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During digestion, different sources of protein are broken down into different types of amino acids. The human body requires 22 different amino acids. Nine of these, called essential amino acids, cannot be produced by the body and must be supplied by food or dietary supplements. The body can make the remaining 13 amino acids (nonessential amino acids) from other nutrients (e.g., fats, carbohydrates, other amino acids). Proteins may be considered complete or incomplete. Complete proteins provide all nine essential amino acids. Incomplete proteins are lacking in one or more of the essential amino acids.

A nonessential amino acid may become essential if the body lacks the substances required to make it or if a medical condition inhibits its production. When this occurs, the typically nonessential amino acid becomes conditionally essential. For example, the genetic condition phenylketonuria inhibits the production of tyrosine, making it a conditionally essential amino acid in people with this condition. Other amino acids which are considered conditionally essential are arginine, cysteine and glycine.

Three essential amino acids are known as branched-chain amino acids. These three, isoleucine, leucine and valine, are important substances in the formation, strengthening and repair of muscle tissues. Other essential amino acids include:

• Histidine. Considered an essential amino acid, but mostly for children. Important for growth and healing, as well as the creation of histamine (a substance that reduces a person’s sensitivity to allergens).
  
  
• Lysine. Important in new tissue formation, calcium absorption and for producing antibodies, enzymes and hormones.
  
  
• Methionine. Helps to maintain the liver and assist in nerve and immune functions. The breakdown of methionine produces another amino acid called homocysteine. Homocysteine may play a role in the development of atherosclerosis, the narrowing of blood vessels that is a major risk factor for heart disease, heart attack and stroke.
  
  
• Phenylalanine. An important amino acid to the human brain, helping to regulate mood. It is needed for growth of children and for protein metabolism in children and adults. It is normally converted to tyrosine in the human body.
  
  
• Threonine. Important in the formation of many body tissues. Also plays a role in immune function.
  
  
• Tryptophan. An important amino acid to the human brain, affecting many functions of mood and perception.

Nonessential amino acids also have a number of different functions in the human body, including growth, the production of new tissues, metabolism and healing.