The evidence that NAD+ declines with age and is biochemically essential stands on solid ground. That NMN and NR raise NAD+ levels in humans has been demonstrated in small trials. But whether that rise actually leads to less disease or slower ageing in humans has not yet been sufficiently studied. The most promising human data exist for NMN, but the studies are small and short in duration, and long-term safety is unknown.
NAD+ is a molecule found in every cell of our body and is indispensable for energy metabolism. But it does more than just supply energy: it is also a required raw material for proteins that repair DNA (such as PARP), carry out epigenetic regulation (sirtuins), and control inflammatory responses (CD38). Without sufficient NAD+, these systems do not function properly. This biochemical foundation is solidly supported by evidence.
As we age, NAD+ levels gradually decline in multiple tissues, including skin, blood, muscle, liver, and brain. This pattern has been consistently described in both mice and humans. Scientists suspect that this decline contributes to age-related phenomena such as muscle loss, cognitive decline, and metabolic diseases, but hard cause-and-effect evidence in humans is still limited. The most compelling data on what a deficiency of NAD+ actually causes come from cell and animal studies.
NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside) are two widely used precursors of NAD+, and both demonstrably raise NAD+ levels. NMN has been tested in small, short-duration clinical trials in humans and shows a measurable increase in the blood. Safety appears acceptable so far. NR has been extensively studied in mice and protected against metabolic problems through activation of sirtuins and improved mitochondrial function. In humans, the translation of those animal studies into real health benefits has not yet been demonstrated.
NAD+ metabolism is also disrupted in heart failure. An unfavourable ratio between NAD+ and its breakdown product NADH contributes to reduced energy production in the heart muscle and epigenetic changes. NAD+ is, however, one of several factors here, and a direct causal role has not yet been proven.
The major caveat is this: in animal models the results of NAD+ supplementation are impressive, but those results have not yet been confirmed in humans. The available clinical studies are small and short in duration, and too little is known about long-term safety. Scientists themselves are explicitly cautious about this. For anyone considering a NAD+ precursor, NMN currently has the most human data (however limited). But long-term health benefits have not yet been proven in humans.
Based on multiple review articles and clinical trials (PMID 33353981, 32694684, 37619764, 33028824, 28648096, 26118927, 22682224, 33983836). The mechanistic basis is strongly supported. Human intervention data are limited to small, short-duration trials.