Why Exercise Might Help You Eat Less—or Not

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


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.