substrate repletion does not retrain a deconditioned muscle. Functional endpoints in this class should be benchmarked against clinically meaningful thresholds — the Perera 2006 0.1 m/s substantial gait-speed improvement, the Bohannon 1997 0.05 m/s annual age-related decline, or the Cruz-Jentoft 2019 27 kg (men) / 16 kg (women) grip-strength cutoffs — and no study in this corpus demonstrates NAD+-driven change at those magnitudes. Resolving the tension requires longer-duration, adequately powered trials in sarcopenic or frail adults with these canonical thresholds as pre-specified endpoints.

The tension is not between NAD+ and longevity directly but between the over-reading of these tagged studies as if they were NAD+-longevity trials, when in fact they are unrelated. The boundary condition is terminological: "NAD+" in this corpus is the nicotinamide-adenine-dinucleotide biology, whereas "NAD" in Simonis 2025 is a clinical-acuity classifier for decompensation. The synthesis must refuse to let acronym collision generate spurious negative longevity claims about NAD+ supplementation. What is genuinely observable for the longevity outcome class in this corpus is essentially nothing — no human RCT with mortality or healthspan as a primary endpoint. Resolution requires explicit acronym disambiguation in any downstream review and, more substantively, human trials long enough and large enough to detect the kind of effect that preclinical lifespan models suggest (Anisimov 2008, ~5% typical preclinical extension) translated to humans. Until then, the longevity claim for NAD+ in humans is unsupported by direct evidence in this corpus, and negative-sounding signals from non-overlapping literature must not be borrowed to fill that gap.

Another tension concerns the dosing pharmacokinetics class, where the evidence for substrate elevation is comparatively strong but the linkage to clinically interpretable benefit is weak. Simic 2020 (NR with pterostilbene in acute kidney injury, stepwise dose-escalation safety study) reports P = 0.05, P = 0.04, P = 0.002. The pattern across these studies is that NAD+ rises on supplementation in most adults, especially healthy middle-aged adults, with some inter-study variance. Yet the effect direction is tagged unclear, not positive, because the PK signal does not by itself establish health benefit. The boundary condition is the gap between pharmacokinetic adequacy and pharmacodynamic sufficiency: a dose that elevates NAD+ may still be subtherapeutic for the indication, or therapeutic only in a subset with baseline depletion. Resolving this tension requires (i) baseline-NAD+-stratified trial designs, (ii) co-measurement of functional endpoints tied to canonical thresholds (e.g., gait speed at Studenski 2011 0.8 m/s, grip strength at Cruz-Jentoft 2019 27 kg / 16 kg, or HbA1c at ADA 2024 7% / 6.5% in applicable comorbid populations), and (iii) explicit acknowledgement that pharmacokinetic success is a necessary but not sufficient condition for clinical translation, consistent with the Ioannidis 2005 surrogate-endpoint caution. The synthesis must therefore describe NAD+-elevating pharmacokinetics as established within the corpus, while declining to extend that finding into clinical-benefit claims without direct functional or hard-outcome data.

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, 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: Across 24 curated reference papers, the evidence base for NAD+ shows a context-dependent profile. Positive signals appear in: contextual other. Negative signals appear in: longevity, contextual other. Null findings dominate: contextual other, dosing pharmacokinetics. The synthesis surfaces cross-study disagreements across outcome classes — see Cross-Domain Synthesis. The NAD+ 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. 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 24 included sources. The evidence-tier distribution is: B2 (n=20), B1 (n=2), A1 (n=2). By directness, the breakdown is: indirect (n=18), review (n=4), direct (n=2). 18 of 24 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 3 distinct summaries across the source set: adults; frail / sarcopenic adults; older 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 curated corpus does not contain any long-term mortality or hard-cardiovascular-endpoint randomized trial of NAD+ precursor supplementation in non-diabetic or generally healthy community-dwelling adults, which is a gap that constrains every downstream causal claim. No source addresses incident cardiovascular events, cancer incidence, or all-cause mortality in a primary-prevention setting, so any inference that biomarker changes translate into longevity benefit must be treated as extrapolative rather than evidence-based. This is a well-known hazard of surrogate-endpoint research (Ioannidis 2005), and the present corpus is not positioned to overcome it.

Several clinically relevant outcomes in this evidence base rest on a single trial, which means they cannot be replicated within the corpus and should be interpreted accordingly. With only one trial apiece, sampling variability, site-specific effects, and chance cannot be disentangled from a true effect, and the field-level confidence in any of these outcome-specific conclusions is necessarily low. A second independent cohort for each endpoint would be the minimum requirement before these signals can be promoted from hypothesis-generating to provisional evidence.

Additional corpus sources included animal/preclinical evidence; the enrolled populations are narrowly defined, which limits external validity beyond the studied subgroups. sources cluster in healthy middle-aged adults (Yi 2022, Xue 2022, Martens 2018, Airhart 2017, Katayoshi 2023, Liao 2021), healthy older adults (Elhassan 2019, Vreones 2022, Christen 2026), and physically compromised older adults (Connell 2021, n = 14). Notably absent are adults with established cardiometabolic disease who are not already on optimized background therapy, adults older than 80, and any population with substantial racial or geographic diversity. Even where sarcopenia cutoffs exist as reference standards (Cruz-Jentoft 2019: 27 kg grip strength for men, 16 kg for women), the corpus does not enroll enough sarcopenic participants to test whether NAD+ precursor effects vary by baseline deficit severity.

Several clinically attractive claims in the surrounding literature are supported in this corpus only by mechanistic or preclinical evidence, and the leap from bench to clinic is not closed within the 24 sources. These sources establish biological plausibility but do not test whether NAD+ precursor supplementation alters disease incidence, progression, or survival in humans. The cross-study disagreement map flags the corresponding cross-domain mechanism-vs-clinical pairs (severity 3), and they remain unresolved because no source bridges the gap for those indications.

Conclusion

Across the corpus, the 24-study evidence base supports a context-dependent and incomplete case for NAD+ as an anti-aging intervention. The integrating thesis — that mechanistic plausibility coexists with mixed or sparse human-RCT evidence, and that boundary conditions remain to be established — is the most defensible reading of the current literature. The strongest supportive signals come from acute clinical contexts in which NAD+ precursor supplementation is layered on top of an active disease process: Gao 2025 reported favorable hearing-recovery responses in sudden sensorineural hearing loss with p-values ranging from P = 0.010 to P < 0.001 across the primary endpoints, and Curran 2025 (a systematic review) found positive aggregate effects for NAD metabolites in infectious-disease animal models, with several comparisons reaching P < 0.01. Translational relevance to humans remains uncertain. These findings are consistent with the more general hypothesis that NAD+ may be detectable when the underlying NAD+ depletion is driven by acute pathophysiology, rather than by the slower drift of healthy aging.

Additional corpus sources included animal/preclinical evidence; the strongest countervailing and unresolved evidence clusters in three places. Third, Zhao 2024 reports negative direction on a contextual outcome, in direct conflict with the positive aggregate finding from Curran 2025 and in partial conflict with several null results — Christen 2026, Holmes 2026, Bai 2022, Gao 2025, Vreones 2022, and Gao 2026 — which collectively prevent any clean classification of effect direction. The cross-study disagreements catalogued in the Cross-Domain Synthesis, and the repeated mechanism-versus-clinical and direct-versus-indirect gaps (e.g., Xue 2022 vs Martens 2018, Yi 2022 vs Elhassan 2019, Xue 2022 vs Membrez 2024), make clear that biomarker and surrogate-endpoint movement, in line with the cautionary framing of Ioannidis 2005, has not been shown to translate into hard functional or longevity outcomes. As a general-health matter, dietary, sleep, and exercise measures that support mitochondrial function are reasonable on their own grounds, but this should be kept separate from marketing any specific NAD+ product as a proven standalone anti-aging intervention.

The recommended next step is a single, adequately powered, pre-registered human RCT with a hard clinical endpoint — gait speed anchored to the Studenski 2011 0.8 m/s frailty-relevant threshold, the Cesari 2009 0.6 m/s severe-frailty cutoff, and the Perera 2006 0.1 m/s substantial-improvement threshold, and grip strength benchmarked against the Cruz-Jentoft 2019 27 kg male and 16 kg female EWGSOP2 sarcopenia cutoffs — to determine whether sustained NAD+ elevation produces a clinically meaningful change rather than only a biomarker shift.

Pending further trials, the evidence does not support off-label geroprotective use of NAD+ supplementation; general-health advice regarding lifestyle remains appropriate, but the current data are insufficient to claim a proven anti-aging benefit at the individual-patient level.

What This Synthesis Adds

This synthesis maps 24 included sources on NAD+ across 6 outcome classes and 61 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.

Prior reviews in the corpus (Curran 2025, Han 2022) emphasize convergent signals on NAD+. 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	negative, unclear	direct interventional hard-endpoint gap
cardiometabolic	0	2	null, unclear	direct interventional hard-endpoint gap
frailty	0	1	null	direct interventional hard-endpoint gap
muscle function	0	3	unclear	direct interventional hard-endpoint gap
contextual adjacent evidence	1	12	negative, null, positive, unclear	conflict-resolution gap
dosing and pharmacokinetics	1	2	null, unclear	replication gap

Evidence-Gap Priority

Priority	Gap	Rationale
P1	longevity: direct interventional hard-endpoint gap	0 direct and 2 indirect sources; direction profile: negative, unclear
P2	cardiometabolic: direct interventional hard-endpoint gap	0 direct and 2 indirect sources; direction profile: null, unclear
P3	frailty: direct interventional hard-endpoint gap	0 direct and 1 indirect source; direction profile: null
P4	muscle function: direct interventional hard-endpoint gap	0 direct and 3 indirect sources; direction profile: unclear
P5	contextual adjacent evidence: conflict-resolution gap	1 direct and 12 indirect sources; direction profile: negative, null, positive, unclear

Next-Study Design Recommendation

The next high-yield study for NAD+ 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

• Yi 2022; tier=A1; directness=direct; endpoint=dosing pharmacokinetics; direction=unclear; representative statistic=p ≤ 0.001.
• Xue 2022; tier=A1; directness=direct; endpoint=contextual adjacent evidence; direction=null; representative statistic=P = 0.003 (off-summary).
• Curran 2025; tier=B1; directness=review; endpoint=contextual adjacent evidence; direction=positive; representative statistic=P < 0.01.
• Han 2022; tier=B1; directness=review; endpoint=longevity; direction=unclear.
• Katayoshi 2023; tier=B2; directness=indirect; endpoint=cardiometabolic; direction=null; representative statistic=P = 0.001 (off-summary).
• Connell 2021; tier=B2; directness=indirect; endpoint=muscle function; direction=unclear; representative statistic=P = 0.001.
• Gao 2025; tier=B2; directness=indirect; endpoint=contextual adjacent evidence; direction=null; representative statistic=P < 0.001 (off-summary).
• Mevenkamp 2024; tier=B2; directness=indirect; endpoint=contextual adjacent evidence; direction=unclear; representative statistic=P = 0.0003.
• Martens 2018; tier=B2; directness=indirect; endpoint=cardiometabolic; direction=unclear; representative statistic=P < 0.006.
• Simic 2020; tier=B2; directness=review; endpoint=dosing pharmacokinetics; direction=null; representative statistic=P = 0.002 (off-summary).

Source Classification Map

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

• The efficacy and safety of β-nicotinamide mononucleotide (NMN) supplementation in healthy middle-aged adults: a randomized, multicenter, double-blind, placebo-controlled, parallel-group, dose-dependent clinical trial: outcome=dosing pharmacokinetics; directness=direct; tier=A1; direction=unclear; claims=104.
• A Combination of Nicotinamide and D-Ribose (RiaGev) Is Safe and Effective to Increase NAD + Metabolome in Healthy Middle-Aged Adults: A Randomized, Triple-Blind, Placebo-Controlled, Cross-Over Pilot Clinical Trial: outcome=contextual adjacent evidence; directness=direct; tier=A1; direction=null; claims=72.
• Meta-analysis of niacin and NAD metabolite treatment in infectious disease animal studies suggests benefit but requires confirmation in clinically relevant models: outcome=contextual adjacent evidence; directness=review; tier=B1; direction=positive; claims=109.
• The impacts of continuous improvements in air quality on mortality in Beijing: A longitudinal comparative study.: outcome=longevity; directness=review; tier=B1; direction=unclear; claims=3.
• Nicotinamide adenine dinucleotide metabolism and arterial stiffness after long-term nicotinamide mononucleotide supplementation: a randomized, double-blind, placebo-controlled trial: outcome=cardiometabolic; directness=indirect; tier=B2; direction=null; claims=177.
• NAD + -Precursor Supplementation With L-Tryptophan, Nicotinic Acid, and Nicotinamide Does Not Affect Mitochondrial Function or Skeletal Muscle Function in Physically Compromised Older Adults: outcome=muscle function; directness=indirect; tier=B2; direction=unclear; claims=148.
• NAD+ Enhanced on Hearing Recovery in Sudden Sensorineural Hearing Loss: Randomized Controlled Trial: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=126.
• Development of a 31 P magnetic resonance spectroscopy technique to quantify NADH and NAD + at 3 T: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=unclear; claims=113.
• Chronic nicotinamide riboside supplementation is well-tolerated and elevates NAD + in healthy middle-aged and older adults: outcome=cardiometabolic; directness=indirect; tier=B2; direction=unclear; claims=97.
• 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: outcome=dosing pharmacokinetics; directness=review; tier=B2; direction=null; claims=86.
• Refining Prognosis in Cirrhosis Patients With Ascites: Impact of Acute vs. Non‐Acute Decompensation: outcome=longevity; directness=indirect; tier=B2; direction=negative; claims=70.
• 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: outcome=dosing pharmacokinetics; directness=indirect; tier=B2; direction=null; claims=66.
• Acupuncture as Add-on Therapy to SSRIs Can Improve Outcomes of Treatment for Anxious Depression: Subgroup Analysis of the AcuSDep Trial: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=negative; claims=55.
• Nicotinamide Riboside Augments the Aged Human Skeletal Muscle NAD + Metabolome and Induces Transcriptomic and Anti-inflammatory Signatures: outcome=muscle function; directness=indirect; tier=B2; direction=unclear; claims=54.
• Effect of Nicotinamide Adenine Dinucleotide on Heart Failure Caused by Ischemic Cardiomyopathy: A Randomized, Placebo-Controlled Trial: outcome=muscle function; directness=indirect; tier=B2; direction=unclear; claims=54.
• Relationship between sperm NAD + concentration and reproductive aging in normozoospermia men:A Cohort study: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=47.
• Nicotinamide mononucleotide supplementation enhances aerobic capacity in amateur runners: a randomized, double-blind study: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=unclear; claims=38.
• Synaptic biomarkers in Alzheimer's disease dementia and mild cognitive impairment: A systematic review and meta‐analysis: outcome=contextual adjacent evidence; directness=review; tier=B2; direction=null; claims=34.
• A Liposomal Formulation Enhances the Anti-Senescence Properties of Nicotinamide Adenine-Dinucleotide (NAD + ) in Endothelial Cells and Keratinocytes: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=20.
• Nicotinamide riboside and pterostilbene reduces frequency and severity of undesirable symptoms of the menopause transition: an open-label, pilot clinical trial: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=19.
• The differential impact of three different NAD + boosters on circulatory NAD and microbial metabolism in humans: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=17.
• Trigonelline is an NAD + precursor that improves muscle function during ageing and is reduced in human sarcopenia: outcome=frailty; directness=indirect; tier=B2; direction=null; claims=13.
• SERPINE1 drives ferroptosis in acute respiratory distress syndrome by disrupting mitochondrial NAD + homeostasis and suppressing Sirt3 activity: outcome=contextual adjacent evidence; directness=indirect; tier=B2; direction=null; claims=7.
• Oral nicotinamide riboside raises NAD+ and lowers biomarkers of neurodegenerative pathology in plasma extracellular vesicles enriched for neuronal origin: 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

• Severity 5 disagreement: Zhao 2024 vs Curran 2025; Zhao 2024 reports negative effect on contextual other; Curran 2025 reports positive on the same outcome — direct conflict
• Severity 4 null vs negative: Zhao 2024 vs Ministrini 2025; Zhao 2024 (negative on contextual other) vs Ministrini 2025 (null on contextual other) — partial conflict
• Severity 4 null vs negative: Zhao 2024 vs Christen 2026; Zhao 2024 (negative on contextual other) vs Christen 2026 (null on contextual other) — partial conflict
• Severity 4 null vs negative: Zhao 2024 vs Gao 2025; Zhao 2024 (negative on contextual other) vs Gao 2025 (null on contextual other) — partial conflict
• Severity 4 null vs negative: Zhao 2024 vs Holmes 2026; Zhao 2024 (negative on contextual other) vs Holmes 2026 (null on contextual other) — partial conflict
• Severity 4 null vs negative: Zhao 2024 vs Gao 2026; Zhao 2024 (negative on contextual other) vs Gao 2026 (null on contextual other) — partial conflict
• Severity 4 null vs negative: Zhao 2024 vs Gaur 2026; Zhao 2024 (negative on contextual other) vs Gaur 2026 (null on contextual other) — partial conflict
• Severity 4 null vs negative: Zhao 2024 vs Bai 2022; Zhao 2024 (negative on contextual other) vs Bai 2022 (null on contextual other) — partial conflict

Additional corpus sources informed the synthesis without anchoring a foregrounded quantitative claim and are catalogued for completeness: Tancredi 2015.

References

• Katayoshi 2023. Nicotinamide adenine dinucleotide metabolism and arterial stiffness after long-term nicotinamide mononucleotide supplementation: a randomized, double-blind, placebo-controlled trial. Scientific Reports, 2023. DOI: 10.1038/s41598-023-29787-3. PMID: 36797393.
• Connell 2021. NAD + -Precursor Supplementation With L-Tryptophan, Nicotinic Acid, and Nicotinamide Does Not Affect Mitochondrial Function or Skeletal Muscle Function in Physically Compromised Older Adults. The Journal of Nutrition, 2021. DOI: 10.1093/jn/nxab193. PMID: 34191033.
• Gao 2025. NAD+ Enhanced on Hearing Recovery in Sudden Sensorineural Hearing Loss: Randomized Controlled Trial. The Laryngoscope, 2025. DOI: 10.1002/lary.70173. PMID: 41035311.
• Mevenkamp 2024. Development of a 31 P magnetic resonance spectroscopy technique to quantify NADH and NAD + at 3 T. Nature Communications, 2024. DOI: 10.1038/s41467-024-53292-4. PMID: 39443469.
• Curran 2025. Meta-analysis of niacin and NAD metabolite treatment in infectious disease animal studies suggests benefit but requires confirmation in clinically relevant models. Scientific Reports, 2025. DOI: 10.1038/s41598-025-95735-y. PMID: 40221506.
• Yi 2022. The efficacy and safety of β-nicotinamide mononucleotide (NMN) supplementation in healthy middle-aged adults: a randomized, multicenter, double-blind, placebo-controlled, parallel-group, dose-dependent clinical trial. GeroScience, 2022. DOI: 10.1007/s11357-022-00705-1. PMID: 36482258.
• 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.
• Xue 2022. A Combination of Nicotinamide and D-Ribose (RiaGev) Is Safe and Effective to Increase NAD + Metabolome in Healthy Middle-Aged Adults: A Randomized, Triple-Blind, Placebo-Controlled, Cross-Over Pilot Clinical Trial. Nutrients, 2022. DOI: 10.3390/nu14112219. PMID: 35684021.
• Simonis 2025. Refining Prognosis in Cirrhosis Patients With Ascites: Impact of Acute vs. Non‐Acute Decompensation. Alimentary Pharmacology & Therapeutics, 2025. DOI: 10.1111/apt.70302. PMID: 40719565.
• 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.
• Zhao 2024. Acupuncture as Add-on Therapy to SSRIs Can Improve Outcomes of Treatment for Anxious Depression: Subgroup Analysis of the AcuSDep Trial. Neuropsychiatric Disease and Treatment, 2024. DOI: 10.2147/NDT.S446034. PMID: 38770535.
• Yu 2025. Effect of Nicotinamide Adenine Dinucleotide on Heart Failure Caused by Ischemic Cardiomyopathy: A Randomized, Placebo-Controlled Trial. American Journal of Cardiovascular Drugs, 2025. DOI: 10.1007/s40256-025-00764-7. PMID: 40954388.
• 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.
• Bai 2022. Relationship between sperm NAD + concentration and reproductive aging in normozoospermia men:A Cohort study. BMC Urology, 2022. DOI: 10.1186/s12894-022-01107-3. PMID: 36182928.
• Liao 2021. Nicotinamide mononucleotide supplementation enhances aerobic capacity in amateur runners: a randomized, double-blind study. Journal of the International Society of Sports Nutrition, 2021. DOI: 10.1186/s12970-021-00442-4. PMID: 34238308.
• Gaur 2026. Synaptic biomarkers in Alzheimer's disease dementia and mild cognitive impairment: A systematic review and meta‐analysis. Alzheimer's & Dementia, 2026. DOI: 10.1002/alz.71501. PMID: 42192211.
• Ministrini 2025. A Liposomal Formulation Enhances the Anti-Senescence Properties of Nicotinamide Adenine-Dinucleotide (NAD + ) in Endothelial Cells and Keratinocytes. Current Issues in Molecular Biology, 2025. DOI: 10.3390/cimb47090722. PMID: 41020844.
• 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.
• Christen 2026. The differential impact of three different NAD + boosters on circulatory NAD and microbial metabolism in humans. Nature Metabolism, 2026. DOI: 10.1038/s42255-025-01421-8. PMID: 41540253.
• Membrez 2024. Trigonelline is an NAD + precursor that improves muscle function during ageing and is reduced in human sarcopenia. Nature Metabolism, 2024. DOI: 10.1038/s42255-024-00997-x. PMID: 38504132.
• Gao 2026. SERPINE1 drives ferroptosis in acute respiratory distress syndrome by disrupting mitochondrial NAD + homeostasis and suppressing Sirt3 activity. Redox Biology, 2026. DOI: 10.1016/j.redox.2026.104146. PMID: 42190562.
• Han 2022. The impacts of continuous improvements in air quality on mortality in Beijing: A longitudinal comparative study. Chemosphere, 2022. DOI: 10.1016/j.chemosphere.2021.132893. PMID: 34780733.
• 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.

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).

• Studenski 2011. Studenski S, Perera S, Patel K, et al. Gait speed and survival in older adults. JAMA. 2011;305(1):50-58. DOI: 10.1001/jama.2010.1923. PMID: 21205966.
• 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.
• Perera 2006. Perera S, Mody SH, Woodman RC, Studenski SA. Meaningful change and responsiveness in common physical performance measures in older adults. J Am Geriatr Soc. 2006;54(5):743-749. DOI: 10.1111/j.1532-5415.2006.00701.x. PMID: 16696738.
• ADA 2024. American Diabetes Association. Standards of Care in Diabetes. Diabetes Care. 2024;47(Suppl 1). DOI: 10.2337/dc24-S006.
• Bohannon 1997. Bohannon RW. Comfortable and maximum walking speed of adults aged 20-79 years: reference values and determinants. Age Ageing. 1997;26(1):15-19. DOI: 10.1093/ageing/26.1.15.
• 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.
• Anisimov 2008. Anisimov VN, Berstein LM, Egormin PA, et al. Metformin slows down aging and extends life span of female SHR mice. Cell Cycle. 2008;7(17):2769-2773. PMID: 18728386.
• Tancredi 2015. Tancredi M, Rosengren A, Svensson AM, et al. Excess mortality among persons with type 2 diabetes. N Engl J Med. 2015;373(18):1720-1732. DOI: 10.1056/NEJMoa1504347. PMID: 26510021.
• 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.