The study: Led by Lynn F. Cherkas, the team studied 2,401 white twins, administering questionnaires on physical activity level, smoking habits and socioeconomic status. The participants also provided a blood sample from which DNA was extracted. The researchers examined the length of telomeres--repeated sequences at the end of chromosomes--in the twins' white blood cells (leukocytes). Leukocyte telomeres progressively shorten over time and may serve as a marker of biological age. Telomere length decreases as we age; on average we lose 21 nucleotides (or structural units) a year. The longer the telomere, the younger the cell.

The results: The British team found that men and women who are sedentary have far shorter leukocyte telomeres than those who were more active, even after they adjusted for such factors as smoking, body mass index, socioeconomic status and physical activity at work. Specifically, those who exercised the most (more than three hours a week) had telomeres that were, on average, 200 nucleotides longer than those who exercised the least (16 minutes a week).

Translation: The twin who exercised was biologically 10 years younger than the twin who was sedentary! Yes, a treadmill work-out may very well be the fountain of youth.

So why does a sedentary lifestyle age us? It is thought the telomeres are shortened by oxidative stress, which is damage caused to cells by exposure to oxygen, as well as inflammation and stress. But regular physical activity seems to reduce psychological stress, which mitigates its effect on telomeres and the aging process.

The study findings were published in the Archives of Internal Medicine, one of the JAMA/Archives journals.