Aerobic activities like jogging and interval training can make our cells biologically younger, according to a noteworthy new experiment. Weight training may not have the same effect, the study found, raising questions about how various types of exercise affect us at a microscopic level and whether the differences should influence how we choose to move.

There is mounting and rousing evidence that being physically active affects how we age, with older people who exercise typically being healthier, more fit, better muscled and less likely to develop a variety of diseases and disabilities. But precisely how, at an interior, molecular level, exercise might be keeping us youthful has not been altogether clear. Past studies have shown that exercise alters the workings of many genes, as well as the immune system, muscle-repair mechanisms and many other systems within the body.

Some researchers have speculated that the most pervasive anti-aging effects of exercise may occur at the tips of our chromosomes, which are capped with tiny bits of matter known as telomeres. Telomeres seem to protect our DNA from damage during cell division but, unfortunately, shorten and fray as a cell ages. At some point, they no longer safeguard our DNA, and the cell becomes frail and inactive or dies.

Many scientists believe that telomere length is a useful measure of a cell’s functional age. But researchers have also found that telomeres are mutable. They can be lengthened or shortened by lifestyle, including exercise. A 2009 study, for instance, found that middle-age competitive runners tended to have much longer telomeres than inactive people of the same age. Their telomeres were almost as lengthy of those of healthy, young people. But that study was associational; it showed only that older people who ran also were people with extended telomeres, not that the exercise necessarily caused that condition.

For the new study, which was published in the European Heart Journal, researchers recruited 124 middle-age 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 (usually used in studies of telomeres because they are 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. Others started a supervised programme of brisk walking or jogging for 45 minutes three times a week, or a thrice-weekly, high-intensity interval programme 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 three times a week.

After six months. 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 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.

These results would seem to indicate that exercise needs to be aerobically taxing to extend telomeres and slow cellular-level aging, said Dr Christian Werner, a cardiologist and researcher at the University of Saarland in Germany, who led the new study.

But the findings do not indicate that weight training does not combat aging, Werner said. Like the other workouts, it improved people’s fitness, he said, which is one of the most important indicators of longevity.

He and his colleagues would like to study the cellular effects of various combinations of endurance and strength training. But for now, the message of the new study, he said, is that exercise of any kind may change the nature of aging, even for people who are already middle-aged. “It is not too late,” he said, “to keep your cells young.