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Nicotinamide adenine dinucleotide (NAD+) extends longevity in experimental organisms, raising interest in its impact on human health. De novo NAD+biosynthesis from tryptophan is evolutionarily conserved yet considered supplanted among higher species by biosynthesis from nicotinamide (NAM). Here we show that a bottleneck enzyme in de novo biosynthesis, quinolinate phosphoribosyltransferase (QPRT), defends renal NAD+and mediates resistance to acute kidney injury (AKI). Following murine AKI, renal NAD+fell, quinolinate rose, and QPRT declined. QPRT+/−mice exhibited higher quinolinate, lower NAD+, and higher AKI susceptibility. Metabolomics suggested an elevated urinary quinolinate/tryptophan ratio (uQ/T) as an indicator of reduced QPRT. Elevated uQ/T predicted AKI and other adverse outcomes in critically ill patients. A phase 1 placebo-controlled study of oral NAM demonstrated a dose-related increase in circulating NAD+metabolites. NAM was well tolerated and was associated with less AKI. Therefore, impaired NAD+biosynthesis may be a feature of high-risk hospitalizations for which NAD+augmentation could be beneficial.