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.

Credit:Mike Harrington

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.