Basically, burning calories through exercise causes some people to eat more, but not everyone does, and a few people actually eat less. Alex Hutchinson discussed this point recently on his blog (1). Part of it depends on how much fat you carry-- if you're already lean, the body is more likely to increase hunger because it very much dislikes going too low in body fat. Most overweight/obese people do not totally make up for the calories they burn through exercise by eating more, so they lose fat. There is a lot of individual variability here. The average obese person won't lose a substantial amount of fat through exercise alone. However, everyone knows someone who lost 50+ pounds through exercise alone, and the controlled trials support that it happens in a minority of people. On the other side of the spectrum, I have a friend who gained fat while training for a marathon, and lost it afterward.
Recent studies, reviewed in the NYT article, have shown that one of the key determinants of who eats more and who doesn't is the activation of reward circuits in the brain. From a neurobiology perspective, these are THE circuits that determine your motivation to seek and consume food. People who have increased reward circuit activation after exercise eat more; people who have less reward circuit activation after exercise eat less. It's nothing revolutionary, because it's exactly what you'd expect, but it's still nice to see it confirmed.
Here's a quote from the end of the article:
“Four or five years ago, it really looked like appetite hormones” controlled what we eat, says Dr. Habogian, who conducted some of the first studies of exercise and the hormones. “But I’m more and more convinced that it’s the brain. Hormones don’t tell you to go eat. Your brain does. And if we can get the dose right, exercise might change that message.”Of course, hormones and other circulating signals influence what the brain decides, so they do have a substantial impact. But the brain is the ultimate arbiter of food intake behaviors, and hormones and other circulating signals are among many factors that it considers. I'll be exploring this in more detail in coming posts.