NAD+Nicotinamide adenine dinucleotide
A coenzyme central to energy metabolism and DNA repair, which declines with age.
NAD, nicotinamide adenine dinucleotide, is a coenzyme present in every cell and central to energy metabolism. It shuttles electrons between reactions, which is how the breakdown of fuel is coupled to the production of ATP.
It has a second role beyond metabolism. NAD is consumed by enzymes involved in DNA repair and in the sirtuin family associated with ageing biology, which is why declining NAD with age became one of the more prominent theories in the longevity field.
That theory drove a large supplement market built on precursors, chiefly nicotinamide riboside and nicotinamide mononucleotide, which the body converts into NAD.
The precursors do what they claim biochemically. Trials reliably show they raise blood NAD levels, and that part of the marketing is accurate.
What has not followed is benefit to the person. Systematic reviews of NMN and NR find no significant improvement in glucose, insulin, HbA1c, lipids or muscle outcomes across mostly short trials. The NAD number moves and the outcomes do not, which is a textbook surrogate endpoint problem.
That does not make the biology wrong, only unproven in humans so far. Longer trials in older or metabolically impaired populations are ongoing, and it is entirely possible that a benefit appears where current trials have not looked.
In practice, this is a category to watch rather than to buy. Exercise raises NAD too, alongside everything else it does, and has outcome evidence the supplements currently lack.