Fasted Cardio: Yes or No?
Fasted cardio is common practice among fitness and bodybuilding enthusiasts, or even recommended as an effective strategy for the loss of fat mass. This myth is deeply rooted among athletes and even some professionals, and has been publicised by Bill Philips, one of the biggest “gurus” in Fitness. The logic is simple. Once we are using fatty acids as energy substrate during the night’s fasting, we are going to have more available fat during exercise if we do it before a meal. This way, we avoid the effect of insulin and the transition for a greater oxidation of carbohydrates. Although it is a strong argument, the truth is that sciences seems not to corroborate it.
It’s very reductive to think that the most important when it comes to lose fat mass is the utilisation of lipid as energy substrate during the training session. If it lasts 45 minutes, we have more 23 hours and 14 minutes in the day. Our metabolism doesn’t stop during this period and the energy balance is by far the determining factor in weight loss. We must look at the big picture and ask: “will doing fasted cardio make me leaner?”. It’s totally different from saying that fasted cardio “burns” more fat.
It is known that high intensity exercise, although depending more from carbohydrates during the session, leads to a higher utilisation of fatty acids during the period that follows. In increasing intensities, blood flow is directed to the muscle and diverted from the fat tissue. Nevertheless, there is lipolysis generated by the high production of catecholamines (adrenaline), releasing fatty acids that are retained in the tissue. After exercise, the diversion of blood decreases and these fatty acids are “fired” into the muscle, where they are preferably oxidised as an energy source. Regardless of intensity and nutritional status, the adipose tissue releases more lipids that those we can use up. They represent the largest part of energy expenditure through the hours following the workout, with carbohydrates being saved for the regeneration of glycogen. If high intensity training is more efficient, it will hardly be maintained in a fasted state.
Even if someone, for any reason, is able to maintain high intensities during fasted training, the effect on energy expenditure and partitioning will not be more favourable because of it. In fact, having a meal before exercise can increase the thermic effect of the session, proven by the larger post-workout oxygen consumption, an indicator of a higher energy expenditure. With fasted training, the utilisation of carbohydrates as energy source seems to be higher in the following 12h and 24h, indicating that, even if assuming a higher utilisation of lipids during the session, this is inferior during the rest of the day comparing with having a pre-workout meal. Besides, the opposite is true for oxygen consumption, indicating a lower peak in post-exercise metabolic rate with previous fasting.
Another important aspect to take into consideration is the high protein catabolism with fasted training. This may not seem relevant for a sporadic session, but when done systematically, it can represent a far too high sacrifice of muscle mass. Besides, the theoretical support defending fasted cardiovascular exercise goes precisely through taking advantage of the moment when the cortisol level is higher. If it is catabolic for fat, it is much more catabolic for muscle tissue, cannibalising the amino acids to produce glucose in the liver in an effort to maintain glycaemia in life compatible levels.
In short, eloquent theories are not always supported by evidence. Fasted training does not seem to be particularly effective for fat mass loss, let alone in recreational athletes or common people. I don’t intend to say that it cannot be used as an occasional strategy. It’s surely preferable to have someone doing fasted training when it’s the only way than having them not training at all. Nonetheless, believing that this strategy will bring better results in terms of fat mass loss, and this is the point we are discussing, is not supported by science nor by our physiology.
Sérgio Veloso – Sérgio Veloso has a degree in Cellular Biology and a post-graduate degree in Clinical Nutrition, having lectured, in recent years, several training activities on Sports and Functional Nutrition. He’s also a consultant in the field of Fitness and Sports Supplements Industry, and a speaker at several national and international congresses.
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