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Intermittent fasting is one of the most popular diets right now.
There are different types, but they all have one thing in common: they last longer than a normal overnight fast.
Studies have shown that intermittent fasting can help you lose fat. But maybe you're one of the people wondering whether it can also cause muscle loss?
In this article, I'll summarize everything you need to know about the effects of intermittent fasting on your muscles. I combine scientific studies with my experience as a personal trainer in Zurich.
Types of intermittent fasting
Intermittent fasting is a broad term that describes many different types of fasting. These are the most common types:
Time-restricted eating
With time-restricted eating, all calories are consumed within a specific time window.
This window ranges from 4 to 12 hours, with 8 hours being the most common.
Alternate-day fasting
With this type, you alternate between fasting days and non-fasting days. Some people eat nothing at all on fasting days, but it's more common to limit yourself to one meal a day.
Periodic fasting
Periodic fasting, also known as whole-day fasting, consists of occasional fasting interrupted by several days or weeks of normal eating.
Although the definitions vary, fasting for one or more days every 1-4 weeks is often referred to as periodic fasting.
The 5:2 diet
The very popular 5:2 diet resembles alternate-day and periodic fasting.
You eat normally five days a week, and on the remaining two (non-consecutive) days, you consume around 25% of your normal calorie needs.
Do you lose muscle while fasting?
Almost all studies on intermittent fasting have been conducted with weight loss in mind.
It's important that you understand that weight loss without training usually leads to a loss of both fat and lean mass. Lean mass includes everything except fat, including muscle.
This applies to any form of weight loss, whether through intermittent fasting or other diets.
Some studies show that small amounts of lean mass (around 1kg) can be lost after several months of intermittent fasting.
Other studies, on the other hand, have shown no loss at all.
Some scientists believe that intermittent fasting is more effective at preserving lean muscle mass than other diets. More studies simply need to be done on this.
At this point, it looks like intermittent fasting probably doesn't cause more muscle loss than other diets.
It's probably not the best method for building muscle
There are very few studies on whether it's possible to build muscle while doing intermittent fasting.
In one study, 18 young men underwent an 8-week weight training program. None of them had trained regularly before.
The men either followed a normal diet or a time-restricted eating program. Four days a week, they consumed their calories within a 4-hour window.
By the end of the study, the intermittent fasting group had maintained their lean body mass and increased their strength. The group that ate normally, on the other hand, gained 2.3kg of lean mass and also increased their strength.
This result could suggest that intermittent fasting isn't optimal for muscle growth. This is probably because less protein is consumed with this approach.
To build muscle, you need to eat more calories than you burn. You should also consume enough protein to build new muscle tissue (sources: 1, 2, 3).
Intermittent fasting makes it harder to consume enough calories and protein.
Some studies also indicate that it's beneficial for muscles when protein is consumed regularly throughout the day (sources: 1, 2).
All of this doesn't mean it's impossible to build muscle while doing intermittent fasting, but it does suggest that it's not the optimal approach for it.
Weight training can help you maintain muscle mass while intermittent fasting
Scientific studies show that weight training during a diet can help prevent muscle loss, including specifically when it comes to intermittent fasting.
An 8-week study, for example, examined the combination of intermittent fasting and weight training. One group of experienced athletes ate normally, while another only consumed food within an 8-hour window each day.
Neither group had lost muscle mass or strength, but the intermittent fasting group lost 1.6 kg of fat.
Another study showed that 25-40 minutes of training on a bike or elliptical trainer can preserve muscle mass during intermittent fasting.
So, in general, physical training is highly recommended alongside intermittent fasting.
Should you train while fasting?
A 4-week study examined treadmill training in both the fasted and non-fasted state.
Both groups lost the same amount of weight and fat, and neither group showed a difference in muscle mass.
This study suggests that it hardly matters whether you do cardio training on an empty or full stomach, as long as you're not doing high-performance sport.
It seems to come down to personal preference. That's also what my experience in personal training in Zurich shows. However, it's advisable to consume at least 20 grams of protein after training to support your muscles.
Nutrition strategies to support your muscles
If you decide to do intermittent fasting, there are a few strategies you can use to help you keep as much muscle mass as possible.
Studies show that you're more likely to lose muscle mass if you lose weight quickly. So try not to restrict your calorie intake too much.
0.5 - 1 kg per week is recommended. If your goal is to preserve muscle mass, less is more.
Regardless of which diet you choose, it's important to consume enough protein. Around 1.6 grams of protein per kilogram of body weight is recommended.
Supplements to support your muscles
Be especially careful with supplements during intermittent fasting, so you don't break your fasting cycle.
Supplements during eating periods
Protein and creatine are the two important supplements.
If your diet already contains enough protein, a protein supplement isn't necessary.
Creatine is a molecule in your body whose levels you can increase with a supplement - which is especially helpful when you're training. The strength gains from creatine are estimated at an average of 5-10% (sources: 1, 2).
Supplements during fasting periods
If you take supplements during your fasting window, even if it's just BCAAs, you signal to your body that you're not fasting.
If you consume enough protein during your eating window, fasting for 16 hours is not a problem for your muscles.
So it's best to take your regular supplements together with your food.
Conclusion
Intermittent fasting, in whatever form, probably doesn't cause more muscle loss than other diets.
Nevertheless, exercise, especially weight training, helps preserve muscle mass during intermittent fasting.
Whether you train during your fasting period or not is up to you - though it probably has no benefits either way.
What's important is that your goal is slow and steady weight loss, since that helps you avoid losing muscle mass.
If you're looking for professional support with your fitness and nutrition plan, I can help you as a personal trainer in Zurich. Just send me an email, I'd love to hear from you!