Many fitness advocates, personal trainers and bodybuilders believe that engaging in cardio on an empty stomach first thing in the morning will burn more fat than if you eat breakfast first. I also thought that this was the case before I started reading up on the subject and would sometimes engage in fasted cardio myself. According to Schoenfeld (2011), this became a popular concept after Bill Phillips published his book, “Body For Life” and stated that “20 minutes of intense aerobic exercise after an overnight fast has greater effects on fat loss than performing an entire hour of cardio” after meal time. The idea behind this is that it will cause your body to shift to using fat for fuel instead of carbohydrates due to lower glycogen levels, however science does not back up this hypothesis (Schoenfeld, 2011).
A point worth mentioning here is that although science does not back up the fasted cardio theory, there are mountains of evidence that support the idea that engaging in high intensity interval training (HIIT) does in fact burn more fat and calories than engaging in moderate training. It also burns calories for hours after you are done working out (Petrofsky, 2011; Schoenfeld, 2011).
Schoenfeld and colleagues (2014) conducted a study using 20 female participants who were utilizing a hypocaloric diet to analyze the differences between exercising when eating before exercise compared to fasting before exercise. The non-eating group was called the FASTED group (n=10) and the eating group was called the FED group (n=10). The study was 4 weeks in duration and subjects performed 1 hour of aerobic exercise 3 times a week. They found that both groups resulted in a significant loss of weight (P=0.0005) and fat mass (P=0.02). However, there were no significant differences between the two groups. Thus, suggesting that one will see the same effects on weight or fat loss when engaging in exercise and adhering to a hypocaloric diet regardless of eating or fasting before exercise. These findings are consistent with previous research conducted by Febbraio and colleages (2000) in which they found no evidence of decreased fat oxidation as a result of ingesting carbohydrates before or during exercise.
One more thing worth noting is that fasted cardio can result in proteolysis (the breakdown of protein) and results in a loss of protein at about 10.4% of the total caloric cost of exercise after cycling for 1 hour at 61% VO2max (Schoenfield, 2011). Therefore, people who wish to build strength or hypertrophy should probably not engage in fasted cardio. In fact, people who want to get serious about building substantial amounts of muscle mass or strength should limit their aerobic exercise regardless of eating before or not (I will explain more in a future blog).
The take home point here is that fasted cardio does not necessarily burn any more fat or calories than eating before exercise. What does matter for burning more fat and calories is engaging in HIIT. Also, if you are looking to gain muscle mass or strength, stay away from fasted cardio. Not to say that fasting itself without cardio will have detrimental effects on muscle mass or strength, in fact I have been recently looking into this after watching a NSCA lecture by Dr. John Berardi titled, “Intermittent Fasting: Science or Fiction?”. I will be posting more about that topic later. Feel free to leave me any comments/questions!
Alkahtani, S. A., Byrne, N. M., Hills, A. P., King, N. A., & Dept. of Health and Fitness,University of Dammam. (2014). Interval training intensity affects energy intake compensation in obese men. International Journal of Sport Nutrition and Exercise Metabolism, 24(6), 595-604.
Febbraio, M. A., Chiu, A., Angus, D. J., Arkinstall, M. J., & Hawley, J. A. (2000). Efects of carbohydrate ingestion before and during exercise on glucose kinetics and performance. Journal of Applied Physiology, 89(6), 2220.
Horowitz JF, Mora-Rodriguez R, Byerley LO, and Coyle EF (1997). Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise. American Journal of Physiology, Endocrinology & Metabolism, 273 E768–E775.
Petrofsky, J., Laymon, M., Altenbernt, L., Buffum, A., Gonzales, K., & Guinto, C. (2011).Post exercise basil metabolic rate following a 6 minute high intensity interval workout. Journal of Applied Research, 11(2), 65.
Racil, G., Ben Ounis, O., Hammouda, O., Kallel, A., Zouhal, H., Chamari, K., & Amri, M. (2013). Effects of high vs. moderate exercise intensity during interval training on lipids and adiponectin levels in obese young females. European Journal of Applied Physiology, 113(10), 2531-2540
Schoenfeld, B. (2011). Does cardio after an overnight fast maximize fat loss? Strength and Conditioning Journal, 33(1), 23-25.