Breaking up an exercise session, by adding a rest period in between, may boost a workout's fat-burning efficiency, a team of Japanese and Danish researchers reports.

When men exercised for two 30-minute stretches, taking a 20-minute rest break in between, they burned more fat than when they exercised for a single 60-minute session, and then rested afterward, Dr. Kazushige Goto of the University of Tokyo and colleagues found.

Current recommendations on exercise for preventing or treating obesity emphasize longer exercise sessions, Goto and his team note in the Journal of Applied Physiology. But there is evidence that following one exercise session with another workout may increase fat metabolism, they add.

To investigate, the researchers had seven healthy men complete one long workout and then two shorter workouts on exercise bicycles, measuring several different indicators of fat metabolism. All exercised at 60 percent of their maximum level of exertion.

When the men performed the two shorter exercise sessions, their blood levels of free fatty acids and other substances rose during the rest period, indicating greater fat metabolism. Levels of these substances also were higher during an hour-long rest period after the two-part exercise session.

Greater fat metabolism was recorded during each of the rest periods in the two-part session than during the rest period following the single, longer workout.

The men also showed lower levels of insulin and blood glucose during the second phase of the two-part exercise session.

While the proportion of total calories burned did not differ between the two workouts, fat represented nearly 77 percent of the calories burned in the recovery period after the two-part exercise session, compared with about 56 percent of calories burned in the recovery period after the single long exercise session.