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One Twin Exercises, the Other Doesn’t


Identical twins in Finland who shared the same sports and other physical activities as youngsters but different exercise habits as adults soon developed quite different bodies and brains, according to a fascinating new study that highlights the extent to which exercise shapes our health, even in people who have identical genes and nurturing.

Determining the precise, long-term effects of exercise is surprisingly difficult. Most large-scale exercise studies rely on questionnaires or interviews and medical records to establish the role of exercise. But these epidemiological studies, while important and persuasive, cannot prove that exercise causes health changes, only that people who exercise tend to be healthier than those who do not.

To prove that exercise directly causes a change in people’s bodies, scientists must mount randomized controlled trials, during which one group of people works out while a control group does not. But these experiments are complicated and costly and, even in the best circumstances, cannot control for volunteers’ genetics and backgrounds.

And genetics and upbringing matter when it comes to exercise. Genes affect our innate endurance capacity, how well we respond to different types of exercise, and whether we enjoy working out at all. Childhood environment also influences all of this, muddying the results of even well-conducted exercise experiments.

All of this makes identical twins so valuable. By definition, these

pairs have the same DNA. If they were raised in the same household, they also had similar upbringing. So they can provide a way to study the effects of changes in lifestyle among people with the same genes and pasts.

Some past studies had found that older identical twins whose workout habits had diverged over the years tended to age differently, with greater risks of poor health and early death among the sedentary twin.

But no studies had looked at young twins and the impacts of different exercise routines on their health. So for the new study, which was published this month in Medicine & Science in Sports & Exercise, researchers at the University of Jyvaskyla and other institutions in Finland turned to that country’s extensive FinnTwin16 database, which contained twins’ answers to questionnaires about their health and medical conditions, beginning when the pairs were 16 and repeated every few years afterward.

The researchers were looking for young adult identical twins in their early- to mid-20s whose exercise habits had substantially diverged after they had left their childhood homes. These twins were not easy to find. Most of the pairs had maintained remarkably similar exercise routines, despite living apart.

But eventually the researchers homed in on 10 pairs of male identical twins, one of whom regularly exercised, while the other did not, usually because of work or family pressures, the researchers determined.