https://www.scientificamerican.com/article/do-amino-acids-build-bigger-muscles/

Let’s start with the basics. The most common muscle-building supplement there is can be found right in your fridge. It’s called protein.

When you eat protein, your body breaks the protein down into amino acids. Those amino acids are then used to repair and grow new muscle fibers. When you consume an adequate amount of protein, your body will experience something called a positive balance of nitrogen.