NAD+ is required for functioning of sirtuins (proteins that contribute to longevity by maintaining the length of critical telomeres) Telomeres are stretches of repetitive DNA strands that cap the ends of chromosomes. Every time the cell replicates, the telomeres at the end of that DNA strand shorten. Once telomeres reach a critical short length, cell renewal virtually halts, leading to accelerated aging or death of the cell.  Telomere shortening is both a marker of cellular aging and a predictor of shortened lifespan