Does Exercise After Fasting Increase Fat Loss?

If you are like most people you have likely heard that doing cardio on an empty stomach in the morning leads to greater fat loss. The theory behind this is that your carbohydrate stores are depleted in the morning from the overnight fast and your body will then utilize stored fat as the energy source¹. This theory sounds plausible but is there any research backing it up? This article will talk about whether or not performing exercise in the morning on an empty stomach leads to greater fat loss than performing the same exercise after eating a meal.

Individual differences affect fat loss during exercise

Often times a theory that seems to make sense from a theoretical stand point does not work in the real world. What goes on in a lab can be drastically different than what takes place in the human body. This appears to be the case for burning more fat during exercise on an empty stomach. There are many factors which influence how much stored fat or carbohydrates we use during exercise including: difference in hormones, the physical fitness of a person and enzyme activity¹. Each person’s body responds differently to exercise which makes it difficult to predict individual fat loss from exercise.

Fat loss is not directly measured by the breakdown of stored fat during exercise

Studies have found that we cannot measure fat oxidation (breakdown of stored fat for energy) just by looking at fat lipolysis (breakdown of stored fat). During HIIT exercise there is a reduction of blood flow to adipose tissue yet fat loss is greater when maintaining this high exercise intensity than low-intensity exercise¹.

A study measured differences in fat oxidation between subjects that fasted or ate during low-intensity exercise². It was found that although fat lipolysis was reduced by 22% in the fed group during exercise there were no differences in fat oxidation between groups at 80-90 minutes of cycling. After 90 minutes fat oxidation was greater in the subjects that fasted.

Another study which measured fat oxidation during moderate-intensity cycling found no differences in fat oxidation between the fasted and non-fasted group despite a 20-25% reduction in fat lipolysis in the non-fasted group.

Changes in body composition following fasted or non-fasted exercise

A 2014 study which measured changes in body composition in women performing aerobic exercise after fasting or not fasting found that while both groups lost a significant amount of weight there were no differences in body composition changes between the groups³.

A 2011 study compared fat loss in subjects performing moderate-intensity exercise while fasting or after eating a small meal⁴. Somewhat surprisingly, the authors found that not only was fat loss not greater in the fasting state, it may be beneficial to eat a light meal before exercising.

Summary

• Individual differences in physiology make it difficult to predict fat loss during exercise
• We cannot measure fat oxidation (breaking down fat molecules for energy) just by measuring the levels of fat lipolysis (breakdown of lipids) seen during exercise as studies have found differences in fat lipolysis during exercise yet no differences in resulting fat oxidation
• Exercise intensity has as strong influence on fat loss, with HIIT producing the greatest amount of fat loss
• Studies have found no differences in body composition change following exercise in a fasted versus non-fasted state while one study found it may be advisable to consume a light meal before exercise to enhance fat loss

Sources

1. Schoenfeld, B. (2011). Does cardio after an overnight fast maximize fat loss?. Strength & Conditioning Journal, 33(1), 23-25.
2. Horowitz, J. F., Mora-Rodriguez, R., Byerley, L. O., & Coyle, E. F. (1997). Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise. American Journal of Physiology-Endocrinology And Metabolism, 273(4), E768-E775.
3. Horowitz, J. F., Mora-Rodriguez, R., Byerley, L. O., & Coyle, E. F. (1997). Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise. American Journal of Physiology-Endocrinology And Metabolism, 273(4), E768-E775.
4. Paoli, A., Marcolin, G., Zonin, F., Neri, M., Sivieri, A., & Pacelli, Q. F. (2011). Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training. International Journal of Sport Nutrition and Exercise Metabolism, 21(1), 48-54.