This is only true up to a point. A colleague put me onto a fasting method called “Buchinger’s Fasting”. I fasted for around a week and a half, muscle maintenance was not a concern. After 3 days, your body burns through the glucose reserves and you get brain fog. After that, indeed, your body tried to break down muscle mass for fuel. The trick is to do some moderate exercise to keep your muscles in good condition. After a few days, your body finally turns to your fat reserves, and rapidly starts burning away fat. After the first 3 or so days, you actually become numb to the hunger and go into a slightly “high”, hunter gatherer alert mode that enabled our ancestors to search for food when hungry. It really brings home that being hungry and not being fed constantly is a normal part of human existence, and the industrial era, shoving food down our gullets constantly is making us sick and prevents natural healing within our bodies.