What sirtuins are and how NAD+ activates them
Sirtuins are a family of enzymes (SIRT1 through SIRT7 in mammals) that regulate cellular processes by removing chemical groups from proteins, most notably acting as NAD+-dependent deacetylases. Their activity depends on the coenzyme NAD+ (nicotinamide adenine dinucleotide), which links sirtuin function to the cell's metabolic and energy state. When NAD+ is plentiful, as during fasting or energy stress, sirtuin activity tends to rise.
Through this NAD+ dependence, sirtuins influence gene expression, DNA repair, mitochondrial biogenesis, inflammation, and glucose and lipid metabolism.
The link to longevity and important caveats
Sirtuins gained attention in aging research because increasing their activity extended lifespan in model organisms such as yeast, worms, and flies, and they help mediate some benefits of caloric restriction. They are studied as part of pathways involving mitochondrial health, DNA repair, and metabolic homeostasis that may support healthspan.
Evidence in humans is far less established. Much of the strongest data comes from non-human models, results have at times been debated, and supplements marketed to raise NAD+ or activate sirtuins are not proven to extend human lifespan. These remain active research areas rather than settled clinical facts.