Protein is a macronutrient (like carbohydrates and fat), with each gram supplying 4 calories. It is not a single, simple substance but a multitude of chemical combinations. The basic units of proteins are amino acids. There are over 500 amino acids, but humans use just 20 of them, combining them to create thousands of different proteins that perform a vast array of vital functions in the body.
The body can manufacture 11 amino acids; the other 9 amino acids are called essential because the body can’t make them and thus they must come from the diet. Proteins that contain all of the essential amino acids in sufficient amounts are called complete; those that are missing one or more amino acid are called incomplete. Animal proteins—as in meats, poultry, fish, dairy products, and eggs—are always complete, whereas plant proteins, with a few exceptions (notably those in soy and quinoa), are incomplete.
The essential amino acids missing in one plant food can be supplied by a food that has them. The two foods—rice and beans, for instance—are said to have “complementary” proteins. It used to be thought that you had to eat complementary proteins at the same meal, but research has shown that consuming them over the course of the day is sufficient.