If working out makes you feel younger, a new study suggests it's no illusion- vigorous exercise can actually slow the aging process on a cellular level, turning back the clock nearly a decade. Researchers analyzed 6,000 adults based on their physical activity and biological markers of aging, Time.com reports. Most importantly, they used DNA samples to measure the length of participants' telomeres, protein caps that protect chromosomes, like the plastic tips of shoelaces. Telomeres shrink with age- we lose bits of them every time a cell divides. "in general, people with shorter telomeres die sooner and are more likely to develop many of our chronic diseases," says study author Larry Tucker. Taking into account risk factors such as smoking, alcohol consumption, and obesity, the researchers found people who exercised strenuously- say, running for 30-40 minutes five days per week- had longer telomeres. That gave them about a nine-year "biological aging advantage" over sedentary adults; those who exercised more moderately had a two-year edge. The researchers speculate physical activity could help preserve telomeres by reducing stress and inflammation. "we all know people who seem younger than their actual age, " Tucker says. "Exercise can help with that, and now we know that part of that may be because of its effect on our telomeres."