Gretchen Reynolds, in here weekly PHYS ED column, wrote Internally Fit: How Exercising Keeps Your Cells Young http://well.blogs.nytimes.com/2010/01/27/phys-ed-how-exercising-keeps-your-cells-young/#comments.
She wrote about a study where four groups of people were studied: sedentary people in their 20s, sedentary people middle aged and over, and runners of the same two age groups. Telomere length was measured in each. Telomeres are ends of DNA that are believed to grow shorter as a cell nears its death. The length of two groups in their 20s was about the same length. But the telomere length of the older runner, fit groups was 40% shorter than that of the sedentary YOUNGER group, and only about 10% shorter than those of the fit younger group.
More studies need to be done to determine how exercise affects teleomere length and exactly what this means. Dr. Werner, who performed the study, said that exercise "at the molecular level has an anti-aging effect".
Disclaimer: None of the above information can be taken as a substitute for advice from a medical professional such as a physician.
My third book, Pocket Guide to Fitness, is available on www.louizapatsis.com, http://www.authorhouse.com, www.bn.com and http://www.amazon.com. If you look up my name on those Web sites, you will find my other books The Boy in a Wheelchair and Life, Work and Play: Poems and Short Stories.