Does exercising at one point during the day make you less active the rest of the time?
The question of whether humans have an innate set point for movement, a so-called activitystat, is of increasing interest and controversy among scientists. One of them is Dr. Terence J. Wilkin, a professor of endocrinology at the Peninsula Medical School in Plymouth, England, who asked himself that question a few years ago while hoping to learn more about the interplay of activity and childhood obesity.
Dr. Wilkin had outfitted about 70 children at three wildly different English elementary schools with an accelerometer, an electronic device that records almost all movement. One of the schools, a private college-preparatory academy with acres of playing fields, required an average of 9.2 hours of physical education classes each week. Another was a village public school, equipped with outdoor facilities and an established sports tradition, but requiring only 2.2 hours of P.E. each week. And the final was an urban school with limited playground options and 1.6 hours a week of P.E. The children wore the devices full time for a week on four separate occasions during the school year.
Dr. Wilkin had expected that the children at the prep school, who spent about 65 percent more time exercising at school than the other students, would be much more active over all. But they weren’t. In fact, when he collated the data, the weekly activity levels of the students from all three schools were remarkably similar. Students who exercised more at school were less active afterward. In a study published this month in The International Journal of Obesity, Dr. Wilkin and his co-authors conclude that, at least in these 8- to 10-year-olds, “activity at one time is met with less activity at another.” The findings, they say, may help to explain why so many children remain overweight, despite programs designed to get them moving.
A similar mechanism may hold for adults. In another notable experiment published this month in the journal Menopause, a group of postmenopausal women completed a 13-week walking program while wearing accelerometers to measure their full daily activity. During that time, some of the women were more active over all than they had been at the start. But almost half had reduced their spontaneous physical activity when they weren’t exercising. The reductions weren’t intentional: The women hadn’t consciously set out to move less. But, as a result, they were no more active, on a daily basis, than they had been before starting the exercise regimen. Their bodies had compensated for the walking and kept their overall energy expenditure about the same.
The implications of such findings are broad and worrisome. “The evidence to date shows that physical activity interventions have not” been able to significantly reduce childhood obesity, Dr. Wilkin says, “and our data suggest that part of the reason” may be that children who exercise at school expend less energy the rest of the time. The same dynamic could be impeding adults’ efforts to use exercise to trim away flab.
In animal studies, rodents bred over generations to voluntarily run for hours will, if deprived of their wheels, race around their cages until they’ve fulfilled their bodies’ seeming imperative for motion, while animals bred to be languorous and avoid activity will, if forced to swim or run, subsequently lie on their cage floors and not move for hours. They are not merely tired, Dr. Wilkin says, but obeying some inner physiological command. The animals seem to have a “genetically determined level of preferred energy expenditure,” he says, to which their bodies default.
But other researchers are not convinced. “Twin studies show that the environment, defined broadly as the physical and cultural environment, has a massive influence on the level of physical activity,” at least in children, says John J. Reilly, a professor of pediatric energy metabolism at the University of Strathclyde in Scotland and the author of a commentary accompanying Dr. Wilkin’s study. Children’s physical activity is determined largely by their living conditions — in other words, not their biology.
In confirmation of that idea, a study of 9- to 12-year-old British twins published last year determined that, while the children’s fidgetiness and enjoyment of activity were dependent on heredity, their actual levels of movement were almost wholly determined by their environment, and in particular by the actions and attitudes of their teachers and parents.
An equally powerful argument against the existence of an activitystat may derive from the findings of studies that reduce people’s habitual activity for a period of time. Presumably, if the body has a preset, preferred amount of energy expenditure, those people should become more active afterward. But in general, they do not. A representative recent study of schoolchildren found that, on days when they were denied recess, they “did not compensate” by running around more after school. They simply expended less energy that day.
Still, almost all researchers agree that science is not close to fully understanding the complex interplay of biology, volition, laziness and modern living conditions in determining how active each of us will be. “Far more work is needed,” Dr. Wilkin says, especially long-term studies. He suspects, he says, that many studies that dispute the idea of an activitystat use time frames that are too short to capture the body’s subtle workings. “Compensation may be happening over the course of weeks or months,” he says, “not hours or days.”
Most important, though, he adds, even if people have a set point for exercise, its existence would not provide carte blanche for us to give up on exercise, or cancel P.E. classes at schools. “Exercise is extremely good for the health of young people, as it is for all of us,” he says. “It improves metabolic profiles and cardiorespiratory fitness. Our results should not be interpreted to mean that exercise is not worthwhile,” he says, only to suggest that how, why and whether we move may be more complicated issues than any of us might wish.
