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The boundary between an essential amino acid and one that is not can sometimes be unclear. Methionine and homocysteine, sulfur-containing amino acids, can be converted into each other, but neither can be synthesized from scratch in humans. Cysteine can be made from homocysteine, but it cannot be synthesized from scratch either. So, for convenience, people will sometimes count the sulfur-containing amino acids as a single pool. Likewise, because of the urea cycle, arginine, ornithine, and citrulline are interconvertible, and therefore form a single pool of nutritionally-equivalent amino acids.
Foodstuffs that are lacking essential amino acids are poor sources of protein equivalents, as the body will tend to deaminate the amino acids obtained and convert proteins into fats and carbohydrateCarbohydrates (literally hydrates of carbon are chemical compounds that act as the primary biological means of storing or consuming energy, other forms being fat and protein. Relatively complex carbohydrates are known as polysaccharides. Carbohydrates ares instead. Therefore, a balance of essential amino acids is necessary for a high degree of net protein utilizationThe net protein utilization or NPU, is the ratio of amino acid converted to proteins to the ratio of amino acids supplied. This figure is somewhat affected by the salvage of essential amino acids within the body, but is profoundly affected by the level of, which is the mass ratio of amino acids converted to proteins to amino acids supplied. This figure is somewhat affected by salvage of essential amino acids in the body, but otherwise is profoundly affected by the limiting amino acid content, which is the essential amino acid found in the smallest quantity in the foodstuff.
| Protein source | Limiting amino acid |
| wheataestivum ''T. aethiopicum ''T. araraticum ''T. boeoticum ''T. carthlicum ''T. compactum ''T. dicoccon ''T. durum ''T. ispahanicum ''T. karamyschevii ''T. militinae T. monococcum ''T. polonicum T. spelta ''T. timopheevii ''T. trunciale ''T. turanicum ''T. | lysineLysine is one of the 20 most common natural amino acids on Earth. Lysine is directly coded for in DNA and molecular biologists tend to abbreviate it to Lys or K . Nutritionally, in humans, lysine is an essential amino acid. Lysine can be used as a nutriti |
| riceRice Rice fields on Java Scientific classification Kingdom: Plantae Division: Magnoliophyta Class: Liliopsida Order: Poales Family: Poaceae Genus: Oryza Species Oryza barthii ''Oryza glaberrima ''Oryza latifolia ''Oryza longistaminata ''Oryza punctata ''O | lysineLysine is one of the 20 most common natural amino acids on Earth. Lysine is directly coded for in DNA and molecular biologists tend to abbreviate it to Lys or K . Nutritionally, in humans, lysine is an essential amino acid. Lysine can be used as a nutriti and threonineThreonine is one of the 20 most common natural amino acids on Earth. Nutritionally, in humans, threonine is also an essential amino acid. The threonine side chain can undergo O-linked glycosylation. Amino acids. |
| maizeMaize Scientific classification Kingdom: Plantae Division: Magnoliophyta Class: Liliopsida Order: Poales Family: Poaceae Genus Zea Species Zea diploperennis ''Zea luxurians ''Zea mays ssp. huehuetenangensis ''Zea mays ssp. mays ''Zea mays ssp. mexicana '' | tryptophan and lysine |
| pulses | methionine |
| beef | methionine and cysteine |
| whey | none |
It is therefore a good idea to mix foodstuffs that have different weaknesses in their essential amino acid distributions. This limits the loss of nitrogen through deamination and increases overall net protein utilization.
8 amino acids are generally regarded as essential, with two others, histidine and arginine, essential only in children:
The question of which amino acids are essential varies from species to species, as different metabolisms are able (or unable) to synthesize different substances. For instance, taurine (which is not, by strict definition, an amino acid) is essential for the cat, but not for the dog; thus, taurine is added to commercial cat food, but not to dog food - and therefore, dog food is not nutritionally sufficient for cats.