OCTOBER 19, 2011, 12:01 AM
Do We Have a Set Point for Exercise?
By GRETCHEN REYNOLDS
Mike Tittel/Getty Images
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.
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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.
