An amino acid is a type of molecule that serves as a building block for proteins, which are some of the most important substances in living things. Just as letters combine to form words, amino acids link together in long chains to form proteins. Your body contains thousands of different proteins, each one made by connecting amino acids in a specific order, like beads on a string.

There are 20 different amino acids that your body uses to build proteins. Some of these your body can make on its own, but nine of them (called essential amino acids) must come from the food you eat. When you eat protein-rich foods like meat, eggs, beans, or nuts, your digestive system breaks down their proteins into individual amino acids. Your body then rearranges these amino acids to build the specific proteins you need: proteins that form your muscles, carry oxygen in your blood, fight off germs, or help you digest food.

The name comes from their chemical structure: they contain nitrogen in a form called an amine group, plus an acid group.