system
Given the following post, respond as if you're commenting on Reddit:
human
ELI: How does muscle growth and decay work?

If I remember from biology class correctly, large muscles are a result from large sarcomeres, not more of them. So how does working out (i.e. tearing and rebuilding muscles) make them bigger? And why do they get smaller if they're not used?
gpt
Your body is like anything in life that needs "fuel".There are 3 Macro - Nutrients. Protien/Fats/Carbohydrates. Fats and carbs are the primary "fuel" and the protein is the building blocks or new cells. Exercise breaks down muscle fibers which causes micro tears. These are repaired stronger and bigger to try to stop the muscle breaking down again if enough protein has been consumed. Lack of protein will cause limited muscle growth and cause them not to repair to the same or or bigger than before. Too little carbs/fats your body uses stored body fat to fuel your body. Too little for long periods of time it then breaks down muscle as a final back-up to keep your body running. Too much fats/carbs creates fat cells in which the "fuel" can be stored for times when food isn't in constant supply.

Hope this helps :)