Some people do lose some muscle when fasting, especially the people that do fasting more often and longer than others.
However not everyone will lose muscle when fasting.
Fasting is the temporary stopping of eating and sometimes drinking for a period of time.
From a purely physiological context, "fasting" can refer to the metabolic status of someone who has not eaten overnight, or to the metabolic state achieved after complete digestion and absorption of a meal.
During the night when we are sleeping our bodies are actually fasting which is why you should eat breakfast in the morning to break the fast.
Common fasting methods involve fasting for 16 hours and having an 8-hour eating window.
Some people who fast do 24-hour fasts twice per week.
Fasting has been a practice throughout human evolution.
As a result of fasting, humans evolved to be able to function without food for extended periods of time.
When you're fasting your body uses your fat to give you energy and when you burn that fat off for energy while fasting then you lose that weight.
Fasting alone does help you lose weight but you should not fast too often as it could make you sick and cause you to faint.
However fasting at least once or twice per week is a good weigh to aid you in weight loss.
You should also watch your calorie intake and get plenty of exercise and eat foods low in calories.
For example eating a salad without any added meat or cheese is a good way to give you energy but also keep your calorie intake down.
The less calories you consume the more calories you should burn off and in turn you should lose the weight.