The NAD+ decline curve

Nicotinamide adenine dinucleotide (NAD+) is a coenzyme that every living cell uses to run the redox reactions of metabolism and to fuel a family of enzymes — sirtuins, PARPs, and CD38 — that govern DNA repair, gene expression, and inflammation. Tissue NAD+ levels fall with age in humans and in every animal model studied so far, and that decline has become one of the more durable observations in aging biology.

Direct measurements in human skin show NAD+ concentrations roughly halve between the third and seventh decades of life. Studies in skeletal muscle, brain, liver, and plasma show similar though tissue-specific declines. The drop is not linear; it accelerates in mid-life, which mirrors the timing of many age-related metabolic changes.

Two mechanisms appear to drive most of the loss. Synthesis from precursors (the salvage pathway) becomes less efficient with age. And consumption rises — particularly through CD38, an NAD-degrading enzyme whose expression climbs in aged tissues and in chronic inflammation.

The best-studied precursors are nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN). Both are converted to NAD+ inside the cell. Multiple randomized human trials have shown that oral NR and oral NMN raise blood NAD+ levels in a dose-dependent way and are well tolerated over weeks to months.

Functional outcomes are less settled. Some trials report improvements in muscle insulin sensitivity, walking distance in older adults, or markers of vascular function; others show NAD+ rises without clear functional change. The field is converging on the view that raising NAD+ is necessary but probably not sufficient — context, tissue, and baseline status all matter.

Estrogen interacts with NAD+ metabolism through several routes, including effects on sirtuin activity and mitochondrial biogenesis. The perimenopausal transition is one of the periods of steepest metabolic change in the female lifespan, and several research groups are now specifically studying NAD+ precursors in women across that transition rather than extrapolating from mixed-sex or male-dominated trials.