NAD+: The Coenzyme at the Center of the Aging-Metabolism Research
Not a peptide, a coenzyme
It's worth saying plainly: NAD+ (nicotinamide adenine dinucleotide) is not a peptide. It's a small dinucleotide coenzyme present in every living cell, and it does two distinct kinds of work that make it interesting to longevity research.
First, the classic role: NAD+ is a redox carrier. It shuttles electrons through the reactions that turn food into ATP, glycolysis, the citric acid cycle, oxidative phosphorylation. Without it, cellular energy metabolism stops.
Second, and the reason it shows up in aging journals: NAD+ is a consumed substrate for a set of regulatory enzymes, the sirtuins, the PARPs (DNA-repair enzymes), and CD38. These don't just borrow NAD+ the way redox reactions do; they break it down to do their jobs. That makes the cell's NAD+ pool a shared currency that energy metabolism, DNA repair, and gene regulation all draw on.
The observation that started the field
NAD+ levels decline with age across many tissues. Because sirtuins and PARPs depend on NAD+, the hypothesis that has driven a decade of work is whether that decline contributes to age-related cellular dysfunction, and whether restoring NAD+ through precursors could counteract it. The influential framing of this argument appears in a *Science* review (Verdin, 2015), and the mechanistic detail, how NAD+ is made, consumed, and recycled, and what goes wrong with age, is laid out in a comprehensive *Nature Reviews Molecular Cell Biology* review (Covarrubias et al., 2021).
Where the evidence is firm and where it isn't
The biochemistry is settled: NAD+ is essential, it's consumed by sirtuins and PARPs, and it falls with age. The open question, and it is genuinely open, is the causal one. A correlation between declining NAD+ and aging is not the same as showing that raising NAD+ changes aging outcomes, and the human interventional data remains an active, unfinished area. The most accurate summary of the field is "promising mechanism, real biology, unproven endpoints."
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