Fasted Cardio

Secret Fat-loss Weapon or Pseudoscience?

Fasted Cardio - Secret Fat-loss Weapon or Pseudoscience?
By Dr. Layne Norton, Ph.D. Nutritional Sciences

Anecdotal data is heavily influenced by the power of perception and placebo. If we have positive thoughts about a variable that we put into our training or nutrition, we are more than likely to have positive results simply due to the power of positivity. This has been shown many times over in research studies. People are given some placebo treatment and told that it will work and so it does, even though they are not taking anything new— they are just thinking more positively. By the same token, if we have a negative perception of something before we try it out, it has a lower chance of being effective even if the treatment itself is based in sound science and has shown to be effective overall with unbiased studies. How does this apply to our current topic of fasted cardio?

Fasted cardio has been a technique that has been suggested by many fitness enthusiasts over the years as a superior method to burn fat. The idea is that by performing cardio in the fasted state before you eat in the morning, the calories you burn will be a greater percentage from fat, and lead to superior overall fat loss. From an intuitive standpoint this makes sense. In fact, I performed fasted cardio for several contest preps when I was younger, as that was the default “secret” for improving fat loss that was impressed upon me by many others in the fitness industry. However, when I stepped back and looked objectively at my results, I felt that they really were not that great compared to what people were saying. Upon doing a bit more digging into the literature, I started to see that the actual research on fasted cardio did not demonstrate superior fat-loss results and furthermore, data showed that training in the fasted state actually impaired anabolism.

Even when I was performing fasted cardio, one of the problems I had with the logic of people who preached fasted cardio was that these were the same people who claimed that if you waited more than two to three hours between meals without eating, then you would spontaneously combust with catabolism. OK, I’m exaggerating, but still I wondered how people could speak out both sides of their mouth with regard to anabolism and catabolism. Somehow it was a terrible idea to go more than two or three hours without eating during the day, but it was OK to perform an hour of cardio when you had not eaten for many hours prior to that? It didn’t jive with me. Over my next several contest preps, I stepped away from fasted cardio and focused more on high-intensity intervals and I noticed improvements in my response to cardio metabolically and with regard to fat loss.

Fasted Cardio - Secret Fat-loss Weapon or Pseudoscience?I’ve talked about my experience with fasted cardio, but what does the research actually say? Brad Schoenfeld actually did a marvelous job of summarizing the data in a review of fasted cardio in the Strength and Conditioning Journal. He makes several great points, including the mention of the problems with focusing on the percentage of calories you burn from fat during cardio. In general, if you burn more fat during exercise, you will burn more carbohydrate at rest and vice versa. Interestingly, he points out that fasting before exercise actually reduces the thermogenic response to exercise compared to not fasting. This shows that eating before exercise actually enhances the calorie burning, thermogenic effect of exercise!

Most importantly, Schoenfeld discusses that fat loss is a process involving two major steps: lipolysis and oxidation. Lipolysis is the process of liberating fatty acids from fat tissue and dumping them into the bloodstream where tissues like heart, liver and muscle can pull them in and oxidize them for energy. During exercise, the rate-limiting step of total fat loss is the oxidation step, NOT lipolysis. That is, during exercise the body tends to liberate far more fatty acids than it is actually able to oxidize for energy. The excess fatty acids that are not oxidized are simply restored back into fat tissue. Fasted cardio indeed causes a greater release of free fatty acids than traditional forms of cardio. It does not, however, increase the rate of oxidation and thus, there is no difference between fat loss between traditional cardio and fasted cardio.

In order to maximize fat loss from cardio, you have to modulate the rate-limiting step of fat loss, which is fatty acid oxidation. So what form of cardio maximizes the rate of oxidation? Evidence suggests that high-intensity interval training cardio (HIIT) causes greater increases in fatty acid oxidation compared to traditional forms of steady-state, low-intensity cardio, increases 24-hour energy expenditure and leads to better overall fat loss compared to low-intensity, steady-state cardio.

If you are trying to build muscle, I’m not saying you should do HIIT every day as it would likely lead to decrements in weight-training performance due to the intensity. I’m also not saying that low-intensity cardio is useless. But you should use a combination of the two to maximize fat loss while still maintaining the intensity of your workouts.

In conclusion, fasted cardio does not appear to be superior for fat loss based on scientific evidence and my own anecdotal evidence with myself and the clients I work with. So instead of suffering as a fasted cardio zombie on a treadmill, have some food beforehand and kick thermogenesis into high gear for improved fat loss!


Schoenfeld B. Does cardio after an overnight fast maximize fat loss? Strength and Conditioning Journal, 2011(33): 23-25.
Dr. Layne Norton is a natural pro bodybuilder with the IFPA and has a Ph.D. in Nutritional Sciences from the University of Illinois. He offers contest prep, nutritional and training consultations through his company, BioLayne LLC. To learn more about Layne and the services he offers, visit