Exercise Primes NAD+ Recycling, but Muscle NAD+ Is Unproven

Exercise reliably turns up a key enzyme that helps muscle recycle NAD+, a coenzyme central to cellular energy metabolism. A 2023 meta-analysis found that structured exercise training increased skeletal-muscle intracellular NAMPT (iNAMPT) by 1.46-fold, with a large standardized effect size (Cohen's d = 0.81). That makes training the best-characterized behavioral way to increase this specific enzyme in humans.

It does not prove that exercise raises NAD+ concentrations inside human muscle. The available training studies did not provide direct pre- and post-training muscle-biopsy NAD+ measurements. Exercise appears to prime the recycling machinery; researchers have not yet read the fuel gauge in the tissue that matters.

Why iNAMPT gets so much attention

iNAMPT is the rate-limiting enzyme in the nicotinamide salvage pathway, the main route by which mammalian cells remake NAD+. Cells use NAD+ in reactions involved in energy production and in signaling systems that include sirtuins. Because iNAMPT constrains a major NAD+ recycling route, more of the enzyme offers a plausible route to greater NAD+ synthesis capacity. Capacity, however, is not the same thing as a measured rise in NAD+.

The 2023 systematic review and meta-analysis by Xu Sun et al. selected five human exercise intervention studies under PRISMA 2020 criteria. The publication does not report a pooled participant total or confidence interval in the evidence summarized here, so those details should not be invented. Its pooled result was a 1.46-fold increase in muscle iNAMPT and Cohen's d of 0.81. For an enzyme-expression endpoint, that is a sizable signal, but it remains a surrogate endpoint rather than a clinical outcome.

Exercise has convincing human evidence for increasing muscle iNAMPT. Direct evidence that the increase raises muscle NAD+ after training is still absent.

Training seems to change the enzyme, across more than one modality

The same meta-analysis found positive signals for both aerobic and resistance training. Its Bayesian analysis assigned a 75.1% posterior probability of iNAMPT upregulation to aerobic exercise and 66.4% to resistance exercise. The 8.7-percentage-point difference invites follow-up research, not a prescription to abandon lifting for cardio.

Why the restraint? Every subgroup analysis came from the same five-study pool. There are no intensity-stratified human intervention data that can tell us whether zone 2, intervals, heavy resistance work, or a mixed program produces the largest iNAMPT response. Any claim to know the optimal NAD+-boosting workout would be premature.

Cross-sectional comparisons point in the same direction but cannot establish cause. Endurance-trained athletes had roughly twofold higher iNAMPT than sedentary, non-obese peers, while lifelong football players had roughly 2.2-fold higher expression than active but untrained comparators in studies covered by the review. Years of training could contribute to those differences. So could baseline biology: people with higher NAMPT expression may be more likely to thrive in endurance sport or stick with it for decades.

Age may change the response

The meta-analysis found a lower posterior probability of iNAMPT upregulation in older adults than in younger adults: 68.7% versus 75.6%. This is a modest difference from limited data, not proof that older muscle cannot respond to training. Still, it matters because older adults are the group most likely to ask whether an NAD+ precursor can fill a gap that exercise leaves behind.

That question has no direct answer. No human trial has randomly assigned people to exercise, an NAD+ precursor, both, or neither and then compared muscle iNAMPT and muscle NAD+ concentrations. The sensible conclusion is narrower: keep exercise in the plan, while recognizing that its enzymatic response may differ with age and that supplementation has a separate evidence base.

Blood-cell findings support the mechanism, not the muscle claim

A 2024 human study by Walzik et al. found that one acute exercise bout increased NAD+ metabolite dynamics in peripheral blood mononuclear cells. The result supports the idea that exercise engages NAD+ metabolism quickly in circulating immune cells.

Peripheral blood mononuclear cells are not skeletal muscle. They differ in function and metabolism, and a blood-cell signal cannot substitute for a muscle biopsy. It is useful mechanistic context, but it does not close the central evidence gap.

Supplements are not the defeated side of this story

Oral nicotinamide riboside (NR), nicotinamide mononucleotide (NMN), and nicotinamide have a broader clinical research literature than the five exercise-iNAMPT intervention studies. A 2026 crossover randomized trial by Christen et al. compared nicotinamide, NR, and NMN over 14 days and found that each changed the whole-blood NAD+ metabolome in distinct ways. It had no exercise arm and no muscle biopsy, so it cannot tell us whether a capsule outperforms a workout in muscle.

The more useful comparison is endpoint by endpoint. Supplements can measurably alter circulating NAD+-related metabolites. Exercise training can measurably increase muscle iNAMPT. Those are different outcomes, and neither result automatically predicts better strength, function, longevity, or muscle NAD+ concentration.

For muscle mass and function, a 2025 systematic review and meta-analysis by Prokopidis et al. found inconsistent results from NMN and NR randomized trials. Inconsistent does not mean useless; it means the present trials do not support a uniform functional benefit across the studied populations and outcomes.

Results may also depend on who takes the supplement. A 2026 meta-analysis by Wang et al. reported measurable effects of NMN on muscle and liver function in middle-aged and older adults, and noted that supplementation may matter especially when exercise responses are diminished. A separate 2026 PRISMA-guided review by Gallagher et al. describes a substantially larger overall supplementation literature. Neither review supplies a head-to-head exercise comparison.

What the conflicting NR evidence actually says

Some findings argue against assuming that exercise and NAD+ precursors stack neatly. In a 2021 human study by Stocks et al., NR supplementation did not change whole-body or skeletal-muscle metabolic responses, or NAD+-sensitive signaling, during a single endurance exercise bout. That was an acute exercise experiment, so it cannot rule out longer-term interactions. It does show that adding NR did not produce an obvious extra muscle-metabolic effect under those conditions.

The Xu Sun review also cites a 12-week NR trial in obese, insulin-resistant men in which iNAMPT fell 14%. That result raises a reasonable feedback question: when more precursor is available, does muscle reduce its own salvage-pathway enzyme expression? The population was narrow, and the finding should not be generalized to healthy, lean, or regularly exercising adults. It is a lead for research, not a verdict on NR.

The practical verdict

Train regularly if you want the strongest human evidence for a behavioral increase in muscle iNAMPT. Aerobic and resistance exercise both have a positive signal, and current data cannot identify a winning intensity or modality. The 1.46-fold pooled increase is meaningful because it targets a rate-limiting part of NAD+ recycling, not because it proves that exercise replenishes muscle NAD+.

Do not frame exercise as a replacement for NAD+ precursors. Supplementation has more extensive human research, changes circulating NAD+-related metabolites, and may have particular relevance for some middle-aged and older adults. Its effects on muscle function remain inconsistent overall, while the interaction between supplements and training remains largely untested.

The experiment the field needs is straightforward: a sufficiently large human trial that measures muscle NAD+ and iNAMPT before and after exercise, precursor supplementation, and their combination. Until then, exercise has earned a strong claim about enzyme regulation—and no stronger claim about muscle NAD+ itself.