There has been a lot of hype around fasted cardio. There are many posts and articles touting its benefits and how fasted cardio will get rid of that “stubborn” fat. For many, it sounds like a dream come true and a simple solution to a very difficult problem. Simply do some cardio after a fast (generally a minimum of 3-4 hours but more often employing an overnight fast) and watch the fat fall off. Of course, it is expected that a person doing fasted cardio will be in a calorie deficit and will still maintain a healthy diet for weight loss. But for many who do those things with cardio the results they want still elude them. They still have issues losing weight or getting rid of that belly fat. So why not turn to fasted cardio especially when it has worked for so many? But does it actually work? We are about to find out. When it comes to results the proof is in the pudding. So we go straight to the studies to find out what science says.
So how does fasted cardio work? Well, the general idea is that after a fast your body’s glycogen levels are low and to supplement the energy needed your body switches gears and starts using fat as a source of energy instead of carbs. So because your body has already burned through the food in your stomach it has to use stored fat instead, since that is the only ample source of energy left. This stored fat is often the fat in unwanted places that people want to get rid of. Sounds great right? It’s like tricking your body into using up the stuff you are trying to get rid of anyway. It’s like a bait and switch. But there are a couple issues with this. The human body is dynamic, it changes constantly and adapts to whatever pressure it is under at that given time. Brad Schoenfeld has done some excellent research in this area, specifically with fasted cardio. He has provided some great insight into the realities of this method and breaks down what is really happening in the body. He points out the dynamic nature of the human body and explains how the principle behind fasted cardio is great but it also works against the practice as well. The idea behind fasted cardio ignores “the dynamic nature of the human body, which continually adjusts its use of substrate for fuel.”[1] So yes you may burn more fat during your fasted cardio, but throughout the day your body will rely heavier on carbs or other sources of energy and will burn less of the fat that you were trying to get rid of.
In one study researchers looked at 20 healthy young female college athletes and compared the rate of fat loss when half of the group fasted before cardio and the other had a meal before cardio. This study showed how the dynamic nature of the body makes cardio after a meal just as effective as fasted cardio. The 20 students were split into 2 groups; half were in the “fasted” group where they did steady state cardio(jogging, biking, etc) after an overnight fast and the other half was a part of the “fed” group where they did the same cardio after eating a meal. Everything was monitored; what they ate, their age difference, the difference in their body composition, and how much exercise they did. On average the “fed” group was about 2.5 years younger than the “fasted” group but all participants were 20-26 years old. In this study, they found no difference in weight loss, fat loss, nor waist size. So there was very little difference between the two groups. And after 4 weeks of testing, they found that fasted cardio did not work any better than cardio after a meal.
This may come as a disappointment to some but there are a lot of factors in play that could actually be helpful. Schoenfeld pointed out in his research how quickly the body will adapt, and because of that, you have to look at how your body burns fat over the course of days, not minutes or hours. But even though fasted cardio doesn’t work any better it does show us that we have to look at our health over the course of days, because our body is always burning calories. And since our body is always burning calories and is constantly adapting then we can use that to our advantage. Y.S. Lee did a study of how the body burns fat both after fasted cardio and after fed cardio. He found that there was a significantly increased thermogenic effect for those who ate before cardio compared to those who did fasted cardio. So after a meal and cardio it was found that the body burns significantly more fat after the workout than those who did cardio after fasting. He also found that High-Intensity Interval Training (HIIT) was far more effective than steady state exercise[2]. It is also important to understand that steady state exercise is the most effective exercise for fasted cardio. A big reason for this is because HIIT requires a significant amount of energy and that is difficult to do while fasting. Schoenfeld sums it up nicely:
“Attempting to engage in a HIIT style routine in a hypoglycemic state almost certainly will impair performance. Studies show that a pre-exercise meal allows an individual to train more intensely compared with exercise while fasting. The net result is that a greater number of calories are burned both during and after physical activity, heightening fat loss.” [3]
Ultimately what you decide to do is up to you. If you are doing HIIT training as a form of cardio there are a lot of reasons to eat a meal before. When you go to the gym or participate in any fitness routine you want to make sure that you can give it your all. You don’t want to go in there moving at half pace or running on empty. In another article, I will go more in-depth on how to pick out the right cardio program but we at least have seen research that shows that you will not gain any significant advantage to doing fasted cardio. So don’t worry about starving yourself before your gym session. And if you feel like you just can’t give it your best when you are doing a fasted routing, you are probably right.
Sources:
- Brad Jon Schoenfeld, A. A. (2014). Body composition changes associated with fasted. Journal of the International Society of Sports Nutrition.
- Lee YS, H. M. (1999). The effects of various intensities and durations of exercise with and without glucose in milk ingestion on postexercise oxygen consumption. Journal of Sports Med Phys Fitness
- Brad Schoenfeld, M. C. (2011). Does Cardio After an Overnight Fast Maximize Fat Loss? Strength and Conditioning Journal.
