Scientists have discovered that exercising could make low-calorie food appear more appetizing.
Scans taken after volunteers jogged for an hour showed that the brain's "reward centres" lit up when they were shown images of low-fat diet foods.
But the same brain regions showed reduced activity when volunteers looked at pictures of calorie-dense fatty food.
The findings, by a team of researchers at the University of Birmingham, suggest that exercise has the power to help change eating habits as well as shed fat.
Although it has long been known that working out can curb hunger pangs by adjusting the balance of hormones in the body, there has been little research into its effects on the central regulation of appetite by the brain.
The Birmingham team asked 15 young, healthy men to jog gently on a treadmill for up to an hour.
The subjects then underwent an MRI scan to analyse activity levels in areas of the brain associated with rewards, first when shown images of low-calorie healthy foods and then with fatty takeaways.
The experiment was then repeated but this time after the men had relaxed for an hour.
The results showed that neuron cells in regions of the brain that become more active when they sense a "reward" responded more to low-fat food images after exercise than when volunteers did nothing.
One such region - known as the ventral pallidum - is thought to be involved in processing thoughts related to rewards such as food.