Amino acids play central roles both as building blocks of proteins and as intermediates in human metabolism. There are 20 amino acids found within human proteins. Proteins not only catalyze all (or most) of the reactions in living cells, they control virtually all cellular process. Unlike fat and starch, the human body does not store excess amino acids for later use—the amino acids must be in food every day.
Humans can produce 10 of the 20 amino acids. The other 10 (essential amino acids) must be supplied in food. Failure to obtain enough of even 1 of the 10 essential amino acids, those that we cannot make, results in degradation of the body's proteins—muscle and so forth—to obtain the one amino acid that is needed.