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Proteins are members of a class of chemical compound that are synthesized by all living organisms. They contain amino acids. In humans, proteins are needed for growth and maintenance of body weight, and to repair injured tissue. Excess protein in the diet can be converted to simple carbohydrates and used as a source of energy.

"Complete" proteins, containing all the essential amino acids, are found in animal products such as milk, eggs, cheese, meat and fish. Some vegetables such as soy also contain complete proteins. Many vegetables contain only some of the essential amino acids; vegetarians must make up for this by a diet which is careful to combine vegetable groups that complement each other in terms of what proteins they contain.