ELI5, what happens when you consume more calories than you use, if you are working out? Do you gain weight as muscles, fat or both?
In: Biology
Lets change around your question slightly to read “What happens when your body absorbs more calories than it uses.” Remember that not everything you eat is digested and absorbed, and some people may absorb less calories from a given amount of food than other people. So it’s more about how many calories a persons body absorbs (even though most of what they eat is absorbed).
That being said, it depends on you. If you have recently been fasting, or haven’t ate in several days, the excess calories will probably go immediately to restocking the liver with glycogen, which is your primary source of ‘fast’ energy.
The liver has several functions but one of the most important for calorie maintenance is that it acts like a kind of battery, storing energy it can quickly deploy to the body. For instance your liver is what is powering your body between meals (muscles also store local energy for muscle needs) , when you’re sleeping, etc. However the liver has a finite amount of energy it can store. Once it’s exhausted, it needs to be replenished either by eating, or by burning fat through a process called ketosis.
This is why the first day or two of a diet are so hard, because the body isn’t actually dieting yet, it’s just burning energy stored in the liver, hoping you’re going to eat soon. Then when you don’t, it finally starts ramping up fat burning production to start replacing the lost energy. Until it switches to fat burning mode, you’re feeling weak and jittery.
Anyway, say your liver is already topped off, then most likely your excess calories are going to get stored as fat. “But won’t some of it be used to rebuild muscles and bones etc?” yes but you said if you have calories that are more than you need, so if the needs of muscle and bone repair have already been met, and you take in more calories, it’s going to be stored as fat. The body doesn’t really have a strategy for storing long term energy as protein or carbohydrates. It stores short term energy in the liver and muscles, but all long term energy is fat.
Eventually if you’re starving to death long enough, the body will start breaking down your muscle mass for energy as well, but that’s not really because it uses muscles to store long term energy, so much as an emergency crisis because you ran out of fat and you’re starving to death.
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