I got an email from a reader yesterday aboutan article I wrote a few months ago on the benefits of exercise for preventing or delaying the progression of Alzheimer's disease. He was wondering, essentially, about the problem of cause and effect. Yes, people who exercise more get Alzheimer's less. But is that because exercise prevents Alzheimer's? Or is it because some people are lucky enough to be born with the genetic predisposition to be healthy—a fact that makes them more likely to run and less likely to get Alzheimer's?
You could ask the same question about virtually all the apparent benefits of exercise, and it's an important question to ask. As the reader put it: "[M]aybe the answer is that they are healthy because they are naturally so, and in some way it is the exercise that confirms that they are healthy. Rather than, since they exercise, they are healthy."
There's undoubtedly some truth to this, and it's a very tricky problem in epidemiological studies. But there are also some ways to get around the problem. Just this month, Finnish researchers published a lovely example in Medicine & Science in Sports & Exercise, using identical twins with different exercise habits to separate the effects of genetic endowment from the direct effects of exercise. Gretchen Reynolds had a nice description of the study in the New York Times last week; here are a few additional details.
The biggest challenge was finding pairs of twins with dramatically different exercise habits: with similar genetics and similar upbringings, most twins end up with similar exercise habits. Thanks to an extensive database of Finnish twins, the researchers were able to locate 10 pairs of twins who were similar in almost every way—including, crucially, diet—but different in exercise. One of the interesting nuggets is that their ratings of "exercise enjoyment" on a five-point Likert scale were almost the same: 3.9 for the non-exercising twin and 4.2 for the exercisers, a non-significant difference. So it's not that one twin liked to exercise and the other didn't—instead, the non-exercising twins told researchers that work or family pressures made it hard for them to exercise.
So what were the results? The twins were between 32 and 36 years old, and had different levels of physical activity for the previous three or more years. And already there were significant differences in body fat percentage and waist-to-hip ratio; in glucose and insulin sensitivity; in aerobic fitness (VO2max of 43.6 vs. 37.3 ml/kg/min); and even in the volume of gray matter in certain regions of the brain.
It's hard to think of a more compelling demonstration of the fact that (a) exercise matters, and (b) genes are not destiny. The trickier question is: how do you convince the non-exercising twins (and, by extension, everyone else who struggles to find time or motivation for physical activity) to change their ways?