What happens when you lose muscle? When you are working to lose weight, about 20% of what you lose will be muscle mass. It can potentially set off a cascade of problems. Muscle burns more calories than fat, so losing it slows metabolism. With a lower metabolic rate, the body needs fewer calories to maintain weight, making regain more likely. Muscle also helps fight inflammation, while fat tissue acts as an inflammatory organ. When muscle disappears, inflammation takes over. Then comes fat infilt