A good workout makes many of us opt for healthier foods, but it doesn't have that effect on everyone.
For those who are generally fit and active, exercise tends to reduce cravings for high-fat, sugar-packed foods. But among some overweight and inactive people, working out can have the opposite effect.
These were the results of two recent studies, as reported in the New York Times health and science blog. In one of these, published last month in the Journal of Applied Physiology, researchers at California Polytechnic State University in San Luis Obispo put a group of 30 active men and women in a functional MRI scanner immediately after either a strenuous hour-long workout, and alternately, after an hour of rest. When they were shown images of fatty foods after resting, the areas of their brains responsible for food cravings lit up. After exercise, the food-reward system in their brains kept relatively calm.
So, for these people, a workout would likely make them hold off on those tempting sugary snacks — letting them kill two bird's with one stone.
While he can't speak to the specifics of what goes on inside the brain after exercise, the tendency for active people to also have a healthy diet is something with which University of Toronto physical education and health professor, Guy Faulkner, is quite familiar.
"Health behaviours cluster together," he says. "They may reflect general attitudes towards health and lifestyle."
In this first study, the participants were all healthy, active people to start off with. Unfortunately, the story is not so straightforward for everyone.
In another study, published last year in the Journal of Obesity researchers put 34 obese and relatively inactive men and women in a supervised exercise program. While 20 of them experienced no change in food cravings following workouts, 14 actually had increased cravings for high-fat high-sugar foods after exercising.
But if this study makes you think your trips to the gym are futile, note that even those whose food cravings got worse were able to lose some weight during the 12-week exercise regime. Just not as much as the others.
"It may be too much for people to try to increase their activity and modify their diet," says Faulkner. "It could set really high expectations or barriers."
He thinks it might be easier to take one step at a time. "Let's be more conservative and help someone change one behaviour," he says, "with the hope being that once they change that behaviour they'll feel confident enough to change another."
But exercise alone could make all the difference to your health and longevity. While it might be cold comfort for those mainly concerned with their waistlines, you can be fat and fit, as University of South Carolina professor Steven Blair eloquently explains.
"The fat guys who are moderately fit have a death rate about one-half that of the thin guys who are not fit," he tells a McGill audience earlier this year.
Faulkner agrees. "You can increase your physical activity, improve your fitness, but not necessarily lose body weight," he says. "There still seem to be health benefits."
For tips on the best machines to use at gyms, watch the video below.