When you're trying to lose weight, you're often told to exercise more. But some people say that exercise just makes you hungrier.
So some scientists decided to test this relationship, using something called functional MRI, or fMRI. This is a technique that can detect the blood flow in different areas of the brain. The more blood flow, the more activity in that portion of the brain. Then they had some people exercise for an hour on a stationary bike, showed them photographs of different kinds of food, and measured the blood flow in areas concerned with "food reward."
Reward is a brain response to that makes you want to repeat whatever produced the reward. For example, recreational drugs light up the reward centers in the brain. Food reward is a reward from food. The food is usually tasty, but it doesn't have to be. For example, if you are deficient in vitamin C, you might crave foods high in vitamin C even if you didn't especially like them when you had plenty of vitamin C in your diet.
What these researchers found was that after the 60 minutes of exercise, the brains of the exercisers showed lower responses to food than those who didn't exercise. Hence they suggested that exercise reduces your appetite.
However, a study done last year showed the opposite, that in some people, exercise increased their appetite.
What's going on here?
Well, for one thing, in the first study, the participants were active, young (about 22 years old), and thin (body mass indexes under 24). In the second study, the participants were sedentary and obese. Some of them reported no change in appetite after the exercise, and these "responders" lost more weight than the "nonresponders." In the nonresponders, appetite increased after exercise, especially for high-fat sweet foods.
The New York Times has described these experiments. And what is really interesting is the readers' comments at the end of the article. They range from people who say that exercise makes them ravenously hungry, to people who say exercise takes away their appetite, to people who say sometimes exercise makes them hungry and sometimes it doesn't, to people who say it depends on what kind of exercise they do.
Of course, what this really says is YMMV: your mileage may vary. What is important is not what some study in a lab shows among a certain group of people. What is important is how it works for you.
There are all kinds of exercise that can be done for different lengths of time. A slow hike up a mountain may affect you differently than an hour in the gym lifting heavy weights. So it's worthwhile to pay attention to your appetite after you've exercised. Did it go up or down? If it went up, can you modify the exercise some way so that won't happen?
Exercise is good for us, and we shouldn't abandon it because of a worry that we'll overeat.