One common concern with Intermittent Fasting is worry that it will cause muscle loss, as your body is put into a fasting state so often.
We put so much time and effort into picking up those heavy things to gain such precious muscle, it makes sense we don’t want to sabotage those efforts!
In fact, studies have shown that indeed you are able to gain muscle just as well while intermittent fasting as with a normal high-protein diet.
The key is to eat THE SAME AMOUNT of calories and protein as you would outside of IF, just eat it all during that shorter window.
Does your body burn fat or muscle first when fasting?
The short answer: fat.
It takes 2-3 days of fasting for your body to start really breaking down its healthy muscle tissue for energy. However, after about 12 hours of fasting, the body will start breaking down tissues that are already damaged. This leads to most of the incredible health benefits I discussed in this previous post.
Can I build muscle AND lose fat at the same time with Intermittent Fasting?
Short answer: Probably. This is the closest you can get to doing both but you still have to prioritize one or the other.
The key is how many calories you eat every day, even with intermittent fasting.
It’s important to know how many calories your body burns in total every day – AKA your total daily energy expenditure (TDEE). (TDEE calculator here)
In general, if you eat more calories than your TDEE, you will gain weight (both fat and muscle) and if you eat less than your TDEE, you will lose weight (both fat and muscle).
You can definitely increase the amount of muscle gained (or decrease muscle lost) by doing more resistance training. (Cardio exercise does not change these ratios)
Note: Many studies have been done to see if changing the ratios of macros (protein, fat, or carbs) you eat can change fat/muscle gained/lost but no conclusive data has been found.
However, early research shows that timing of eating (intermittent fasting) can also affect how much fat/muscle is gained/lost.
Seven scientific studies (summarized in Varady, 2011) showed that calorie restriction (eating less than your TDEE) using intermittent fasting retained lean muscle mass better than calorie restriction that did not limit time. All subjects lost weight with calorie restriction but those that ate in intermittent fasting period lost more weight from fat than muscle. These studies did not involve resistance training or any specific exercise activity.
One study (Tinsley, 2017) showed that when subjects ate intermittent fasting in calorie deficit AND did strength training, they gained strength (measured by 1-rep max or 1RM) and maintained the same amount of muscle they started with.
Finally, the best study on this topic had subjects eat in slight calorie excess (as one would when trying to build muscle but minimize fat gained). One group ate all their calories only during an 8 hour period each day and the other group ate the calories throughout the day. Both groups did the same strength-training routine. At the end of the study, both groups gained the same amount of muscle (and strength) but the intermittent fasting group gained less fat.
Interestingly, a study done in mice (Panda, 2016) showed that in mice fed the same number of calories they expended in a day, the mice in the IF group did increase their lean muscle mass while simultaneously decreasing their fat mass. So it definitely can be done in mice and with good diet control and solid strength training routine, you probably can do it in your human body as well.
Is Intermittent Fasting Good for Body Building?
Short answer: yes!
This is similar to above but the goals of body building are generally to gain muscle in specific ways/areas and to decrease the subcutaneous fat overlying the muscle to make it show more.
As we saw in the section above, intermittent fasting (particularly daily IF) is great for losing fat while retaining or even gaining muscle. And the best way to gain muscle while losing fat in intermittent fasting is by intentional and challenging strength training.
How many calories do I need to eat to build muscle while intermittent fasting?
The short answer: Depends on your starting body fat percentage.
If your body fat percentage is >30%, then you should eat slightly less than your TDEE while strength training. (roughly calories eaten daily = TDEE*0.9 or 90% of TDEE)
If your body fat percentage is 20-30%, you should eat about equal TDEE while strength training.
If your body fat percentage is <20%, you should eat slightly more than your TDEE while strength training. (roughly TDEE*1.1 or 110% of TDEE)
**Safety note: If you’ve been told a specific diet or calorie goal by your own personal physician or dietician, then you can ask them if these numbers would work for you but ultimately go by what they recommend for you specifically.
The great news is you CAN build muscle while intermittent fasting.
AND, intermittent fasting is the best way (as shown by science) to build the most muscle while minimizing fat gain as well as minimize muscle loss while trimming.
Moro, T., Tinsley, G., Bianco, A., Marcolin, G., Pacelli, Q. F., Battaglia, G., Palma, A., Gentil, P., Neri, M., & Paoli, A. (2016). 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), 290. https://doi.org/10.1186/s12967-016-1044-0
Panda S. (2016). Circadian physiology of metabolism. Science (New York, N.Y.), 354(6315), 1008–1015. https://doi.org/10.1126/science.aah4967
Tinsley, G. M., Forsse, J. S., Butler, N. K., Paoli, A., Bane, A. A., La Bounty, P. M., Morgan, G. B., & Grandjean, P. W. (2017). 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
Varady K. A. (2011). Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?. Obesity reviews : an official journal of the International Association for the Study of Obesity, 12(7), e593–e601. https://doi.org/10.1111/j.1467-789X.2011.00873.x