NAD+ and Nicotinamide Riboside: Physiology, Decline, and What Human Trials Show

What NAD+ actually does

Nicotinamide adenine dinucleotide (NAD+) is a coenzyme present in every living cell. Its best-known job is as an electron carrier: NAD+ accepts electrons during glycolysis, fatty acid oxidation, and the citric acid cycle, becoming NADH, which then donates those electrons to the mitochondrial electron transport chain to generate ATP. Without an adequate NAD+/NADH ratio, oxidative metabolism slows.

Beyond redox chemistry, NAD+ is consumed as a substrate — not merely recycled — by three families of enzymes: sirtuins (SIRT1–7), which regulate mitochondrial biogenesis, inflammation, and metabolic adaptation; PARPs, which are activated by DNA strand breaks and coordinate DNA repair; and CD38, an ectoenzyme involved in immune signaling and calcium mobilization. Because these enzymes cleave NAD+, cellular demand for continuous NAD+ resynthesis is high.

How NAD+ is made — and why NR matters

Cells generate NAD+ through three routes: de novo synthesis from tryptophan, the Preiss-Handler pathway from nicotinic acid, and the salvage pathway, which recycles nicotinamide back into NAD+ via nicotinamide phosphoribosyltransferase (NAMPT). The salvage pathway supplies the majority of NAD+ in most human tissues, and NAMPT is its rate-limiting step.

Nicotinamide riboside (NR) is a pyridine-nucleoside form of vitamin B3 that enters the salvage pathway downstream of NAMPT. Nicotinamide riboside kinases (NRK1 and NRK2) phosphorylate NR to nicotinamide mononucleotide (NMN), which is then adenylylated to NAD+. Bypassing the rate-limiting NAMPT step is the mechanistic rationale for using NR rather than plain nicotinamide to raise NAD+.

The age-related decline

Tissue NAD+ concentrations fall with age across multiple species and in human tissue samples, including skeletal muscle, liver, brain, and skin. The decline appears to be driven by both reduced synthesis and increased consumption. A leading contributor is age-associated inflammation, which upregulates CD38 in immune and stromal cells; CD38 is a highly efficient NAD+-consuming enzyme. Chronic DNA damage also increases PARP activity, further draining the pool.

Lower NAD+ availability constrains sirtuin activity, which in turn is associated with reduced mitochondrial biogenesis, impaired oxidative capacity, and blunted stress resistance. This chain of reasoning — falling NAD+, reduced sirtuin signaling, mitochondrial dysfunction — is why NAD+ repletion has become one of the most studied interventions in geroscience.

What randomized human trials show

The most consistent finding is pharmacokinetic: NR reliably and dose-dependently raises blood NAD+. In a randomized, double-blind, placebo-controlled trial in healthy middle-aged and older adults, six weeks of NR at 1,000 mg per day roughly doubled whole-blood NAD+ concentrations, was well tolerated, and produced no serious adverse events (Martens et al., Nature Communications, 2018). That trial also reported a modest reduction in systolic blood pressure and aortic stiffness in participants with elevated baseline values — a secondary, exploratory outcome that has not yet been confirmed in a dedicated trial.

In aged human skeletal muscle, 21 days of NR at 1,000 mg per day increased the muscle NAD+ metabolome and downregulated markers of muscle inflammation, but did not measurably improve mitochondrial bioenergetics or physical performance over that short window (Elhassan et al., Cell Reports, 2019). This is the central nuance of the current literature: raising NAD+ is straightforward, while demonstrating downstream functional benefit is not.

Safety data are reassuring at studied doses. Trials up to 2,000 mg per day for 12 weeks report tolerability comparable to placebo, with mild gastrointestinal complaints being the most common side effect. Long-term safety beyond roughly one year has not been established, and NR has not been studied in pregnancy, in active cancer, or in children.

How we read the evidence

The physiology is well characterized and the biomarker effect is real: NR raises NAD+ in humans. What remains unproven is whether that translates into clinically meaningful improvements in strength, endurance, cognition, or healthspan. Most positive functional findings so far come from animal models or from secondary endpoints in small human trials.

For anyone considering NR, the reasonable framing is a well-tolerated intervention with strong mechanistic rationale and immature outcome data. Doses used in trials cluster between 250 and 1,000 mg per day. Foundational inputs with far stronger outcome evidence — resistance training, aerobic volume, sleep, and protein intake — also support mitochondrial function and should not be displaced by a supplement.