After exercise, each rider received the drink he had not previously swallowed. The riders who had fasted got the shake and the other group the placebo.

This routine continued for six weeks. Afterward, the scientists crunched numbers and turned up some telling differences between the groups. As expected, the control group’s fitness and insulin sensitivity remained unbudged, while the men in both exercise groups had improved their fitness and narrowed their waistlines, although few had lost weight.

The riders who had pedaled on an empty stomach, however, 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.

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,” says Javier Gonzalez, a professor of physiology and nutrition at the University of Bath, who oversaw the new study, which appeared in the Journal of Clinical Endocrinology and Metabolism.

The reasons for this extra metabolic boost are complex but most likely involve slimming of muscle fat, he says. The fasted riders’ bodies had to turn to internal energy stores for fuel, including fat from their muscles. (Interestingly, the fasted riders did not feel as if their workouts were more draining than the other group, according to everyone’s subjective ratings of their exertions.)

This study looked primarily at insulin sensitivity, though, and not other aspects of exercise and metabolism, including weight loss. (Other research by some of the same investigators have looked at how eating before exercise may affect appetite.) The researchers also cannot tell whether skipping lunch before an early-evening workout would have the same effects, although it seems likely, Dr. Gonzalez says.