Exercising before eating breakfast burns more fat and improves how the body responds to insulin, new research reveals.
Scientists at the universities of Bath and Birmingham found that changing the time when people eat and exercise can help control their blood sugar levels.
The six-week study involved 30 men classified as obese or overweight who were split into two intervention groups - one that ate breakfast before exercise and one after - as well as a control group.
Those who exercised before breakfast burned double the amount of fat than the group that did so after eating, the research found.
Scientists say the increased fat use is mainly due to lower insulin levels during exercise when people have fasted overnight, meaning they can use more of the fat in their fat tissue and muscles as fuel.
The study was published in the Journal of Clinical Endocrinology and Metabolism.
"Our results suggest that changing the timing of when you eat in relation to when you exercise can bring about profound and positive changes to your overall health," Dr Javier Gonzalez, of the Department for Health at the University of Bath, said.
"We found that the men in the study who exercised before breakfast burned double the amount of fat than the group who exercised after.
"Importantly, whilst this didn't have any effect on weight loss, it did dramatically improve their overall health.
"The group who exercised before breakfast increased their ability to respond to insulin, which is all the more remarkable given that both exercise groups lost a similar amount of weight and both gained a similar amount of fitness.
"The only difference was the timing of the food intake."
Over the six weeks, scientists found that muscles in the group who exercised before eating were more responsive to insulin than those who exercised after.
The two groups performed identical training sessions and had matched food intake.
Muscles in those who exercised before breakfast also showed greater increases in key proteins, specifically those involved in transporting glucose from the bloodstream to the muscles.
Dr Gareth Wallis, of the University of Birmingham, said the research suggests that "performing exercise in the overnight-fasted state" can increase the health benefits of exercise for individuals, "without changing the intensity, duration or perception of their effort".
"We now need to explore the longer-term effects of this type of exercise and whether women benefit in the same way as men."