Exercise Can Help Prevent Weight Gain, but It Won't Be Easy

In women following their usual diet, only the nonoverweight avoided weight gain with exercise.

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There's a lot of attention paid to what works when it comes to losing weight. But that's not really the hard part; anyone can diet or exercise in the short term. Maintaining a loss, avoiding age-related weight creep, and keeping up healthful habits over time are much more difficult. That's why the researchers behind a new study, published this week in the Journal of the American Medical Association, wanted to examine the habits of people who were eating what they considered a normal diet and were "living life as usual," says one of the authors, I-Min Lee, an associate epidemiologist at Brigham and Women's Hospital in Boston and associate professor of medicine at Harvard Medical School. And they made some interesting discoveries about the power of exercise.

When researchers followed more than 34,000 nondieting women (average age 54.2) over many years, they found that regular physical activity was associated with gaining less weight over time—but only in women who weren't overweight or obese. (That means a BMI of lower than 25, or less than 150 pounds for a 5-foot, 5-inch woman.) And those women had to exercise quite a bit: an average of an hour a day of moderately intense activity—such as a brisk walk—or the equivalent (if you exercise more strenuously, less time is required) during a week was the amount of activity recorded for the normal-weight women who gained less than 5 pounds during the 13-year study. (Just 13.3 percent of women studied filled that bill.)

That may sound like bad news for people of normal weight who aren't exercising that much, not to mention everyone who is overweight or obese, says Lee. But it doesn't mean that physical activity of less than 420 minutes a week is worthless. Working out at a moderate intensity for 150 minutes a week, as the government recommends, is associated with a lower risk of many chronic diseases, no matter your weight.

[Read: 7 Tips From Uncle Sam's New Fitness Guidelines. And try our 10-Week Workout.]

But it does underline that exercise on its own, with no attention paid to calories, is unlikely to carve away excess weight or prevent gain. (Even people training for a marathon can gain weight; it's far easier to eat than to burn off what you eat.) Remember that none of these women were consciously dieting. The study can't say whether exercise is useless for overweight or obese women who are actively attempting to lose weight by also changing their eating habits. And that "usual diet" consumed by the women probably differs by their weight. It's likely that normal-weight women eat fewer calories as a matter of routine, without thinking of it as a diet.

Yet exercise also appears to be an important factor in maintaining a healthy weight, according to this and other studies. Research published in 2008 found that women who dieted and were then able to keep off 10 percent of their body weight for two years also exercised more than the government recommends, about 275 minutes a week. The amount of exercise needed to sustain a healthy weight may be in question, but the principle is consistent: Get out there and move.

[Read: Do Program Diets Work? Rarely. Here Are 7 Tips to Shed Pounds.]