Warning, couch potatoes: resting on your laurels may be hazardous to your health, not to mention make you old before your time.

"A sedentary lifestyle increases the propensity to aging-related disease and premature death," researchers at King's College London report today in the journal Archives of Internal Medicine. "Inactivity may diminish life expectancy not only by predisposing to aging-related diseases but also because it may influence the aging process itself."

Researcher Lynn Cherkas and colleagues reached their conclusions by examining the genetic material extracted from blood samples of some 2,400 twins. They specifically studied the length of telomeres (repeated DNA sequences) on the ends of chromosomes in leukocytes (white blood cells); the protective caps are believed to be markers of biological aging, because they shrink over time.

Their findings: the telomeres of subjects who exercised the most (an average of 199 minutes weekly) were longer than those of volunteers who worked out the least (a mere 16 minutes or less a week). The discrepancy was enough, researchers wrote, to suggest that the exercise mavens were on average as much as a decade biologically younger than the slackers.

"Such a relationship between leukocyte telomere length and physical activity level remained significant after adjustment for body mass index, smoking, socioeconomic status and physical activity at work," the authors report. "The mean difference in leukocyte telomere length between the most active and least active subjects was 200 nucleotides (chemical structural units of DNA and RNA), which means that the most active subjects had telomeres the same length as sedentary individuals up to 10 years younger, on average."

The scientists speculate that stress, inflammation and oxidative stress (cell damage caused by oxygen exposure) may be responsible for shortened telomeres in physically inactive people. Exercise is among the factors found to help alleviate stress. Previous research has linked regular workouts to lower rates of cardiovascular disease, type 2 diabetes, cancer, high blood pressure, obesity and osteoporosis.

The researchers note that their findings support U.S. guidelines calling for individuals to exercise moderately for 30 minutes at least five days a week. "Our results. . . show that adults who partake in regular physical activity are biologically younger than sedentary individuals," they say. "This conclusion provides a powerful message that could be used by clinicians to promote the potential anti-aging effect of regular exercise."