This newly published study was performed on 12 young male endurance athletes and was conducted as cross-over by which all participants underwent both types of tests.
These tests consisted of two phases; the first phase was exercising before breakfast on an empty stomach. And the second, exercising after breakfast.
The exercise consisted of cycling at 50 percent of their maximal oxygen uptake (VO2max) for 60 minutes. After which the participant had to stay and live for 24 hours inside a metabolic chamber, a sealed laboratory constantly monitored by the researchers.
All participants had to eat the same standardized meals over the two days. So there was no difference in total energy “in” for the athletes.
Even though the energy consumption did not differ during the 24 hours, the researchers did observe a difference in total energy balance. They also observed a difference in both fat and carbohydrate balance between the trials, but with a zero net balance.
As they noticed that if you used more fat during the exercise, the body then compensated by using more carbohydrates after the exercise. So it, therefore, appeared to be a net balance between stored fat and carbohydrates at all times.
But the researchers also noticed that the above did not apply if you exercised on an empty stomach, before breakfast instead of after. Then the body did not compensate for the increased fat burned during the activity.
There was at least a difference of ~ 12 grams less fat mass for the group who trained during fasting in the morning, before breakfast.
So it does indeed seem to be benefits of exercising during fasting if you want to lose fat mass.
Fat oxidation during exercise depends on nutritional state, and exercise performed in the post-absorptive state oxidizes more fat than that performed in the postprandial state. However, the effects of exercise on energy metabolism continue during the post-exercise period, and the difference in fat oxidation during exercise may be compensated for during the post-exercise period. The present study compared the effects of an acute exercise bout in the post-absorptive or postprandial state on 24h fat oxidation.
Twelve young male athletes stayed twice in a room-size metabolic chamber for 24h indirect calorimetry in a randomized repeated-measure design. Before or after breakfast, i.e. in the post-absorptive or postprandial state, subjects exercised at 50% VO(2) max for 60min.
During the 60min of exercise, energy expenditure in the two exercise trials was equivalent, but exercise in the post-absorptive state was performed with lower RQ compared with that in the postprandial state (P<0.01). The time of exercise relative to breakfast did not affect 24h energy expenditure (P>0.5). However, accumulated 24h fat oxidation was higher (P<0.05) and that of carbohydrate oxidation was lower (P<0.05) when exercise was performed in the post-absorptive state.
Compared with exercise performed in the postprandial state, exercise performed in the post-absorptive state oxidized more fat and saved more carbohydrate in the body, without affecting 24h energy expenditure.
Effects of post-absorptive and postprandial exercise on 24 h fat oxidation.