There are a lot of myths surrounding fasting and muscle loss. Fasting is a popular way of losing weight. People find that if they simply ‘don’t eat’ for a while, they are less likely to end up suffering from cravings, and they are less likely to want to snack, which means that it’s easier for them to stay under their daily calorie goal.
Some people are wary of fasting, because they worry that when their glycogen stores get depleted, their bodies will enter a catabolic state, burning muscle for energy instead of fat. In the long term, this means poorer body composition, a significantly slower metabolism, and an increased likelihood that you would end up regaining the weight that you had lost.
For the most part, this is a myth. It’s true that if you starve yourself, your body will start burning both muscle and fat for energy, and the closer you get to your ideal weight the more muscle you will lose (and the more fat you will hold onto), but your body doesn’t go into a “starvation mode” just because you eat a little less or a little less often. Meal timing does not start having an impact on body composition to any noticeable degree until you are already fairly lean.
In addition, it’s possible to mitigate a lot of the muscle loss simply by lifting heavy weights as a part of your exercise regimen. If you are burning a lot of calories doing cardio, and not eating very much, you will lose muscle at a noticeable rate. If you eat at a slight calorie deficit over the course of the day, and you are lifting weights, your body will preferentially burn fat, because that’s what it needs to do.
Fasting typically involves just ‘not eating’ for something like 16 hours or maybe 20 hours, and putting all of your calories into one ‘eating window’ at a set time of day. Some people might not eat at all for a whole day, but eat normally other days, or eat a very small number of calories on their fast days, and eat normally the rest of the week. Various intermittent fasting reviews discuss these methods and their constraints. None of these eating methods are going to have much of an impact on you compared to normal eating patterns, other than making it easier to sustain a calorie deficit. They are simply ways of eating less.
The human body is amazingly efficient at getting fuel from multiple sources. Yes, glycogen is more readily available than fat, but when you run out of glycogen you transition to fat burning pretty smoothly and quickly – if you didn’t, you would faint every time you skipped a meal. Healthy people don’t do that; they may feel a little drained for a few minutes, but nothing more.
Don’t agonize over nutrient timing. Our ancestors ate when food was available, and felt hungry when it wasn’t. Our bodies are designed around that way of survival, and skipping a meal or to will not harm us in the long term.