What's the best exercise for you? Twins can provide an answer

If you start exercising and do not seem to be gaining as much endurance or strength as you had hoped, you might want to switch up your routine, according to a fascinating new study. The study, which involved twins, shows that almost everyone responds to the right exercise program, but the right exercise program can differ from person to person.

The findings also indicate that genetics may play less of a role than we imagine in how our bodies respond to workouts.

Recent research indicates that our physical responses can be idiosyncratic. After a few months of running, one person may gain considerable aerobic fitness, while another's endurance hardly budges. Ditto for weight training, in which some people build far more strength than others, even if they lift the same amount of weight.

Few studies, though, have delved into whether people's responses are monolithic — meaning someone who barely responds to one kind of exercise will gain little from other types of exertion — or supple, so that swapping routines should raise response.

The effect of genetics likewise has been uncertain. In some past studies, exercise response has seemed to run in families, with parents and siblings sharing similar physiological exercise gains or droughts, a potentially disheartening finding for anyone whose familial history suggests low response. But those studies tended to be epidemiological, not experimental; they asked about people's exercise histories but did not ask people actually to exercise while tracking whether and how their familial genetics affected responses.

So, for the new study, which was published in the Journal of Physiology, researchers at the University of Western Australia in Perth and the University of Melbourne studied both the variability and genetics of exercise response by asking pairs of twins to work out together.

Twins can be uniquely useful to science because identical pairs share 100% of their DNA, while fraternal twins share about 50%, like any siblings.

If twins try something new and the identical pairs react almost identically and the fraternal ones do not, then the intervention has a strong genetic component. If, on the other hand, everyone's reactions are mixed, then genetics is less foundational to these responses than lifestyle factors such as diet, education, weight, mental states and so on.

The researchers recruited 42 sets of young, healthy and sedentary twins in Perth, 30 of the pairs identical and 12 fraternal. (Several of the pairs believed they were fraternal, but genetic testing at the start of the study revealed they, in fact, were identical, to everyone's surprise.) Some participants were male, some female. The scientists measured the twins' endurance and leg strength and then directed them to start exercising.

Each of the sets of twins worked out together for two three-month periods. During one, they ran or cycled for an hour three times a week. During the other, they lifted weights three times a week. At the end of each three-month session, the twins' aerobic capacities and muscle power were checked. Then they compared how various twins' legs and lungs had changed during these exertions.

The results were telling. Most of the 84 participants increased their endurance during their three months of jogging and biking, but not all. So, too, leg strength rose in most of the twins after three months but stayed relatively puny in a few.

Almost no one, however, responded poorly to both the endurance and strength training. Those volunteers who gained little fitness bang from running substantially pumped up their strength after lifting, and vice versa.

"There were very few recalcitrant nonresponders," said Daniel Green, a professor of exercise science at the University of Western Australia, who conducted the study with doctoral students Channa Marsh and Hannah Thomas and others. "Almost all of those who failed to respond to one form of training were capable of gaining benefit by switching to the other."

Interestingly, the benefits gained could be broad. In some cases, nonresponders to aerobic exercise added endurance when they turned to weight training, even though lifting mostly targets muscles.

At the same time, there was little evidence that genes shaped outcomes, because the twins' responses varied wildly, even among identical pairs.

The upshot of this data is that we should not be discouraged if prolonged running or lifting does not produce the results we hope for. We might, in that case, want to noodle about with different approaches. "There is an optimal exercise strategy for everyone," Green said, "but it differs between people" and will not be determined or limited only by your DNA.