This seems counterintuitive. I mean, if my bills exceed my income, *but* I have a million dollars in the bank, I can still buy new stuff.
I can spend the energy from fat reserves on any amount of exercise, after all – so why can’t I spend it on building muscle? Why does *that* energy have to come from a caloric surplus?
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Your body isn’t built like that. Calorie deficit tells your body to cut spending (stop building muscle and/or cut muscles), because fat reserves are last-ditch resources. Hence, why burning fat is so difficult.
The body doesn’t treat this as just one big pool of energy. It treats different types of energy differently. Fat is long term storage. It doesn’t want to spend this if it doesn’t have to, and building muscle beyond a certain level required for basic function isn’t perceived by cells as a good use of stored energy. At a calorie deficit, the body is in longevity mode, not growth mode. It wants to keep you alive for as long as possible so it’s going to spend less energy on other things.
It isn’t just calories taken in in that creates muscle mass, it’s calories consumed in specific forms. Fat cannot be converted into muscle because muscle is made from different materials. To stick with your analogy, protein is needed to create muscle mass and fat is like a foreign currency that doesn’t have an exchange rate for protein.
You actually can gain some muscle on a calorie deficit.
*But* there are serval caveats:
You need to be in only a minor calorie deficit.
You need a diet that is still high enough in protein. (You need to eat protein in order to build muscle, your body can’t turn fat into muscle.)
You need to have a large enough fat reserve.
You need to not have a lot of muscle in the first place.
You need to follow a good lifting program. (duh)
So if all of those line up you can indeed gain some muscle while losing a bit of weight, but for a lot of people it is really difficult to ensure that all of those are perfectly lined up.
Muscle protein synthesis is a pretty low priority objective for the body, behind everything it takes to keep you alive. Fat is energy, and your body uses that to keep you alive. More important to keep the organs and the muscle you have energized so you can go get some food
According to those over on r/ketogains this is possible to do. Like another commenter said it comes down to everything lining up properly. So let’s say you only ate lean steak or lean chicken to ensure the correct amount of protein needed and were lifting appropriately, your body can build muscle while being in a deficit. But most don’t have the discipline to eat that much protein while being in a deficit. But it is possible. Just difficult for most people. You also have to have a surplus of fat or else find a way to eat fat while being in a deficit.
Muscle is made of protein. Proteins are made up of amino acids. Fat is made up of fatty acids, (triglycerides) which are long carbon chains. So, even with energy in “reserve” fat will not break down into the necessary building blocks to make proteins, therefore, no muscle building is possible.
Open to corrections/clarifications
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