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NAD+ and physical function
NAD+ research has an unusually clear chemical success: oral precursors raise blood levels. The more interesting question is what a person can do with the change.
The walking result
The middle-aged-adult NMN trial published in 2023 deserves more than a passing mention of blood NAD+. Walking distance and reported health and wellbeing improved compared with placebo. Blood NAD+ rose in all NMN groups at 30 and 60 days, with larger study amounts producing larger rises. Participants received 300-900 mg/day for 60 days; no safety problems appeared during the trial.
Yet the researchers’ preference for one tested amount does not establish a personal dose, and a result over 60 days leaves years of health unanswered. The 2025 review of human ageing trials judged efficacy limited overall, with little tissue-specific data and only a limited number of human studies consistently showing an age-related NAD+ decline.
Insulin response in muscle
The 2021 NMN trial in prediabetic, postmenopausal women offers a narrower kind of progress. Over ten weeks, the study’s 250 mg/day oral NMN improved muscle insulin sensitivity: muscle responded better to the hormone that helps control blood sugar. Body composition and HbA1c, a longer-term blood-sugar measure, remained unchanged.
The split interests me. A more responsive muscle is a real physiological finding, while weight change and diabetes prevention remain separate questions. I’m comfortable calling this an encouraging muscle result. Calling it rejuvenation would lose nearly everything useful about it.
NAD+, NMN and nicotinamide riboside
NAD+ stands for nicotinamide adenine dinucleotide. It cycles with its partner form, NADH, in energy metabolism, and is consumed by enzymes involved in DNA repair, inflammation and gene regulation. The 2021 review of NAD+ metabolism describes competing demands from sirtuins, PARPs and CD38. That biology supplies a plausible reason to investigate the supply.
Most cells take up intact NAD+ poorly. Oral trials therefore commonly use precursors: NMN or nicotinamide riboside, shortened to NR, which feed the body’s NAD+ production. The substance swallowed matters when interpreting a result.
In the 2019 NR trial in healthy overweight adults, the tested 100, 300 and 1000 mg/day amounts over eight weeks raised blood NAD+ by 22%, 51% and 142%, respectively. The trial reported no flushing, no significant adverse-event difference from placebo, no LDL cholesterol rise and no disruption of one-carbon metabolism. Those are useful findings within that trial’s population and duration.
Capsules, infusions and safety
The controlled oral-precursor literature has more support than intravenous or injected NAD+. Infused NAD+ clears rapidly from plasma, and infusions run too quickly can cause chest or abdominal discomfort, flushing and nausea. A capsule result cannot establish an infusion’s effects.
The injectable contamination problem is already concrete: FDA issued a Class I recall, its most serious category, for a compounded NAD+ injection with elevated bacterial endotoxin. Compounded NAD+ injections have no FDA drug approval. Supplement content and purity also vary, and a label alone supplies no assurance of third-party testing.
FDA has taken the position that NMN is excluded from the dietary-supplement definition because it was investigated as a drug; the status has been contested. Another unresolved issue concerns existing cancers: NAD+ also supports their metabolism, so extra supply could help them in some circumstances. This is a theoretical, context-dependent concern, with no demonstrated conclusion here that supplementation causes cancer.
I’m enthusiastic about experiments that measure function over a useful span of time. Blood chemistry has already supplied an answer; daily life still has several questions open.