There are also natural compounds that elevate sirtuins—one is resveratrol, which is already sold as a dietary supplement today. Another is called NAD. NAD—Nicotinamide adenine dinucleotide—is one of the most compelling bits of chemistry related to aging. Its presence in the body is directly correlated with the passage of time: An elderly man will have about half the levels of NAD is his body as a young person. There’s no amount of healthy eating or exercise that can stop the decline. But in a scientific