Are you planning to lose weight but you don’t know whether you should go the “normal” way of calorie restriction with eating small frequent meals or whether you should try some kind of fasting approach?
The fitness industry has been very successful in promoting the idea that we have to eat every 2 hours to protect our muscle mass and even though I followed this approach for many years, it didn’t work very well for me.
Eating small portions throughout the day was just enough to keep me hungry all the time but when I heard about intermittent fasting I got pretty excited. In general, intermittent fasting means to fast for a some time and then eat all calories in a short eating window. Common examples are fast of 12, 16 or 20 hours but also every other day fasts are becoming more popular.
Before I started with intermittent fasting, however, I needed to be convinced that it would not somehow lead to increased muscle loss during dieting and therefore, I did what every scientist would do and looked into the research. Here is what I found:
Intermittent fasting and muscle loss
A study that was published in 2016 came in really handy for me. 
In an 8-week trial, researchers split 34 men who were experienced in weight training into two groups and both groups were assigned the same amount of calories and protein per day. The groups only differed in the meal timing. One group performed intermittent fasting with an eight hours eating window, while the second group followed a normal diet with breakfast, lunch and dinner.
By the end of the study, neither group had lost lean mass or strength. However, the time-restricted group lost on average 1.6 kg of fat, while there was no significant fat loss in the normal diet group.
This shows that intermittent fasting might actually be a better tool to protect muscle mass and at the same time speed-up fat loss but of course bigger studies are needed to really validate this.
There is a potential mechanism behind that explains why fasting protects from muscle loss. Researchers found, that fasting increases the production of the human growth hormone, which stimulates muscle growth and fat burning. 
A study found that a 24-hours fast more than doubled the growth hormone release with an even further increase after 5 days fasting. 
The study also found that the concentration of the ketone body ß-hydroxybutyrate increased extremely during fasting.
When the body is in a state of starvation or when on a ketogenic diet, the body starts to break down fat to produce ketone bodies and besides their role as an alternative energy source to glucose, ketone bodies seem to have additional benefits, including the fact that they protect muscle loss.
Prolonged Fasting - The Tim Ferriss Method
An N=1 example for the powerful muscle-protective properties of ketone bodies is provided by the “human guinea pig” and bestselling author, Tim Ferriss. In his book, Tool of Titans, Tim talks about two different fasts he did:
One was a 7 day fast with clinical supervision. He was stationed in a hospital and not allowed to exercise during the fast and he actually ended up losing about 6kg of muscles mass.
During another fast, however, Tim went without eating for 10 days and was able to maintain all his muscles by applying some small tricks. He consumed small amounts of branch chain amino acids and about 300 to 500 calories of pure fat mainly in form of exogenous ketones to fasten the transition into the muscle preserving state of ketosis.
To get into ketosis as fast as possible, Tim also went on a 3-4 hour walk the first day of fasting to deplete his glycogen stores and further speed up the transition into ketosis.
The muscle preserving function of ketone bodies is by no means a new concept. A study published in 1988 used intravenous infusions of the ketone body ß-hydroxybutyrate and looked at the effect on protein synthesis by tracking the metabolism of the amino acid leucine. The researchers found that leucine oxidation was decreased by an average of 30% and incorporation of leucine into skeletal muscle protein increased by 5-17%, meaning that ß-OHB reduced muscle degradations and promoted muscle synthesis compared to the control. 
Somebody who helped Tim to create his successful fasting approach is Dr. Dominic D’Agostino, who is a researcher studying the effect of ketone bodies on human health, including the anticatabolic effects of ketone bodies and I am sure there will be more research coming out soon. 
For me personally, intermittent fasting with a daily 16 hours fast paired with a ketogenic diet is probably the best way to lose some body fat while minimizing muscle loss and I also crave food less.
Longer fasts like the one Tim Ferriss did seem to be another way to accelerate fat loss but longer fasts should be performed carefully to avoid potential negative side effects that might appear. So, I suggest that you inform yourself before you start a prolonged fast. In this video, Dr. Phinney talks about potential side effects of prolonged fasting - just to give you both sides of the medal.
Moro 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, J Transl Med, 2016
Espelund et al., Fasting Unmasks a Strong Inverse Association between Ghrelin and Cortisol in Serum: Studies in Obese and Normal-Weight Subjects, J Clin Endo & Meta, 2005
Ho et al., Fasting Enhances Growth Hormone Secretion and Amplifies the Complex Rhythms of Growth Hormone Secretion in Man, J. Clin. Invest. 1988
Nair et al., Effect of beta-hydroxybutyrate on whole-body leucine kinetics and fractional mixed skeletal muscle protein synthesis in humans., J Clin Invest, 1988
Koutnik et al., Anticatabolic effects of ketone bodies in skeletal muscle, Trends in Endocrinology & Metabolism, 2019