Why Exercise Might Help You Eat Less—or Not

Working out often seems to cut the appetite. But that may not hold for everyone.

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I've written before about how most (but not all) studies show that exercise suppresses appetite. But outside of a scientist's lab, some people say they're not any less hungry after working out; in fact, many have E-mailed me to report that they actually gain weight when they start an exercise routine because they are eating more than before. A small study presented today at the annual meeting of the Endocrine Society in San Francisco may point to why: The hunger-damping effects of exercise may not apply to obese women. The study looked at 20 women, half of them lean and half obese, and through a series of experiments found that obese women reported no appetite suppression during exercise, while lean women did.

The probable suspect is leptin, an appetite-regulating hormone described by my colleague Sarah Baldauf in a story about the biology of fat. The thought is that obesity throws off the ability of leptin to regulate appetite. Again, this was a small study, so this theory hasn't been nailed down yet. But it may mean, as the study's lead author, Katarina Borer of the University of Michigan, put it in a press release, "Obese women perhaps need to consciously watch their calories because some of the hormonal satiety [fullness] signals don't seem to work as well." That's a bummer, because it means the advice to just eat when you're hungry may not be productive for the already obese. I wrote earlier this year about how to more objectively figure out how many calories you should eat each day based on your weight and activity level.