Not only does working up a sweat on an empty stomach burn off more body fat - helping you lose those extra inches around the waist - it also triggers a bigger reduction in artery-clogging blood fats.

Although the benefits of exercise are well-known, people have been unsure about whether it is better to do it on an empty stomach or a full one.

Dr. Jason Gill, who conducted the research at Glasgow University, said that while exercise in itself is good, any done before breakfast may be extra beneficial, because it forces the body to rely on its stores of fat for energy. "When people talk about losing weight, they really mean losing fat," he said.

Ten men made three visits to his laboratory at the university's Institute of Cardiovascular and Medical Sciences, and were given breakfast each time. On the first visit, the men didn't do any exercise. On another, they did an hour's brisk walk ahead of breakfast and on the third they did the walk after breakfast. Those who exercised burnt off more fat than those who simply sat around. And those who did their walk before breakfast used up 33 percent more fat than those who exercised after eating.

Tests revealed they also experienced a bigger drop in the blood fats that raise the risk of heart disease. While the men only did a brisk walk, more strenuous activity can also be safely carried out on an empty stomach, Dr. Gill said.

This is because the body has enough reserves for about 90 minutes to two hours of exercise.