From Today’s New York Times (Is Aerobic Exercise the Key to Successful Aging? - The New York Times):
… researchers began by recruiting 124 middle-aged men and women who were healthy but did not exercise. They determined everyone’s aerobic fitness and drew blood to measure telomere length in their white blood cells (which usually are used in studies of telomeres, because they are so readily accessible). They also checked blood markers of the amount and activity of each person’s telomerase, an enzyme that is known to influence telomere length.
Then some of the volunteers randomly were assigned to continue with their normal lives as a control or to start exercising.
Others started a supervised program of brisk walking or jogging for 45 minutes three times a week, or a thrice-weekly, high-intensity interval program consisting of four minutes of strenuous exercise followed by four minutes of rest, with the sequence repeated four times.
The final group took up weight training, completing a circuit of resistance exercises three times a week.
Researchers monitored people’s heart rates during their workouts, and the exercisers continued their programs for six months. Afterward, everyone returned to the lab, where the scientists again tested fitness and drew blood.
At this point, the volunteers who had exercised in any way were more aerobically fit.
There were sizable differences, however, between the groups at a molecular level. Those men and women who had jogged or completed intervals had much longer telomeres in their white blood cells now than at the start, and more telomerase activity. The weight trainers did not. Their telomeres resembled those of people in the control group, having remained about the same or, in some instances, shortened during the six months.
These results would seem to indicate that exercise needs to be aerobically taxing to extend telomeres and slow cellular-level aging, says Dr. Christian Werner, a cardiologist and researcher at the University of Saarland in Germany, who led the new study.