n his laptop computer, biology professor Leonard Guarente plays a video clip of 29-month-old mice hobbling around a cedar-chip-filled cage. They?re scruffy, fat, slow moving, and over the hill by rodent standards. Then he plays a clip of another group of 29-month-old mice. They?re svelte, frisky, and scrambling around like adolescents. What?s their secret? These mice have eaten about two-thirds as many calories as their portly peers. Not only does the meager diet seem to keep them light in the limbs, but they tend to live 30 percent longer than their well-fed friends and are less likely to contract age-related diseases, such as diabetes and cancer.