Studies show that exercising while you are fasting helps your muscle cells take in glucose and help you burn fat. These are great things if your goal is to lose weight. But what is happening to your muscles?
Are muscles breaking down to fuel your exercise session? The answer to this question is not black and white. But in this post, I will share what research has uncovered about exercising in a fasted state.
Exercise and Fasting Summary
- Study result: Aerobic exercise in a fasted state improved fat burning, glucose uptake by your muscles, and insulin sensitivity
- Study results: Men and women retained or gained muscle when fasting for 16 hours. They used whey protein supplements and worked out during their eating window.
- Extended fasts lasting more than a couple of days may result in muscle loss.
Can You Exercise During a Fast? [Video]
In this video, you’ll learn…
- The result of intermittent fasting and resistance training in both men and women.
- If it is advisable to supplement your diet with whey protein.
- The effects of long-term fasting vs short-term fasting.
Exercise Enhances Diet
Exercise has always been looked at as a good complement to a healthy diet. It burns calories and also improves insulin sensitivity, which means that your cells have an easier time taking in glucose.
Research shows that these benefits may be enhanced when you exercise in a fasted state.
Aerobic Exercise in a Fasted State
A 6-week study had healthy men perform four 60 to 90-minute exercise sessions per week (1).
Some of the men ate before and during the exercise session while others exercised in a fasted state.
The study revealed that early morning exercise in the fasted state is more potent than an identical amount of exercise in the fed state to improve whole‐body glucose tolerance and insulin sensitivity, which are both things that are going to help your body become a more efficient fat burner.
That is good news, but are these positive results coming at the expense of muscle? For that answer, we need to go a bit farther into the research.
The study that I just mentioned had the participants run and ride bike, which are endurance-style exercises that are not aimed at muscle building.
Resistance Training + Intermittent Fasting
There are intermittent fasting studies that had participants perform muscle-building or resistance exercises.
One study took men with a history of performing regular bodybuilding workouts and split them into two groups (2).
Each group ate three meals a day. One group consumed those three meals in a 12-hour eating window and the other in an 8-hour window. At the end of the study, the group with the shorter eating window had a significant decrease in fat mass, with no loss of muscle.
Two points are worth noting, however:
- The participants exercised within their eating window, so they were not in a fasted state.
- Both groups were given 20 grams of whey proteins 30 minutes after each training session.
Whether that supplementation or the timing of the workouts was a factor in the muscle retention is not clear.
A similar study that utilized women bodybuilders had similar results. The female athletes who restricted their daily eating window down to 8-hours experienced muscle gain rather than muscle loss (3).
Here again, it’s worth noting that the women were instructed to consume a whey protein supplement on each day of the study and their exercise sessions took place during their eating window.
Just like in the male study, it is not clear if the supplementation or exercising with food in their system impacted muscle retention.
Supplementing with Whey Protein
There needs to be substantial research performed on people who are weight training in a fasted state to evaluate if muscle can be retained under those parameters.
Many of you are likely wondering if supplementing with whey protein while in a fasting state would be beneficial.
There is a possibility that is true, but that brings up the question of whether consuming a protein supplement during a fast, breaks your fast. Here again, the research is lacking, so we have to rely on speculation.
In my opinion, supplementing with protein would break a fast, but if you are simultaneously supporting your muscles, then the benefits might outweigh the costs.
Short-Term Fasts vs Extended Fasts
It is also worth noting that these studies had participants performing short-term fasts, so they were eating every day and this fact may help explain why muscle was not lost.
We know that the body doesn’t like to use protein as fuel because protein has so many other important functions. However, that doesn’t mean it can’t happen and there have been studies that show that fasts that last for a few days result in some muscle loss (4).
If you are fasting for longer than a day and you want to push your body to lose more weight by adding intense exercise, you might be sacrificing muscle.
There are also open questions about whether calories or the nutrient composition of your diet matter.
The bottom line on whether you can exercise during a fast is that the answer depends on a number of factors and more research is needed.
Based on the research presented, if you are eating a nutrient-rich diet then you can perform aerobic exercise during a fasted state. In fact, you may improve fat burning, glucose uptake by your muscles, and insulin sensitivity.
Aerobic exercises include things like riding a bike, walking, jogging, and swimming.
If you work out against resistance, you can restrict your eating window to practice intermittent fasting. The timing of your workout might matter.
It seems clear that working out during your eating window protects your body from muscle loss. Until more research is done, it is not clear how muscle is affected when resistance exercise is performed in a fasted state.
However, I will add an anecdotal story here. I recently completed a 12-week fitness program that included alternating between training with weights and high-intensity interval training. I worked out every morning on an empty stomach and was able to build four pounds of muscle.
Working out in a fasted state worked fine for me. If your schedule requires you to work out in a fasted state, then you’ll want to closely monitor your body fat and lean muscle percentage to make sure you are gaining and not losing muscle mass.
I hope that helps you plan your workout routines. Exercise is a good thing as is a healthy diet. Working to find the balance between those two elements is a good goal to strive for.
(1) Van Proeyen, Karen, et al. “Training in the fasted state improves glucose tolerance during fat‐rich diet.” The Journal of physiology 588.21 (2010): 4289-4302.
(2) Moro, Tatiana, et al. “Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males.” Journal of translational medicine 14.1 (2016): 290.
(3) Tinsley, Grant M., et al. “Time-restricted feeding plus resistance training in active females: a randomized trial.” The American journal of clinical nutrition 110.3 (2019): 628-640.
(4) Tinsley, Grant M., and Paul M. La Bounty. “Effects of intermittent fasting on body composition and clinical health markers in humans.” Nutrition reviews 73.10 (2015): 661-674.
About the Author:
Dr. Becky Gillaspy, DC graduated Summa Cum Laude with research honors from Palmer College of Chiropractic in 1991.