Cardiovascular exercise in a fasted state is common among fitness enthusiasts, bodybuilders, or even recommended as an effective fat loss strategy. This myth is very ingrained among practitioners and even some professionals, mediated by Bill Phillips, one of the greatest fitness “gurus”. The logic is simple. Since we've been using fatty acids as an energy substrate throughout the overnight fast, we'll have more fat available during exercise if we do it before a meal. We thus avoid the effect of insulin and the transition to greater oxidation of carbohydrates. Despite being a strong argument, the truth is that science does not seem to support it.
It is very reductive to think that the most important thing when we want to lose fat mass is the use of lipids as a substrate during the training session. If it lasts 45 minutes, we have 23 hours and 15 minutes more in the day. Our metabolism does not stop in this period, and energy balance is by far the determining factor in weight loss . We have to look at the big picture and ask, “Will doing fasted cardio make me leaner?”. Totally different from saying that cardio exercise in a fasted state “burns” more fat.
Fatty acids as a source of energy
It is known that high-intensity physical exercise, despite being more dependent on carbohydrates during the session, leads to a greater use of fatty acids during the period that follows. At increasing intensities, blood flow is shifted to muscle and diverted away from adipose tissue. However, there is lipolysis generated by the high production of catecholamines (adrenaline), releasing fatty acids that are retained in the tissue. After exercise, the blood shunt alleviates and these fatty acids are “fired” into the muscle where they are oxidized preferentially as a source of energy . Regardless of the intensity and nutritional status, adipose tissue releases more lipids than we can spend. In the hours following training, they represent the largest share of energy spent, with carbohydrates being saved for the regeneration of glycogen. If high-intensity training is more efficient, it will hardly be maintained in a fasted situation.
Even if someone, for some reason, manages to maintain high intensities during fasted training, the effect on energy expenditure and partition will not be more favorable for that. In fact, a pre-exercise meal can increase the thermic effect of the session, as evidenced by higher post-workout oxygen consumption, an indicator of higher energy expenditure. Exercising in a fasted state, the use of carbohydrates as energy appears to be higher in the 12h and the following 24h, indicating that, even assuming a greater use of lipids during the session, it is lower in the rest of the day compared to a pre-workout meal. . In addition, the inverse is verified for oxygen consumption, indicating a lower elevation of the metabolic rate in the post-exercise period with previous fasting.
Another important aspect to consider is increased protein catabolism with exercise in a fasted state . This may not be relevant as a sporadic session, but per system it may represent too high a sacrifice of muscle mass. In fact, the theoretical support in defense of cardiovascular exercise in a fasted state is precisely to take advantage of the moment when cortisol is at its highest. If this is catabolic to fat, it is much more catabolic to muscle tissue which cannibalizes amino acids for glucose production in the liver in an effort to keep blood glucose at levels compatible with life.
In short, eloquent theories are not always supported by evidence. Exercising in a fasted state does not appear to be particularly effective for fat loss, much less in recreational athletes or the average person. This is not to say that it cannot be used as a one-off strategy. It is certainly better for someone to train fasted when they only have that chance than not training at all. However, believing that this strategy will give better results in terms of fat loss, and this is the point we are making, is not supported by science or our physiology.