eli5: why does having more muscle mass increases bmr (Basal metabolic rate)
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Muscle takes energy to maintain, and supplying muscles means your body keeps more energy in the form of short-term storage molecules (so that it can burn more energy on demand) rather than long-term storage like fat.
All the tissues of the body need a constant supply of energy to stay alive and carry out their functions.
Muscle is no exception. All muscle consumes energy, so the more muscle you have, the more energy your body consumes.
To build on other replies, the reason muscle in particular increases BMR is because it takes more energy to maintain than fat. To simplify, complicated things need more maintenance to stay functional than simple things do. Like how an iPhone breaks in a couple years, but your home phone likely lasted for a decade or more. Or why a tall tower of blocks takes more energy to build than a pile of blocks. Muscle cells need to be able to contract in response to nerve signals, moving part or all of your body in the process. Fat cells mostly need to fill themselves full of fat and then sit there until your blood sugar drops and they need to release some of it. So muscle takes more energy to keep repaired, because it’s more complicated and doing more stuff than fat.
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