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After every workout, there's a crucial decision to be made. Will you inhale the burger and fries you crave – thereby undermining any calorie burn – or limit yourself to a sensible snack until your next portion-controlled meal?

The answer depends on your fitness level and how your brain lights up after workouts, The New York Times reports.

In healthy people who have sat for an hour, the brain's food-reward system revs up at the sight of sugary, high-fat foods, according to a recent study conducted at California Polytechnic State University in San Luis Obispo.

But after an hour of vigorous exercise, the same people show little interest in treats – even sundaes – according to brain scans.

"Responsiveness to food cues was significantly reduced after exercise," Todd A. Hagobian, a professor of kinesiology at California Polytechnic, told the Times. That reduction is spread in different regions of the brain, he explained, "including those that affect liking and wanting food, and the motivation to seek out food."

However, volunteers in the study were fit, in their 20s and of normal weight.

In couch potatoes, the exercise-food connection may be reversed, according to earlier research published in the Journal of Obesity.

That study, conducted at the University of Leeds in the United Kingdom, found that exercise stimulated the food-reward systems in some overweight, inactive people – but not others.

Out of 34 participants, both male and female, 20 lost an average of 11 pounds after 12 weeks of exercising five days a week. But 14 "non-responders" dropped nary a pound.

The participants who didn't lose weight showed the highest brain activity in response to food cues both at the beginning and end of the study.

The results may be discouraging for those who most need to lose weight.

"It's likely that, in order to achieve weight loss and weight maintenance, you need to do a fair amount of exercise and do it often," Dr. Hagobian said.

With time, he added, brain research may help single out the best exercise program for the overweight, as well as the lean.

Meanwhile, those who are famished after exercise should not despair: "Being fit can have psychological effects," he said.

In other words, you might not lose weight - but you could get a mood boost.

Does exercise whet your appetite – or keep cravings at bay?