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Phys Ed Gretchen Reynolds on the science of fitness.

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.

The dissimilarities in their exercise routines had mostly begun within the past three years, according to their questionnaires.

The scientists invited these twins into the lab and measured each young man’s endurance capacity, body composition and insulin sensitivity, to determine their fitness and metabolic health. The scientists also scanned each twin’s brain.

Then they compared the twins’ results.

It turned out that these genetically identical twins looked surprisingly different beneath the skin and skull. The sedentary twins had lower endurance capacities, higher body fat percentages, and signs of insulin resistance, signaling the onset of metabolic problems. (Interestingly, the twins tended to have very similar diets, whatever their workout routines, so food choices were unlikely to have contributed to health differences.)

The twins’ brains also were unalike. The active twins had significantly more grey matter than the sedentary twins, especially in areas of the brain involved in motor control and coordination.

Presumably, all of these differences in the young men’s bodies and brains had developed during their few, brief years of divergent workouts, underscoring how rapidly and robustly exercising — or not — can affect health, said Dr. Urho Kujala, a professor of sports and exercise medicine at the University of Jyvaskyla who oversaw the study.

Of course, the study was small and not a formal randomized trial, although it involved identical twins.

But Dr. Kujala said he believes that the results strongly imply that the differences in the twin’s exercise habits caused the differences in their bodies.

More subtly, the findings also point out that genetics and environment “do not have to be” destiny when it comes to exercise habits, Dr. Kujala said. For these particular twins, whether their genes and childhoods nudged them toward exercising regularly or slumping on the couch, one of the pair overcame that legacy and did the opposite (for better and worse).

The rest of us can do likewise, Dr. Kujala said. Even if the input from our DNA and upbringing urges us to skip the gym, we can “move more,” he said, and, based on this study, rapidly and substantially improve the condition of our bodies and brains.

Related: To Jump-Start Your Exercise Routine, Be Mindful

Ask Well: Making Exercise a Habit



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