Every cell in your body runs on NAD+. It powers over 500 enzymatic reactions — energy metabolism, DNA repair, sirtuin activation, cellular signaling. Without it, your mitochondria can't produce ATP. Your DNA repair enzymes can't function. Your sirtuins — the proteins linked to longevity in every model organism studied — go silent.

And your levels are dropping. By 50% between ages 40 and 60, according to human tissue studies (Massudi et al., PLOS ONE, 2012).

This is not fringe science. NAD+ decline is published in Cell, Nature, and Science. The question isn't whether it happens. The question is whether you can fix it with a pill — and whether that actually changes anything.

Three mechanisms, all accelerating with age:

CD38 goes up. CD38 is an enzyme that consumes NAD+. A 2016 Cell Metabolism study (Camacho-Pereira) showed that CD38 expression rises with age due to chronic low-grade inflammation — "inflammaging." CD38 is now considered the primary driver of age-related NAD+ decline. More inflammation = more CD38 = less NAD+. A vicious cycle.

DNA damage accumulates. PARP enzymes repair your DNA — and each repair event consumes NAD+. As DNA damage piles up with age, PARP activity increases, further depleting the NAD+ pool (Zhu et al., Cell Metabolism, 2015).

Synthesis slows. NAMPT, the rate-limiting enzyme in NAD+ production, declines with age. Less production + more consumption = the supply-demand imbalance that defines cellular aging.

Two supplements have emerged as the leading NAD+ precursors: NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside). Both ultimately become NAD+ inside your cells, but they take slightly different paths.

NR enters cells through nucleoside transporters and gets phosphorylated into NMN, which then converts to NAD+. It's been commercially available longer (as Niagen, by ChromaDex) and has more published human trials.

NMN is one step closer to NAD+ in the biosynthetic pathway. It was long thought to be too large to enter cells directly, but a 2019 Nature Metabolism study identified a dedicated NMN transporter (Slc12a8) in the small intestine. This was a pivotal finding — it meant oral NMN could potentially be absorbed and used without first being broken down to NR.

Bottom line: There is no definitive human trial proving one is superior. Both reliably raise NAD+ levels in blood and tissues. The rivalry between NMN and NR is more commercial than scientific — Charles Brenner (NR/ChromaDex advisor) and David Sinclair (NMN advocate) have a well-known public feud. The molecules don't care.

This is where most NAD+ content gets sloppy — mixing mouse studies with human results. Here's what we actually know from clinical trials in humans:

Insulin sensitivity (NMN). Yoshino et al., Science, 2021. 250mg NMN daily for 10 weeks in postmenopausal women with prediabetes. Result: ~25% improvement in skeletal muscle insulin sensitivity. Did not change body weight, blood pressure, or lipids. This is the strongest single finding for NMN.

Exercise performance (NMN). Liao et al., JISSN, 2021. Amateur runners given 300, 600, or 1,200mg NMN daily for 6 weeks. The 600mg and 1,200mg groups showed significant improvements in aerobic capacity and oxygen utilization. Notably, 1,200mg wasn't meaningfully better than 600mg — suggesting diminishing returns.

Physical function (NMN). Yi et al., GeroScience, 2023. 250mg NMN daily for 12 weeks in adults aged 40-65. Significant improvements in 6-minute walking distance, reduced drowsiness, and better health survey scores. NAD+ levels rose significantly.

The METRO Trial (NMN). This is the pharmaceutical-grade study to watch. Metro International Biotech (David Sinclair is co-founder) ran MIB-626, a crystalline form of NMN, through Phase I and Phase II trials. Phase I confirmed safety and dose-dependent NAD+ increases. Phase II data in older adults showed improvements in physical performance metrics. This trial is significant because it used pharmaceutical-grade NMN at controlled doses — unlike most supplement studies.

Cardiovascular trends (NR). Martens et al., Nature Communications, 2018. 1,000mg NR daily for 6 weeks in healthy older adults. NAD+ in blood cells rose ~60%. Trend toward reduced aortic stiffness and lower systolic blood pressure, particularly in those with mild hypertension.

Neurological promise (NR). Brakedal et al., Cell Metabolism, 2022. 1,000mg NR daily for 30 days in Parkinson's patients. Cerebral NAD+ levels increased (measured by MRI). Mild clinical improvement. Small study, needs replication — but the fact that oral NR raised brain NAD+ levels is significant.

The disappointment (NR). Dollerup et al., AJCN, 2018. 2,000mg NR daily for 12 weeks in obese, insulin-resistant men. NAD+ metabolism improved. But: no significant improvement in insulin sensitivity, mitochondrial function, or body composition versus placebo. Not every trial hits.

The mouse data is dramatic. The human data is promising but modest. Be honest about this gap. Anyone selling NAD+ precursors as a proven anti-aging intervention is outrunning the evidence.

In 2022, the FDA pulled NMN from the supplement market, ruling it couldn't be sold as a dietary supplement because it was being investigated as a pharmaceutical drug (by Metro International Biotech, in which David Sinclair holds a stake). The supplement industry pushed back with citizen petitions and legal challenges.

As of 2026, the situation is technically unresolved — the FDA never formally reversed its position — but enforcement has ceased. NMN is widely available on Amazon and in supplement stores. The practical reality: you can buy it, but the regulatory ground could shift again.

If you're under 35 with no metabolic issues, the evidence doesn't strongly support NAD+ precursor supplementation. Your NAD+ levels haven't declined enough for the intervention to matter. Focus on the basics: exercise, sleep, and the foundational supplement stack first.

If you're 40+ and interested, start with 250mg NMN or 300mg NR daily. This is the most-studied dose range. Take in the morning — NAD+ metabolism follows circadian patterns, and NAMPT expression peaks in the AM.

Consider TMG (trimethylglycine) alongside. NMN and NR metabolism produces nicotinamide, which must be methylated for excretion. This could theoretically deplete methyl donors. TMG at 500-1,000mg daily is a common precautionary co-supplement. No clinical study has confirmed this is necessary, but the biochemistry is sound.

Pair with resveratrol — maybe. David Sinclair has long advocated combining NMN with resveratrol, arguing resveratrol activates sirtuins while NAD+ fuels them. The synergy makes biochemical sense, but no human RCT has tested the combination head-to-head against either alone. Thorne's ResveraCel combines NR with resveratrol, betaine, and quercetin in one capsule — a convenient option if you want the combination approach.

Don't exceed 1,000mg/day without reason. The Liao exercise study showed no benefit of 1,200mg over 600mg. More is not reliably better, and long-term safety data at high doses doesn't exist.

If you have active cancer, skip it. NAD+ fuels all cells — including cancer cells. No human study has shown increased cancer risk, but the theoretical concern is real enough to warrant caution. Talk to your oncologist.

Who else should be cautious:

Life Extension NAD+ Cell Regenerator — combines nicotinamide riboside with resveratrol and other NAD+ support compounds. Third-party tested. A reasonable entry point if you want to try NAD+ precursor supplementation without navigating the NMN regulatory ambiguity.

For the deep science behind all of this, David Sinclair's Lifespan: Why We Age and Why We Don't Have To remains the most accessible overview — with the caveat that Sinclair is both researcher and commercial stakeholder in the NMN space. Read it for the biology, not the product recommendations.

If you want to go deeper into the science of cellular aging and longevity optimization, the School of Biohacking has structured courses covering NAD+ metabolism and other advanced topics.

The CD38 angle nobody talks about. If CD38 is the primary driver of NAD+ decline, then reducing CD38 activity might be more effective than brute-forcing NAD+ synthesis with precursors. Apigenin (found in chamomile and parsley) and quercetin are natural CD38 inhibitors showing promise in preclinical studies. This is speculative — no human RCTs yet — but it's the direction the cutting-edge research is heading. Attack the drain, not just the faucet.

NAD+ supplementation is in the "strong rationale, early evidence" category. The mechanistic science is solid. The mouse data is compelling. The human data is real but modest. If you're 40+ and the foundational stack is locked in, it's a reasonable addition. If you're 28 and looking for a shortcut — save your money and go for a run.

Have you tried NMN or NR? Did you notice anything? Hit reply and tell me.

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