Working out before breakfast leads to more fat loss throughout the day
Training at different intensities has been shown to affect the amount of fat burnt during the workout. These differences have been shown to balance out throughout the day when fat balance and carbohydrate balance are followed for longer periods. This study, however, is the only one we know of which specifically investigates differences in fat balance (the balance of fat oxidation and fat storage). It compared individuals training at a relatively low intensity before vs after breakfast.
The result, surprisingly, found that individuals had a lower fat balance throughout the day and a higher carbohydrate balance throughout the day (meaning less fat and more glycogen energy storage) even though total energy balance was the same between groups. Subjects were only followed for 24 hours, long term conclusions can therefore not be be drawn.
What does this mean?
You burn more fat from training before breakfast. Possibly it may also mean that low intensity morning exercise can leave you with more energy for an afternoon workout. These differences are, however, minor, meaning around 12 grams less fat and 30 grams more carbohydrates at the end of the day. So don’t be discouraged if you cannot make a morning workout, as the practical implications of this study are minor. Note that this does not apply when in a caloric deficit.
Shimada K. Effects of post-absorptive and postprandial exercise on 24 h fat oxidation. Metabolism. 2013 Jun;62(6):793-800.