Why Exercise Won't Make You Thin

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Self-Control Is like a Muscle
Many people assume that weight is mostly a matter of willpower — that we can learn both to exercise and to avoid muffins and Gatorade. A few of us can, but evolution did not build us to do this for very long. In 2000 the journal Psychological Bulletin published a paper by psychologists Mark Muraven and Roy Baumeister in which they observed that self-control is like a muscle: it weakens each day after you use it. If you force yourself to jog for an hour, your self-regulatory capacity is proportionately enfeebled. Rather than lunching on a salad, you'll be more likely to opt for pizza.

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Some of us can will ourselves to overcome our basic psychology, but most of us won't be very successful. "The most powerful determinant of your dietary intake is your energy expenditure," says Steven Gortmaker, who heads Harvard's Prevention Research Center on Nutrition and Physical Activity. "If you're more physically active, you're going to get hungry and eat more." Gortmaker, who has studied childhood obesity, is even suspicious of the playgrounds at fast-food restaurants. "Why would they build those?" he asks. "I know it sounds kind of like conspiracy theory, but you have to think, if a kid plays five minutes and burns 50 calories, he might then go inside and consume 500 calories or even 1,000." (Read "Why Kids' Exercise Matters Less Than We Think.")

Last year the International Journal of Obesity published a paper by Gortmaker and Kendrin Sonneville of Children's Hospital Boston noting that "there is a widespread assumption that increasing activity will result in a net reduction in any energy gap" — energy gap being the term scientists use for the difference between the number of calories you use and the number you consume. But Gortmaker and Sonneville found in their 18-month study of 538 students that when kids start to exercise, they end up eating more — not just a little more, but an average of 100 calories more than they had just burned.

If evolution didn't program us to lose weight through exercise, what did it program us to do? Doesn't exercise do anything?

Sure. It does plenty. In addition to enhancing heart health and helping prevent disease, exercise improves your mental health and cognitive ability. A study published in June in the journal Neurology found that older people who exercise at least once a week are 30% more likely to maintain cognitive function than those who exercise less. Another study, released by the University of Alberta a few weeks ago, found that people with chronic back pain who exercise four days a week have 36% less disability than those who exercise only two or three days a week.

But there's some confusion about whether it is exercise — sweaty, exhausting, hunger-producing bursts of activity done exclusively to benefit our health — that leads to all these benefits or something far simpler: regularly moving during our waking hours. We all need to move more — the Centers for Disease Control and Prevention says our leisure-time physical activity (including things like golfing, gardening and walking) has decreased since the late 1980s, right around the time the gym boom really exploded. But do we need to stress our bodies at the gym?

Look at kids. In May a team of researchers at Peninsula Medical School in the U.K. traveled to Amsterdam to present some surprising findings to the European Congress on Obesity. The Peninsula scientists had studied 206 kids, ages 7 to 11, at three schools in and around Plymouth, a city of 250,000 on the southern coast of England. Kids at the first school, an expensive private academy, got an average of 9.2 hours per week of scheduled, usually rigorous physical education. Kids at the two other schools — one in a village near Plymouth and the other an urban school — got just 2.4 hours and 1.7 hours of PE per week, respectively.

To understand just how much physical activity the kids were getting, the Peninsula team had them wear ActiGraphs, light but sophisticated devices that measure not only the amount of physical movement the body engages in but also its intensity. During four one-week periods over consecutive school terms, the kids wore the ActiGraphs nearly every waking moment.

And no matter how much PE they got during school hours, when you look at the whole day, the kids from the three schools moved the same amount, at about the same intensity. The kids at the fancy private school underwent significantly more physical activity before 3 p.m., but overall they didn't move more. "Once they get home, if they are very active in school, they are probably staying still a bit more because they've already expended so much energy," says Alissa Frémeaux, a biostatistician who helped conduct the study. "The others are more likely to grab a bike and run around after school."

Another British study, this one from the University of Exeter, found that kids who regularly move in short bursts — running to catch a ball, racing up and down stairs to collect toys — are just as healthy as kids who participate in sports that require vigorous, sustained exercise.

See nine kid foods to avoid.

Read "Our Super-Sized Kids."