In a nutshell, every tissue in your body needs a certain amount of calories to keep it alive throughout the day...to keep oxygen going to the cells, to fuel that cell's metabolic processes, etc. Different types of tissues require different amounts of fuel. Fat, for example, needs very little fuel to keep it alive...it doesn't do much. Muscle, on the other hand, needs comparatively a LOT of fuel to keep it alive, even if you're not using it. So if you have more muscle, you burn more calories all throughout the day, even when you're in bed or sitting on the couch, because the muscle needs additional calories to keep it going.
For comparison, a pound of fat burns something like 2-5 calories per day to keep it going. A pound of muscle, on the other hand, burns an estimated 20-50 calories a day.
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