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Basic organic acid found in proteins, essential in a healthy diet. There are 24 known kinds, although there are others that occur infrequently in nature. Eight of these - the essential amino acids, cannot be synthesized by humans and must be obtained from diet. These are Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine and Tryptophan. Children need another two that aren't essential for adults.
Amino acids link together into the various chain structures (peptides) that give each kind of protein - whether in blood, bone, hair or nails - its specific characteristics.
If even one is absent, or in a disproportionately low ratio to the others, then protein synthesis is halted or reduced. This can happen in the diets of strict vegetarians or vegans. The full range is contained in egg yolk, milk, liver and kidney.
Individual free amino acids have been found to have specific beneficial effects, such as the treating of baldness, the burning-off of fat, muscle building and relieving stress - as such they are used as nutritional supplements.