Can Exercise Make You Gain Weight?

A reader complains she overeats after working out. But most people don't, studies suggest.

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A reader E-mailed me with a complaint: Exercise, she says, makes her ravenous to the point that she overeats and gains weight whenever she starts an exercise program. That's discouraging, to say the least. But studies show that working out typically does the opposite.

Most, but not all, studies show that appetite is suppressed both during and immediately after a workout. (The exception would be if you're working out so hard that you really deplete your blood sugar and get weak, hungry, and lightheaded—though that's beyond what most people do.) Catia Martins, a researcher now at the Norwegian University of Science and Technology, has studied the effects of an exercise program on appetite and energy intake over the longer term. One small study published last year in the British Journal of Nutrition suggests that a six-week exercise program can improve the body's ability to appropriately regulate appetite, at least in men. After working out an average of four days a week for about 45 minutes at a time, previously sedentary men (but not women) ate less at a buffet after they'd had a substantial snack. Before starting the exercise program, on the other hand, the men consumed the same number of calories at the buffet, whether it followed a calorie-rich snack or only a light one. That finding indicates that having an exercise program made their appetites more closely attuned to the amount of calories they were consuming.

The bad news for my frustrated correspondent? In Martins's study, instituting an exercise regimen didn't seem to affect women's eating habits at the buffet. Other studies of this concept have also shown a gender difference.

Another small study Martins published last year measured participants' hunger levels and food intake on two different days, once after sitting around quietly for an hour, and once after riding a stationary bike for an hour. After exercising, participants reported feeling less hungry. And although they then ate about 200 calories more at a buffet, they enjoyed a net calorie loss because they'd burned about 500 calories while spinning away on the bike.

This encouraging research also might not apply to the reader who wrote me, however. Like the first study, the biking study involved volunteers who were sedentary but not overweight. As my colleague Sarah Baldauf wrote recently, hormones governing appetite may be totally out of whack in the overweight and obese, and the same appetite-suppressing effect may not be present. Martins is planning studies in just this population to see if the results are different.

There are a couple of other points to keep in mind. If your primary exercise is swimming, you may be particularly susceptible to eating more than you burn off. Research has shown that exercising in cold water can make you hungrier than usual; I know that when I first started swimming again after a decade-long hiatus, I was ready to eat my arm after getting out of the pool. Also, as the cycling study shows, exercisers may eat slightly more, then burn off all that extra food and more. But that beneficial caloric deficit isn't likely to happen if you celebrate the end of your spinning class with a sleeve of Oreos.

"People think they can exercise and reward themselves," says Cassandra Forsythe, author of the Women's Health Perfect Body Diet (Rodale). You may get a little more leeway with food when you're a regular exerciser—a woman in my running club once confessed she ran not for the cardiac benefits, but for the extra chocolate and wine she could consume—but you can easily eat more than you burned if you aren't paying attention. Know the calories in what you're eating, and while the occasional splurge is fine, try to focus on eating healthy foods, not junk, says Forsythe. Try to reward yourself with non-food treats, like a massage.

Also keep tabs on how many calories you're actually expending, because it may be less than you think. Look here to calculate how many calories you work off by doing various activities, and don't forget to subtract from that the calories you'd be burning if you weren't exercising. For example, I weigh about 130 pounds. Running for an hour at about 9 minutes a mile will burn off about 680 calories...but watching TV would burn 59. So the extra calorie burn for my day is closer to 600 calories. That's about one less Oreo—and yes, I have counted.