Proteins are present in every cell in the body. Protein is used to build, repair and maintain most tissues in the body. Proteins are also used to make enzymes, hormones and other substances the body can use.

Proteins are composed of chains of amino acids. There are generally a few dozen to several hundred amino acids in a single protein. The human body requires 22 different amino acids. Nine of these, called essential amino acids, must be supplied by the diet. The body can make the remaining 13 amino acids (nonessential amino acids) from other nutrients (e.g., fats, carbohydrates, other amino acids).

Animal sources and soy are considered complete proteins because they provide all nine essential amino acids. Plant sources other than soy are considered incomplete proteins because they lack one or more of the essential amino acids. Incomplete proteins lacking in one amino acid may be eaten with another incomplete protein that is missing a different amino acid. Incomplete proteins that provide all nine essential amino acids when eaten together are called complementary proteins.

The amount of protein needed may vary from one person to another. However, most people can get their recommended dietary allowance of protein by eating a 3-ounce serving of meat or meat substitute twice every day. The majority of Americans get more than enough protein.

Protein deficiency may lead to a condition known as protein-energy malnutrition (PEM). PEM causes severe weight loss, wasting and, in children, poor growth.

Consuming protein beyond amounts the body requires provides no additional health benefit. For example, eating more protein will not cause additional muscle development. Any excess protein in the diet is removed from the body through urine and stool, or is converted into fat. Excessive amounts of protein have been associated with a number of health problems, including heart disease and osteoporosis.