An example of mutual supplementation. Athletes take them to build muscle. Dieters take them to spare their bodies protein while losing weight. People also take individual amino acids, mixtures of two or more amino acids, or products that combine amino acids with other nutrients. Some takers believe the products will cure herpes, induce restful sleep, or relieve pain or depression.
Protein supplements are less well digested than protein-rich food and they cost more than food, too.
When used as a replacement for such food, they are often downright dangerous. The liquid protein diet, advocated some years ago for weight loss, caused deaths in many users. Amino acid supplements are also unnecessary. The body is designed to handle whole proteins best. An excess of one amino acid can tie up a carrier, and temporarily prevent the absorption of another similar amino acid.