The activitystat hypothesis: do we have an exercise set point?

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If you do a vigorous workout in the morning, will you be correspondingly less active for the rest of the day, so that your total physical activity ends up being the same as if you hadn’t worked out at all? That’s the basic gist of the “activitystat” hypothesis, which Gretchen Reynolds described in a New York Times article last week (thanks to Ed for the heads-up!). It’s also the topic of a pro vs. con [EDIT: had the links backward before -AH] debate in the current issue of the International Journal of Obesity (full text free available).

Reynolds describes several interesting studies that line up in favour of or against the theory, including one (from the same issue of IJO) that compared three British elementary schools with very different amounts of in-school physical activity. Here’s what that study found:

You can see that, for both “total physical activity” and “moderate and vigorous physical activity,” one group had much higher levels in school than the other two, but compensated by doing less outside school. On the surface, it seems like a pretty compelling argument in favour of the activitystat hypothesis.

My take: somewhere in the middle, as usual. It would be ludicrous to claim that the body doesn’t regulate physical activity based on previous exertions to some degree. Do a one-day study of “voluntary movement” among people who have run a marathon that morning, and of course you’re going to find that they chill out more. At the opposite extreme, it would be equally silly to argue that all people everywhere in the world do exactly the same amount of physical activity. Or that any given person’s physical activity stays essentially constant over long periods of time  — again, think of someone who goes from sedentary to marathon training: no amount of fidgeting or taking the stairs will add up to the exertions of 100-mile weeks. (For more examples of the role of environment in determining activity level, read the “con” commentary I linked to above. E.g. Nandi children in Kenya who grow up in the countryside are more active overall than Nandi city kids — an obvious result, but one that clashes with the activitystat idea.)

So the relevant question isn’t “Do compensatory mechanisms exist?” It’s “Do they matter, and are they insurmountable?” As lovely as the data from the British school study is, I don’t find it convincing. The school with the highest in-school physical activity was a fancy boarding school in the countryside, while the other two schools were urban. If the boarding-school kids play an hour of cricket in phys ed every day, the fact that they don’t choose to go play an hour after school doesn’t necessarily mean that the activitystat is limiting them. Maybe they just want to (or have to, depending on the other extracurricular requirements of the school) do something else.

One final point: it would be interesting to stratify those results based on the activity levels of the kids. Does the apparent activitystat mechanism apply equally to the most active and least active kids? Because if there are some kids who, left to their own devices, only get a total of 50 minutes of moderate/vigorous activity per week, then giving them 100 minutes a week in school is going to benefit them — and there’s nothing any activitystat can do to stop it!