Some critics of fasting claim that it leads to muscle loss. But the scientific literature does not support this claim. Protein utilization actually declines during a fast, as the body protects functional tissue. The breakdown of muscle only occurs once available fat stores are depleted.
One claim is that the body needs to break down protein to supply the brain with glucose. On a standard diet with unrestricted carbohydrates, the brain needs somewhere between 80-140g of glucose per day. But the body can break down triglycerides from fat to produce glucose without any use of protein in a process called gluconeogenesis .
The body actively preserves lean muscle mass (via growth hormone). Which seems to make logical sense: muscle is required to find food, while fat is highly effective energy storage. In periods of fasting or famine, the body burns fat, not muscle.
David Blaine DB David Blaine fasted for 44 days suspended in a plastic box suspended near the Thames in London. Blaine’s physical activity was severely restricted because he was confined to a very small space. Blaine lost 10kg of muscle. This was more likely due to the lack of physical activity — atrophy — than due to the fasting.