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When you lose weight too quickly, your body cannot maintain its muscle. Because muscle requires more calories to sustain itself, your body begins to metabolize it so that it can reserve the incoming calories for its survival. It protects it fat stores as a defense mechanism to ensure your survival in case of future famine and instead use lean tissue or muscle to provide it with calories it needs to keep its vital organs such as your brain, heart, kidneys and liver functioning. If you reach a point where you have very little fat or muscle, your body will metabolize your organs to keep your brain functioning leading to heart attack, stroke and liver and kidney failure.
As the body loses more muscle mass, the body's overall metabolic rate decreases. The metabolic rate is the rate at which the body burns calories and is partly determined by the amount of muscle you have.
So the more muscle you have, the higher your metabolic rate; the less muscle you have, the lower your metabolic rate and fewer calories you burn. This explains why it is crucial to protect your metabolic rate and not have muscle loss.