Devoting your free time to vegging out won't just burden your waistline -- it could cause you to age faster. In a study published in the "Archives of Internal Medicine," researchers discovered that adults who kept active during their leisure time were biologically younger than sedentary folks. The study measured biological age by focusing on white-blood-cell telomeres, chromosomal structures in the body that progressively shorten over time.

Working with DNA samples from 2,401 volunteers, researchers found that those who exercised an average of 199 minutes weekly had telomeres about the same length as those of sedentary people up to 10 years younger. The high levels of oxidative stress, inflammation, and mental stress that accompany a sedentary lifestyle may contribute to shortened telomere length, the authors say.

You don't need to hit the gym, says Elissa Epel, Ph.D., of the Osher Center for Integrative Medicine at the University of California, San Francisco. Activities like gardening, walking, or gentle yoga can improve fitness while lowering stress hormones thought to play a role in the shortening of telomeres.