Intermittent fasting is a popular approach to weight loss based on time-restricted eating, and numerous studies have found that intermittent fasting diets can indeed help people lose weight.
For example, a review of 27 intermittent fasting dietary interventions found that all 27 studies resulted in weight loss of 0.8% to 13.0% of the starting weight with no serious adverse effects.1Welton, S., Minty, R., O’Driscoll, T., Willms, H., Poirier, D., Madden, S., & Kelly, L. (2020). Intermittent fasting and weight loss. Canadian Family Physician, 66(2), 117–125. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7021351/
But, if you lose weight intermittent fasting, are you losing muscle mass and body fat equally? In other words, is intermittent fasting weight loss just fat loss or also muscle loss? Do you lose muscle mass from intermittent fasting if you don’t eat enough calories?
In this intermittent fasting guide, we will discuss the evidence surrounding the question: Does intermittent fasting cause muscle loss or muscle gain?
We will cover:
Let’s dive in!
does fasting burn fat or muscle?
Numerous studies have demonstrated the weight loss benefits of intermittent fasting diets.
Both time-restricted intermittent fasting diet schedules, where you limit your feeding window to a confined number of hours per day and extend the overnight fasting period accordingly, as well as alternate day intermittent fasting diets wherein you have days of regular eating followed by days of complete fasting or significantly reduced calorie intake, have been shown to help support weight loss.2Tinsley, G. M., & La Bounty, P. M. (2015). Effects of intermittent fasting on body composition and clinical health markers in humans. Nutrition Reviews, 73(10), 661–674. https://doi.org/10.1093/nutrit/nuv041
But, what kind of weight loss is seen with intermittent fasting diets?
The term “body weight“ refers to your total body weight with all types of tissue lumped together.
Most people who want to lose weight want to lose fat mass, not lean body mass.
Body fat is a specific type of tissue known as adipose tissue, whereas lean body mass is all of the other types of cells and tissues in your body, such as muscle, nerve, bone, organs, cartilage, tendons, etc.3Chaston, T. B., Dixon, J. B., & O’Brien, P. E. (2006). Changes in fat-free mass during significant weight loss: a systematic review. International Journal of Obesity, 31(5), 743–750. https://doi.org/10.1038/sj.ijo.0803483
The good news is that many of the intermittent fasting weight loss studies have demonstrated that intermittent fasting decreases body fat, meaning that you are losing body fat specifically.
However, as with most weight loss diets or weight loss from dieting, you will lose some amount of lean muscle from intermittent fasting along with body fat.
In most cases of caloric restriction or generating a calorie deficit to lose weight, you will lose lean body mass and body fat, particularly if you are not working out to maintain as much lean body mass as possible with heavy resistance training.4Phillips, S. M. (2014). A Brief Review of Critical Processes in Exercise-Induced Muscular Hypertrophy. Sports Medicine, 44(S1), 71–77. https://doi.org/10.1007/s40279-014-0152-3
Indeed, some intermittent fasting diet research studies have shown that people lose some lean body mass after several months of intermittent fasting, along the lines of 1-2 kg or about 2-5 pounds.5Tinsley, G. M., & La Bounty, P. M. (2015). Effects of intermittent fasting on body composition and clinical health markers in humans. Nutrition Reviews, 73(10), 661–674. https://doi.org/10.1093/nutrit/nuv041
This is a fairly negligible loss of lean body mass relative to the amount of fat loss.
However, this particular research evaluating the changes in body composition from intermittent fasting diets looked at alternate-day fasting rather than time-restricted intermittent fasting.
Even more promising is the fact that some intermittent fasting diet studies6Varady, K. A., Bhutani, S., Klempel, M. C., Kroeger, C. M., Trepanowski, J. F., Haus, J. M., Hoddy, K. K., & Calvo, Y. (2013). Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutrition Journal, 12(1). https://doi.org/10.1186/1475-2891-12-146 have found that relative to other types of weight loss diets, the intermittent fasting weight loss approach may help spare muscle loss more so.7Klempel, M. C., Kroeger, C. M., Bhutani, S., Trepanowski, J. F., & Varady, K. A. (2012). Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women. Nutrition Journal, 11(1). https://doi.org/10.1186/1475-2891-11-98
This is to say that intermittent fasting muscle loss may be less significant than a regular caloric restriction weight loss diet.8Varady, K. A. (2011). Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss? Obesity Reviews, 12(7), e593–e601. https://doi.org/10.1111/j.1467-789x.2011.00873.x
The intermittent fasting diet approach may therefore help preserve lean body mass while losing weight more so than a traditional weight loss diet wherein you cut calories (eat fewer calories) every day or restrict certain food groups.
However, one potential concern in terms of intermittent fasting leading to muscle loss is that a review of studies found that intermittent fasting decreases testosterone levels in lean, physically active, young males and decreases androgen markers (i.e., testosterone and the free androgen index (FAI)) in premenopausal females with obesity.9Cienfuegos, S., Corapi, S., Gabel, K., Ezpeleta, M., Kalam, F., Lin, S., Pavlou, V., & Varady, K. A. (2022). Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials. Nutrients, 14(11), 2343. https://doi.org/10.3390/nu14112343
Because testosterone plays a key role in building muscle and preserving lean body mass, this evidence suggests that there may be a risk of losing muscle with intermittent fasting.
Can You Build Muscle While Intermittent Fasting?
There have not been very many studies looking at whether you can gain muscle while intermittent fasting.
Unless you are in a caloric surplus, generally along the lines of about 10% over your daily caloric expenditure, it is generally quite difficult to build muscle.10Tipton, K. D., & Wolfe, R. R. (2001). Exercise, Protein Metabolism, and Muscle Growth. International Journal of Sport Nutrition and Exercise Metabolism, 11(1), 109–132. https://doi.org/10.1123/ijsnem.11.1.109
Therefore, typically fat loss and muscle gains are at odds in terms of the nutritional approach; losing fat requires being in a caloric deficit while building muscle is best supported by being in a caloric surplus.
This is why bodybuilders go through a bulking phase to build muscles first and then a cutting phase to lose body fat because the diet and exercise approach to losing fat vs building muscle are almost opposite.
That said, there are advanced training tactics and dieting approaches where you can have some degree of body recomposition. This refers to losing fat while building muscle simultaneously.
Evidence suggests that the most effective diet to follow when trying to build muscle in a caloric deficit (which is necessary during the cutting phase of bodybuilding) is to consume 2.3-3.1 g/kg of lean body mass per day of protein, 15-30% of your total calories from fat, and the remainder from carbohydrate.11Helms, E. R., Aragon, A. A., & Fitschen, P. J. (2014). Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. Journal of the International Society of Sports Nutrition, 11(1). https://doi.org/10.1186/1550-2783-11-20
However, many people follow intermittent fasting diets not for weight loss or fat loss but for the potential health benefits of intermittent fasting and increased energy.
For these individuals, weight loss may be the opposite of their goal, and rather, they might be trying to build muscle.
Studies are limited on the effectiveness of intermittent fasting for muscle growth, but it is likely not the best diet plan to build muscle unless you are strategically timing your eating windows and fasting windows around your workouts.
Most research looking at the most effective dietary approach to building muscle suggests that it is important to consume adequate protein and carbs immediately after heavy resistance training and to continually take in about 20 to 30 g of protein every four hours rather than in large doses only a few times per day.12Mamerow, M. M., Mettler, J. A., English, K. L., Casperson, S. L., Arentson-Lantz, E., Sheffield-Moore, M., Layman, D. K., & Paddon-Jones, D. (2014). Dietary Protein Distribution Positively Influences 24-h Muscle Protein Synthesis in Healthy Adults. The Journal of Nutrition, 144(6), 876–880. https://doi.org/10.3945/jn.113.185280
By nature, if you are restricting your eating windows with periods of fasting, it will be impossible to spread out your protein intake.
Protein absorption may be compromised if you are only eating one or two big meals per day and fasting the rest of the day since there seems to be a limit on how much protein can be effectively utilized for muscle protein synthesis at one time.
For example, one study looked at the effects of intermittent fasting coupled with resistance training and found that it might be harder to gain muscle from intermittent fasting.13Tinsley, G. M., Forsse, J. S., Butler, N. K., Paoli, A., Bane, A. A., La Bounty, P. M., Morgan, G. B., & Grandjean, P. W. (2016). Time-restricted feeding in young men performing resistance training: A randomized controlled trial. European Journal of Sport Science, 17(2), 200–207. https://doi.org/10.1080/17461391.2016.1223173
This study involved untrained young men who tried intermittent fasting alongside an eight-week resistance training program.
Those who were on the intermittent fasting diet were able to maintain lean body mass and increase strength but those who ate normally with the weight training gained 5 pounds (2.3 kg) of lean mass, while also increasing their strength.
The intermittent fasting diet seemed to prevent this hypertrophy benefit.
One potential way in which intermittent fasting may help you build muscle (as long as you are resistance training) is because prolonged fasting has been shown to increase levels of human growth hormone (HGH).14Ho, K. Y., Veldhuis, J. D., Johnson, M. L., Furlanetto, R., Evans, W. S., Alberti, K. G., & Thorner, M. O. (1988). Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. Journal of Clinical Investigation, 81(4), 968–975. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC329619/
Human growth hormone (HGH) supports enhanced muscle building and cellular repair and accelerates fat burning.15Chikani, V., & Ho, K. K. Y. (2013). Action of GH on skeletal muscle function: molecular and metabolic mechanisms. Journal of Molecular Endocrinology, 52(1), R107–R123. https://doi.org/10.1530/jme-13-0208
Indeed, one of the benefits of heavy resistance training is that it can increase levels of HGH, which is why strength training can put you in an anabolic state (to build muscle).
Shorter intermittent fasting protocols may help boost HGH levels, but generally, it takes fasting for a 24-48 hour window to see a significant spike.16Kerndt, P. R., Naughton, J. L., Driscoll, C. E., & Loxterkamp, D. A. (1982). Fasting: the history, pathophysiology and complications. The Western Journal of Medicine, 137(5), 379–399. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1274154/
Keep in mind that with any diet, to prevent muscle loss, it’s important to eat enough protein,17Pasiakos, S. M., Cao, J. J., Margolis, L. M., Sauter, E. R., Whigham, L. D., McClung, J. P., Rood, J. C., Carbone, J. W., Combs, G. F., & Young, A. J. (2013). Effects of high-protein diets on fat-free mass and muscle protein synthesis following weight loss: a randomized controlled trial. FASEB Journal: Official Publication of the Federation of American Societies for Experimental Biology, 27(9), 3837–3847. https://doi.org/10.1096/fj.13-230227 spread out your protein intake,18Areta, J. L., Burke, L. M., Ross, M. L., Camera, D. M., West, D. W. D., Broad, E. M., Jeacocke, N. A., Moore, D. R., Stellingwerff, T., Phillips, S. M., Hawley, J. A., & Coffey, V. G. (2013). Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis. The Journal of Physiology, 591(9), 2319–2331. https://doi.org/10.1113/jphysiol.2012.244897 strength train at least 3 times per week,19Campbell, W. W., Haub, M. D., Wolfe, R. R., Ferrando, A. A., Sullivan, D. H., Apolzan, J. W., & Iglay, H. B. (2009). Resistance Training Preserves Fat-free Mass Without Impacting Changes in Protein Metabolism After Weight Loss in Older Women. Obesity. https://doi.org/10.1038/oby.2009.2 and only create a slight caloric deficit—no more than 500 calories a day.20Chaston, T. B., Dixon, J. B., & O’Brien, P. E. (2006). Changes in fat-free mass during significant weight loss: a systematic review. International Journal of Obesity, 31(5), 743–750. https://doi.org/10.1038/sj.ijo.0803483
Having occasional refeeding days (aka “cheat days”) may also help prevent your body from going into starvation mode and catabolizing muscle.
If you are looking to build muscle, check out our hypertrophy rep range guide:
