This is a question that is asked very often. To answer we must first explain what happens inside our body during starvation, that is, the total deprivation of food in a definitive way. Suppose we stop consuming food at 8:00 a.m. On a Monday, for the next 24 hours, the body will be using, as usual, the glycogen stored in the muscles and liver to stabilize blood glucose levels as it is used as a fuel source. Muscle glycogen provides energy for the first 24 hours or so before being depleted. At the same time, body fats are used to also transform into glucose by a process called gluconeogenesis. As glycogen is depleted, the body increases the use of fats, until it is depleted and fats become the main fuel for the body's processes. During this time, proteins are also used, but very little.
After about three or four days, which in our case would be from Wednesday to Thursday, the amount of ketone bodies begins to increase due to the conversion of fats into energy. The brain, which normally uses glucose as a fuel source, has an affinity for ketone bodies and begins to use them. The brain uses approximately 80 grams of glucose per day for its proper functioning, ketone bodies cover about 50 grams of that glucose, so there would be 30 grams left to cover. In the liver, glycerol is produced from the lipolysis process, which covers about 20 grams of fuel for the brain. That means that we are left with a daily deficit of 10 grams. The body proceeds to obtain those 10 grams from proteins.
As the period of total starvation stretches for weeks, and the body fat reserve drops to less than 3-4%, the body will begin a process of massive muscle catabolism to meet the demands of the most important organs until the death. In short, the body requires about 50 days of starvation to consume one pound of muscle mass. It is physiologically impossible for a person to lose a considerable amount of muscle mass through diet and exercise. This tiny loss of muscle is even less if we consume the adequate amount of protein when we are eating low-calorie diets to lose weight