An age-old debate in the health and fitness industry is the argument between fasted vs. fed workouts, specifically cardio. I hear it from clients and I see it every morning… dozens of gym-goers plodding along on the treadmill at 5:00 in the morning in a “fasted” state, meaning they consumed zero calories before hitting the dreadmill (not a typo). The thought is that if your body has less immediately available and stored carbohydrate, aka glycogen, to be used as fuel, you will use and burn more fat.
Unfortunately, it is not that straight forward.
First of all, our bodies are very efficient at storing carbohydrate (glycogen) in our muscles and other tissues. In fact, muscle can store as much as 300-400 grams or 1,200 to 1,600 calories of glycogen, the liver can store 75-100 grams or 300-400 calories, and blood glucose can account for 25 grams or 100 calories. Therefore, total body glycogen stores can range from 1,400 to 2,100 calories, which can fuel a body through approximately 12-20 miles of running. Most people are not running 12-20 miles on the treadmill in the morning, so they are not depleting glycogen enough to “burn more fat”.
Second, most studies on this subject have found that there is no significant difference in weight loss or body composition changes when subjects exercise in a fed or fasted state. In addition, a very important finding is that individuals are able to work harder and longer in the fed state, which leads to better workouts overall. To make matters worse, if your goal is maintaining or gaining muscle, fasted cardio or training in a chronically glycogen suppressed state can have detrimental effects on your goals.
On the other hand, a couple studies have shown that exercising in a fasted state may decrease appetite and increase fat breakdown in the cells when fasted cardio is performed over a long period of time.
There are a few reasons that I do not train in a fasted state:
Personally, I do get to the gym around 5:00 a.m. with coffee + creamer and usually half a banana in my system.
Here is a treadmill workout I did this morning after eating that exact combo:
I don’t recommend doing this fasted, as it is pretty intense. Give it a try and let me know what you think!
Gillen, J. B., Percival, M. E., Ludzki, A., Tarnopolsky, M. A., & Gibala, M. (2013). Interval training in the fed or fasted state improves body composition and muscle oxidative capacity in overweight women. Obesity, 21(11), 2249-2255.
Gonzalez, J. T., Veasey, R. C., Rumbold, P. L., & Stevenson, E. J. (2013). Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males. British Journal of Nutrition, 110(04), 721-732.
Paoli, Antonio, et al. "Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training." International journal of sport nutrition and exercise metabolism 21.1 (2011): 48-54.
Rosenbloom, C. (2012). Sports nutrition: a practice manual for professionals. Academy of Nutrition and Dietetics.
Schoenfeld, B. (2011). Does cardio after an overnight fast maximize fat loss?. Strength & Conditioning Journal, 33(1), 23-25.
Van Proeyen, K., Szlufcik, K., Nielens, H., Ramaekers, M., & Hespel, P. (2011). Beneficial metabolic adaptations due to endurance exercise training in the fasted state. Journal of Applied Physiology, 110(1), 236-245.
Allison Tropf, MS, RD, CSSD
Allison is a Sports Dietitian in Michigan. She enjoys helping others reach their nutrition and fitness goals through reliable and trustworthy recommendations.
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