Weight loss and muscle loss can be the result of extreme dieting, malnutrition or disease. Muscle loss eventually occurs when the body has to burn muscle tissue to provide vital energy in the absence of fat. Fat is the main source of energy for the body under healthy conditions, but muscle mass contains more energy than the same amount of fat tissue. The conversion of muscle tissue to energy is called catabolysis.
Extreme dieting or starving can cause muscle loss. Each person's body has a set calorie requirement to fulfill base metabolic needs for survival and carry out non-vital functions. All foods and liquids put into the body contain calories, which provide immediate energy or are stored as fat. Weight loss occurs when the energy input is less than the energy expenditure. An energy input/output equilibrium, a high-protein diet and an exercise regimen are necessary to prevent muscle loss.
Weight loss and muscle loss can begin taking place several days into a period of fasting. After short-term reserves of carbohydrates are depleted, the body taps into the fat reserves and uses them for fuel. The exact time until muscle loss begins varies depending on the person's existing fat reserves, but it generally takes place in two or three days. Starvation mode, in which metabolic rates drop and the body begins consuming muscle, occurs when the body receives less than half its recommended calorie intake. Generally, a daily intake of 1,200 calories or fewer triggers starvation mode.
A person's preexisting body composition can affect weight loss and muscle loss. People who have a higher percentage of fat lose fat more easily, and people who have a low body fat percentage lose lean mass more easily, such as cardiac muscle tissue. Obese people tend to have a higher overall amount of lean body mass, such as bones, muscles and cardiac tissue, to aid in moving and supplying the body with enough blood.
Excessive exercise, or over-training, causes weight loss and muscle loss. For example, aerobic exercise causes existing muscle proteins to be broken down and used for energy during exercise. Cardiovascular exercise in particular causes weight loss; it places a large stress on the heart and skeletal muscles of the body, increasing metabolism to keep up with the energy demands. Training excessively takes away from the time the body uses to rebuild and replace torn or catabolized muscle tissue, eventually leading to muscular atrophy. Eating a balanced diet and exercising in moderation prevents unhealthy weight loss.