Bodybuilders are big on the notion that early-morning cardio in the fasted state will burn more fat compared to longer cardio in the fed state.
Studies do not give us conclusive evidence to support this. Carbohydrates do blunt the body’s ability to burn fat because they prevent fatty acids from reaching the mitochondria (Schoenfeld, 2020). This seems to only be evident during low intensity cardio sessions because high intensity cardio actually mobilizes more fat than your body can process. This means that in the fasted state, high intensity cardio is actually counterproductive (Schoenfeld, 2020).
Consuming a meal before exercise actually creates an increased thermic effect during exercise. This contributes to greater post-exercise oxygen consumption. In a study by Paoli et al, participants who consumed a pre-exercise meal utilized more fat as an energy substrate than people who fasted. The differences remained significant 24 hours after the moderate intensity workout protocol (Paoli et al., 2011).
The cold hard truth about fat-burning is that your body wants to adapt. So out of the total fat that our body burns, a little more than half of it comes from sub-cutaneous stores and the rest come from the muscle (van Loon, 2004). It should also be noted, that if you are a trained individual, your body is more likely to use intramuscular triglycerides as fuel as opposed to sub-cutaneous fat (Schoenfeld, 2020). During long-duration cardio sessions, trained people use up to 80% of their energy from intramuscular triglycerides (Hurley et al., 1986). This is because they lie closer to the mitochondria (Romijn et al., 1993). Since acute studies do not distinguish between the types of fat that are burned, we can theorize that fat burning becomes less relevant as you become more trained (Schoenfeld, 2020).
Schoenfeld, B. (2020, July 10). Does Fasted Cardio Maximize Fat Loss? Retrieved September 29, 2020, from https://newsdome.co.za/does-fasted-cardio-maximize-fat-loss/
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. doi:10.1123/ijsnem.21.1.48
van Loon, L. (2004). Use of intramuscular triacylglycerol as a substrate source during exercise in humans. Journal Of Applied Physiology, 97(4), 1170–1187. doi: 10.1152/japplphysiol.00368.2004
Hurley, B., Nemeth, P., Martin, W., Hagberg, J., Dalsky, G., & Holloszy, J. (1986). Muscle triglyceride utilization during exercise: effect of training. Journal Of Applied Physiology, 60(2), 562–567. doi: 10.1152/jappl.19184.108.40.2062
Romijn, J., Coyle, E., Sidossis, L., Gastaldelli, A., Horowitz, J., Endert, E., & Wolfe, R. (1993). Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal Of Physiology-Endocrinology And Metabolism, 265(3), E380-E391. doi: 10.1152/ajpendo.1993.265.3.e380