Donald Ingram at the National Institute of Aging investigates the effects of a lean diet (which is 30 percent reduced in calories) on monkeys. It’s too soon to tell if the animals will live longer; the study began in 1987, and the monkeys typically live for 40 years, but so far, the animals do seem to be somewhat healthier. “But we just don’t know enough yet about how much longer large animals might live on a calorie-restricted diet,” Ingram said.