NAD flux is maintained in aged mice despite lower tissue concentrations.
NAD is an essential coenzyme for all living cells. NAD concentrations decline with age, but whether this reflects impaired production or accelerated consumption remains unclear. We employed isotope tracing and mass spectrometry to probe age-related changes in NAD metabolism across tissues. In aged mice, we observed modest tissue NAD depletion (median decrease ∼30%). Circulating NAD precursors were not significantly changed, and isotope tracing showed the unimpaired synthesis of nicotinamide from tryptophan. In most tissues of aged mice, turnover of the smaller tissue NAD pool was modestly faster such that absolute NAD biosynthetic flux was maintained, consistent with more active NAD-consuming enzymes. Calorie restriction partially mitigated age-associated NAD decline by decreasing consumption. Acute inflammatory stress induced by LPS decreased NAD by impairing synthesis in both young and aged mice. Thus, the decline in NAD with normal aging is relatively subtle and occurs despite maintained NAD production, likely due to increased consumption.