That sound you hear is another illusion shattering. The twin pillars of advice about maintaining a healthy weight, or losing enough weight to get back to where you should be, are diet and exercise, as Claudia Kalb ably explains in her article on childhood obesity. Well, the second pillar just toppled over and smashed itself to smithereens: a 13-year study of 34,079 middle-aged women by researchers at Harvard Medical School and published in the Journal of the American Medical Association found that the only ones who avoided weight gain were those who exercised seven hours a week, not the 30 minutes, five-times-a-week goal that federal guidelines recommend.
I feel like gyms should start displaying signs with the warning Dante said hung at the gates of hell: abandon all hope, ye who enter (spin, walk, jog . . . ) here. Unless you're doing it 60 minutes a day. Seven days a week. At least.
The researchers, led by epidemiologist I-Min Lee
of Brigham and Women's Hospital and Harvard Med, began with the fact
that once you gain weight, it's nearly impossible to get it off and keep
it off. They therefore explored ways to prevent that gain. All the
women ate normally—that is, they weren't on a weight-reducing diet.
Bottom line: the only ones who avoided gaining weight were those who
averaged at least 60 minutes a day. Every day. For the whole 13 years.
With, as far as I can tell, no time off for virtuous behavior.
The depressing details are these: Women exercising 3.5 to seven hours per week—more than the federal guidelines recommend—gained an average of 5.7 pounds, indistinguishable from what women exercising less than 150 minutes per week gained. In other words, attaining the recommended level of exercise—in fact, almost doubling it—did not keep the pounds off. The only women who kept weight gain under about five pounds were those who started out with a BMI below 25 (155 if you're 5 feet 6, for instance—not overweight) and averaged at least 60 minutes a day of moderate exercise.
The researchers' depressing conclusion: "Compared with women who engaged in the equivalent of 420 minutes per week of moderate-intensity physical activity, those carrying out 150 to less than 420 minutes per week of such activity, as well as those engaged in less than 150 minutes per week, gained significantly more weight with no difference in weight gain between these 2 lesser active groups." That there was essentially no difference in weight gain between women who got twice as much exercise as federal guidelines recommend and those who got half the recommended exercise—a fourfold spread—is enough to make one cancel one's gym membership.
One more quote: "Once overweight, it may be too late because physical activity—at least, at levels carried out by study participants—was not associated with less weight gain. Second, sustaining high levels of physical activity (~60 minutes a day) is needed to successfully maintain normal BMI and prevent weight gain."
Other studies have also cast doubt on the power of exercise to fight obesity, including this one from 2009. So the current study is well within the emerging consensus.
Why is exercise so impotent at melting away pounds? No one is sure, but Time had a terrific cover story last year that floated some hypotheses, including that exercise makes us thirsty for sweet drinks (so we guzzle Gatorade), makes us hungrier (so we eat more), makes us feel like we deserve a reward (so we eat more), makes us feel like we can afford to take in more calories (so we . . . well, you get it). And that whole "exercise raises my metabolism so I will burn more calories just sitting around" thing? Not so much.
Interestingly, the Institute of Medicine concluded in a 2002 study that 60 minutes a day (seven hours per week) of moderate exercise may be needed to prevent becoming overweight or obese. I wouldn't be surprised if the feds took one look at that and concluded that if they made it the official recommendation, everyone would give up before so much as walking around the block. The 2008 recommendation of 150 minutes per week is enough to lower the risks of cardiovascular disease and other chronic illnesses, Lee and her colleagues conclude, but "is insufficient for weight gain prevention absent caloric restriction." There is no way to avoid the obvious: to avoid gaining weight, the only recipe is to eat less.