NAD+ Precursors — What the Human Trials Actually Show

Drew, I'm seeing patients walk in with NMN supplements, printed quotes for IV NAD+ infusions, and a lot of confidence. What am I actually dealing with here?

Three separate questions dressed as one. What is NAD+, does supplementing it do anything measurable in humans, and is it safe for this specific patient. The biology matters because it changes which question is even answerable.

Give me the biology in the version that actually determines the clinical answer.

NAD+ does two jobs. One is redox cycling — it shuttles electrons in glycolysis and the TCA cycle, gets reduced to NADH and oxidized back. That job doesn't consume it. The second job is substrate: enzymes called PARPs, sirtuins, and CD38 cleave NAD+ stoichiometrically, one molecule destroyed per reaction. That pool has to be continuously rebuilt.

So the chart showing NAD+ falling with age — what's actually driving that?

Consumption, not failed synthesis. CD38, which is expressed on immune and endothelial cells, rises with age and inflammatory burden and it's a very efficient NAD+ hydrolase. The pool falls because destruction accelerates, not because we stop making it. That reframes the whole supplement question.

How so?

Because NR and NMN — nicotinamide riboside and nicotinamide mononucleotide, the two oral precursors people are buying — they feed into the salvage pathway and do raise blood NAD+ in humans. That's replicated. But raising blood NAD+ in a setting where CD38 is continuously draining the pool is a different problem than simple deficiency.

What do the human trials actually show beyond the blood level?

Mostly biomarker and safety data. Several randomized trials — Washington University did NMN work in older women and in men with prediabetes — showed muscle NAD+ increases and some insulin sensitivity signals. Mechanistic, not clinical endpoints. No trial has shown reduced cardiovascular events, cognitive preservation, or mortality benefit in humans. That's the honest answer.

What about the patient three years out from breast cancer asking if NMN is safe for her?

That's the one question with a more cautious answer. PARP1 is a DNA damage sensor, sirtuins regulate cell survival pathways — these are the enzymes NAD+ feeds. There is preclinical signal in both directions, some suggesting NAD+ repletion could support cancer cell survival. No human oncology safety data exists for this indication. My answer to her is: we don't know, and the absence of evidence is not evidence of absence.

And the IV NAD+ infusions being marketed at several thousand dollars a course?

No randomized controlled trial evidence for any clinical endpoint. It's marketed without evidence. The pharmacokinetics are also not obviously better than oral — blood NAD+ rises with oral NR or NMN comparably. I'd say so plainly.

What do I tell the patient holding the NMN bottle who just wants a yes or no?

It's not dangerous if you're otherwise healthy, it does raise NAD+ levels, and we don't yet know whether that translates to any outcome you'd care about clinically. No guideline body recommends it. If you want to take it, that's a reasonable personal decision on current evidence.

What's the single thing you'd want me to hold onto from this?

The biology is real and interesting, the biomarker data is replicated, and there are zero human clinical endpoint trials. Don't let mechanistic plausibility carry more weight than that gap deserves — and apply extra caution in anyone with a cancer history until we have oncology-specific safety data.