Essential amino acid
Six amino acids are non-essential (dispensable) in humans, meaning they can be synthesized in sufficient quantities in the body.
Pyrrolysine (considered the 22nd amino acid),[3] which is proteinogenic only in certain microorganisms, is not used by and therefore non-essential for most organisms, including humans.
[7] Of the twenty amino acids common to all life forms (not counting selenocysteine), humans cannot synthesize nine: histidine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
Additionally, the amino acids arginine, cysteine, glutamine, glycine, proline and tyrosine are considered conditionally essential,[8] which means that specific populations who do not synthesize it in adequate amounts, such as newborn infants and people with diseased livers who are unable to synthesize cysteine, must obtain one or more of these conditionally essential amino acids from their diet.
Estimating the daily requirement for the indispensable amino acids has proven to be difficult; these numbers have undergone considerable revision over the last 20 years.
The following table lists the recommended daily amounts currently in use for essential amino acids in adult humans (unless specified otherwise), together with their standard one-letter abbreviations.
Graduate students at the University of Illinois were fed an artificial diet so that there was a slightly positive nitrogen balance.
[18] Modern techniques make use of ion exchange chromatography to determine the actual amino acid content of foods.
[23] Scientists had known since the early 20th century that rats could not survive on a diet whose only protein source was zein, which comes from maize (corn), but recovered if they were fed casein from cow's milk.
[24] Through manipulation of rodent diets, Rose was able to show that ten amino acids are essential for rats: lysine, tryptophan, histidine, phenylalanine, leucine, methionine, valine, and arginine, in addition to threonine.
[2] The physical signs of protein deficiency include edema, failure to thrive in infants and children, poor musculature, dull skin, and thin and fragile hair.
[2] The amino acids that are essential in the human diet were established in a series of experiments led by William Cumming Rose.
These diets consisted of corn starch, sucrose, butterfat without protein, corn oil, inorganic salts, the known vitamins, a large brown "candy" made of liver extract flavored with peppermint oil (to supply any unknown vitamins), and mixtures of highly purified individual amino acids.
Rose noted that the symptoms of nervousness, exhaustion, and dizziness were encountered to a greater or lesser extent whenever human subjects were deprived of an essential amino acid.
[17] Essential amino acid deficiency should be distinguished from protein-energy malnutrition, which can manifest as marasmus or kwashiorkor.
Kwashiorkor was once attributed to pure protein deficiency in individuals who were consuming enough calories ("sugar baby syndrome").
[26] Still, for instance in Dietary Reference Intakes (DRI) maintained by the USDA, lack of one or more of the essential amino acids is described as protein-energy malnutrition.
