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. [Display omitted] •NAD+ pool size decreased in aged tissues without a change in circulating precursors•Whole-body synthesis of NAD+ is not impaired in aged mice•Maintained NAD+ turnover despite lower concentration indicates higher consumer activity•Acute inflammatory stress impairs NAD+ synthesis NAD+ is an essential redox cofactor and a substrate for signaling enzymes that influence many aspects of cellular physiology. NAD+ concentration falls with age in some tissues, which has been attributed to decreased synthesis or increased consumption. Here, we provide the first direct measurements of changes in NAD+ turnover flux with age, finding that synthesis is maintained despite the lower NAD+ concentrations in many aged tissues. We further characterize the effects of caloric restriction and inflammatory stress on NAD+ turnover.