A dietary supplement protects aging muscle The oxidized form of cellular nicotinamide adenine dinucleotide (NAD ⁺ ) is critical for mitochondrial function, and its supplementation can lead to increased longevity. Zhang et al. found that feeding the NAD ⁺ precursor nicotinamide riboside (NR) to aging mice protected them from muscle degeneration (see the Perspective by Guarente). NR treatment enhanced muscle function and also protected mice from the loss of muscle stem cells. The treatment was similarly protective of neural and melanocyte stem cells, which may have contributed to the extended life span of the NR-treated animals. Science , this issue p. 1436 ; see also p. 1396