See more of the story

Working out on an empty stomach could amplify the benefits of exercise, according to a new study of the interplay of meal timing, metabolic health and moving.

In general, any exercise improves our health. But even if everyone completes the same amount of exercise, some people become more fit than others, or lose more weight, or gain greater control of their blood sugar. Exercise scientists attribute the differences to our genetics, diets, physiques, temperaments and other subtle aspects of our lives. But some researchers suspect meal timing matters, as well.

The theory behind the study was that working muscles need fuel during exercise, mostly in the form of sugar (glucose) or fat. That fuel can come from our most recent meal, once its component sugars and fats reach our bloodstreams, or from our bodies’ stores of fats and sugars.

Researchers at the University of Bath in England recruited 30 overweight, sedentary men. (They plan to do a follow-up study with women.) The researchers tested the men’s fitness and insulin sensitivity and then divided them into three groups.

One, as a control, continued their usual lives; as expected, not much changed for them during the study. The other two groups started supervised exercise in the morning three times a week, riding stationary bicycles at a moderate pace while wearing monitors and masks that tracked their heart rates and the amount of fat and sugar they burned.

One exercise group downed a vanilla-flavored shake two hours before the ride (with no other breakfast), while the other group swallowed a similar-tasting placebo drink, containing flavored water and no calories. In other words, the placebo group rode on an empty stomach, but did not know it.

After six weeks, some telling differences turned up. While both exercise groups had improved their fitness and narrowed their waistlines, the riders who had pedaled on empty stomachs had incinerated about twice as much fat during each ride as the men who consumed the shake first.

The riders all had burned about the same number of calories while pedaling, but more of those calories came from fat when the men did not eat first. The study concluded that the fasted riders’ bodies had to turn to internal energy stores for fuel.

Those riders also showed greater improvements in insulin sensitivity at the end of the study and had developed higher levels of certain proteins in their muscles that influence how well muscle cells respond to insulin and use blood sugar.

On the whole, these findings suggest that “you can probably get more out of your workout without increasing its intensity or duration by exercising before breakfast,” said Javier Gonzalez, a professor of physiology and nutrition at the University of Bath, who oversaw the study, which appeared in the Journal of Clinical Endocrinology and Metabolism.

The study came with several caveats. It focused primarily on insulin sensitivity and not other aspects of exercise, including weight loss. And it dealt with breakfast and morning exercise. The researchers think that the same fasting theory would apply with lunch and afternoon workouts, but they can’t say so definitively.

And there’s one last — but crucial — caution, Gonzalez said. If your schedule or preferences prevent you from exercising in the morning or on an empty stomach, don’t interpret the study as meaning the exercise won’t help.

“Any physical activity,” he said, whenever you can fit it in, “is better than none.”