Anessential amino acid, orindispensable amino acid, is anamino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life forms, the nine amino acids humans cannot synthesize arevaline,isoleucine,leucine,methionine,phenylalanine,tryptophan,threonine,histidine, andlysine.[1][2]
Six other amino acids are consideredconditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severecatabolic distress.[2] These six arearginine,cysteine,glycine,glutamine,proline, andtyrosine. Six amino acids are non-essential (dispensable) in humans, meaning they can be synthesized in sufficient quantities in the body. These six arealanine,aspartic acid,asparagine,glutamic acid,serine,[2] andselenocysteine (considered the 21st amino acid).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.
Thelimiting amino acid is the essential amino acid which is furthest from meeting nutritional requirements.[4] This concept is important when determining the selection, number, and amount of foods to consume: Even when total protein and all other essential amino acids are satisfied, if the limiting amino acid is not satisfied, then the meal is considered to be nutritionally limited by that amino acid.[3]
| Essential | Conditionally essential[5][6] | Non-essential |
|---|---|---|
| Histidine (H) | Arginine (R) | Alanine (A) |
| Isoleucine (I) | Cysteine (C) | Aspartic acid (D) |
| Leucine (L) | Glutamine (Q) | Asparagine (N) |
| Lysine (K) | Glycine (G) | Glutamic acid (E) |
| Methionine (M) | Proline (P) | Serine (S) |
| Phenylalanine (F) | Tyrosine (Y) | |
| Threonine (T) | Selenocysteine (U) | |
| Tryptophan (W) | ||
| Valine (V) | Pyrrolysine* (O) |
(*)Pyrrolysine, sometimes considered the "22nd amino acid", is not used by the human body.[7]
Of the twenty amino acids common to all life forms (not countingselenocysteine), humans cannot synthesize nine:histidine,isoleucine,leucine,lysine,methionine,phenylalanine,threonine,tryptophan andvaline. Additionally, the amino acidsarginine,cysteine,glutamine,glycine,proline andtyrosine are consideredconditionally 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.[9][10] For example, enough arginine is synthesized by the urea cycle to meet the needs of an adult but perhaps not those of a growing child. Amino acids that must be obtained from the diet are calledessential amino acids.
Eukaryotes can synthesize some of the amino acids from othersubstrates. Consequently, only a subset of the amino acids used in protein synthesis areessential nutrients.
Nonessential amino acids are produced in the body. The pathways for the synthesis of nonessential amino acids come from basic metabolic pathways.Glutamate dehydrogenase catalyzes the reductive amination ofα-ketoglutarate toglutamate. Atransamination reaction takes place in the synthesis of most amino acids. At this step, thechirality of the amino acid is established.Alanine andaspartate are synthesized by the transamination ofpyruvate andoxaloacetate, respectively.Glutamine is synthesized from NH4+ and glutamate, andasparagine is synthesized similarly.Proline andarginine are both derived from glutamate.Serine, formed from3-phosphoglycerate, which comes fromglycolysis, is the precursor ofglycine andcysteine.Tyrosine is synthesized by the hydroxylation ofphenylalanine, which is an essential amino acid.
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 inadult humans (unless specified otherwise), together with their standard one-letter abbreviations.
| Essential (+ conditional) amino acid(s) | Daily intake in mg per kg body mass | ||
|---|---|---|---|
| WHO[11] | USNAM[12] | FAO (2018) young children catch-up growth[13] | |
| Histidine (H) | 10 | 14 | 66 |
| Isoleucine (I) | 20 | 19 | 95 |
| Leucine (L) | 39 | 42 | 198 |
| Lysine (K) | 30 | 38 | 183 |
| Methionine (M) +Cysteine (C) | 10.4 + 4.1 (14.5 total) | 19 total | 88 |
| Phenylalanine (F) +Tyrosine (Y) | 25 (total) | 33 total | 177 |
| Threonine (T) | 15 | 20 | 103 |
| Tryptophan (W) | 4 | 5 | 29 |
| Valine (V) | 26 | 24 | 130 |
The recommended daily intakes for children aged three years and older is 10% to 20% higher than adult levels and those for infants can be as much as 150% higher in the first year of life.Cysteine (or sulfur-containing amino acids),tyrosine (or aromatic amino acids), andarginine are always required by infants and growing children.[11][14]Methionine andcysteine are grouped together because one of them can be synthesized from the other using the enzymemethionineS-methyltransferase and the catalystmethionine synthase.[15]Phenylalanine andtyrosine are grouped together because tyrosine can be synthesized from phenylalanine using the enzymephenylalanine hydroxylase.[16]
Historically, amino acid requirements were determined by calculating the balance between dietarynitrogen intake and nitrogen excreted in the liquid and solid wastes, because proteins represent the largest nitrogen content in a body. A positive balance occurs when more nitrogen is consumed than is excreted, which indicates that some of the nitrogen is being used by the body to build proteins. A negative nitrogen balance occurs when more nitrogen is excreted than is consumed, which indicates that there is insufficient intake for the body to maintain its health. Graduate students at theUniversity of Illinois were fed an artificial diet so that there was a slightly positive nitrogen balance. Then one amino acid was omitted and the nitrogen balance recorded. If a positive balance continued, then that amino acid was deemed not essential. If a negative balance occurred, then that amino acid was slowly restored until a slightly positive nitrogen balance stabilized and the minimum amount recorded.[17][18]
A similar method was used to determine the protein content of foods. Test subjects were fed a diet containing no protein and the nitrogen losses recorded. During the first week or more there is a rapid loss oflabile proteins. Once the nitrogen losses stabilize, this baseline is determined to be the minimum required for maintenance. Then the test subjects were fed a measured amount of the food being tested. The difference between the nitrogen in that food and the nitrogen losses above baseline was the amount the body retained to rebuild proteins. The amount of nitrogen retained divided by the total nitrogen intake is callednet protein utilization. The amount of nitrogen retained divided by the (nitrogen intake minus nitrogen loss above baseline) is calledbiological value and is usually given as a percentage.[18]
Modern techniques make use ofion exchange chromatography to determine the actual amino acid content of foods. TheUSDA used this technique in their own labs to determine the content of 7793 foods across 28 categories. The USDA published the final database in 2018 to the public.[19]
The limiting amino acid depends on the human requirements and there are currently two sets of human requirements from authoritative sources: one published byWHO[11] and the other published byUSDA.[12]
| Based on WHO Requirements | Based on USDA Requirements | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Category | Tryptophan | Threonine | Isoleucine | Leucine | Lysine | Methionine+Cystine | Phenylalanine+Tyrosine | Valine | Histidine | Tryptophan | Threonine | Isoleucine | Leucine | Lysine | Methionine+Cystine | Phenylalanine+Tyrosine | Valine | Histidine | |
| American Indian/Alaska Native Foods | 4 | 0 | 0 | 10 | 4 | 0 | 0 | 15 | 0 | 7 | 2 | 0 | 3 | 6 | 5 | 0 | 0 | 10 | |
| Baby Foods | 2 | 1 | 0 | 7 | 35 | 8 | 0 | 11 | 1 | 5 | 1 | 0 | 5 | 34 | 13 | 0 | 0 | 7 | |
| Baked Products | 0 | 1 | 0 | 5 | 338 | 1 | 0 | 5 | 1 | 0 | 1 | 0 | 0 | 339 | 2 | 0 | 0 | 9 | |
| Beef Products | 276 | 0 | 6 | 2 | 0 | 0 | 0 | 649 | 2 | 289 | 1 | 0 | 176 | 6 | 300 | 0 | 159 | 4 | |
| Beverages | 0 | 0 | 0 | 2 | 11 | 5 | 0 | 1 | 1 | 0 | 0 | 0 | 1 | 12 | 5 | 0 | 0 | 2 | |
| Breakfast Cereals | 0 | 0 | 1 | 1 | 40 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 40 | 1 | 0 | 0 | 0 | |
| Cereal Grains and Pasta | 0 | 0 | 0 | 9 | 143 | 0 | 0 | 3 | 1 | 2 | 1 | 0 | 4 | 148 | 0 | 0 | 0 | 1 | |
| Dairy and Egg Products | 19 | 6 | 4 | 21 | 16 | 122 | 0 | 12 | 3 | 19 | 19 | 0 | 0 | 11 | 122 | 0 | 0 | 32 | |
| Fast Foods | 4 | 3 | 0 | 9 | 39 | 8 | 0 | 62 | 1 | 6 | 4 | 0 | 10 | 82 | 15 | 0 | 1 | 8 | |
| Fats and Oils | 0 | 0 | 0 | 4 | 4 | 4 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 4 | 0 | 0 | 6 | |
| Finfish and Shellfish Products | 3 | 3 | 0 | 15 | 0 | 0 | 0 | 228 | 0 | 5 | 3 | 0 | 174 | 0 | 0 | 0 | 0 | 67 | |
| Fruits and Fruit Juices | 15 | 0 | 9 | 54 | 12 | 31 | 3 | 3 | 14 | 15 | 1 | 7 | 40 | 11 | 35 | 3 | 1 | 28 | |
| Lamb | 10 | 0 | 5 | 254 | 3 | 2 | 0 | 155 | 0 | 10 | 0 | 2 | 207 | 9 | 112 | 0 | 2 | 87 | |
| Legumes and Legume Products | 0 | 0 | 0 | 1 | 26 | 154 | 0 | 22 | 0 | 0 | 0 | 0 | 1 | 27 | 175 | 0 | 0 | 0 | |
| Meals | 1 | 0 | 0 | 1 | 15 | 0 | 0 | 14 | 0 | 2 | 2 | 0 | 2 | 24 | 0 | 0 | 1 | 0 | |
| Nut and Seed Products | 0 | 0 | 1 | 24 | 96 | 8 | 0 | 0 | 0 | 0 | 0 | 1 | 13 | 103 | 10 | 0 | 0 | 2 | |
| Pork Products | 11 | 0 | 1 | 54 | 0 | 2 | 0 | 249 | 0 | 20 | 0 | 0 | 197 | 0 | 73 | 0 | 15 | 12 | |
| Poultry Products | 6 | 12 | 6 | 58 | 1 | 0 | 0 | 287 | 0 | 36 | 22 | 0 | 167 | 5 | 8 | 0 | 99 | 33 | |
| Restaurant Foods | 0 | 9 | 3 | 14 | 24 | 3 | 0 | 41 | 1 | 1 | 25 | 0 | 9 | 33 | 12 | 0 | 0 | 15 | |
| Sausages and Luncheon Meats | 5 | 0 | 1 | 31 | 0 | 2 | 0 | 78 | 0 | 14 | 11 | 1 | 68 | 1 | 11 | 0 | 4 | 7 | |
| Snacks | 2 | 0 | 0 | 6 | 83 | 6 | 0 | 4 | 1 | 2 | 1 | 0 | 6 | 81 | 9 | 0 | 0 | 3 | |
| Soups | 0 | 0 | 2 | 7 | 10 | 28 | 0 | 7 | 0 | 0 | 0 | 0 | 1 | 21 | 31 | 0 | 0 | 1 | |
| Spices and Herbs | 3 | 0 | 0 | 6 | 11 | 3 | 0 | 1 | 1 | 3 | 2 | 0 | 3 | 12 | 4 | 0 | 0 | 1 | |
| Sweets | 0 | 1 | 0 | 3 | 17 | 47 | 0 | 1 | 2 | 0 | 1 | 0 | 1 | 17 | 47 | 0 | 0 | 5 | |
| Vegetables and Vegetable Products | 7 | 0 | 8 | 238 | 114 | 199 | 0 | 18 | 19 | 13 | 28 | 0 | 112 | 144 | 246 | 0 | 2 | 58 | |
Various attempts have been made to express the "quality" or "value" of various kinds of protein. Measures include thebiological value,net protein utilization,protein efficiency ratio,protein digestibility corrected amino acid score and thecomplete proteins concept. These concepts are important in thelivestock industry, because the relative lack of one or more of the essential amino acids inanimal feeds would have a limiting effect on growth and thus onfeed conversion ratio. Thus, various feedstuffs may be fed in combination to increase net protein utilization, or asupplement of an individual amino acid (methionine, lysine, threonine, or tryptophan) can be added to the feed.
Protein content in foods is often measured in protein per serving rather than protein per calorie. For instance, the USDA lists 6 grams of protein per large whole egg (a 50-gram serving) rather than 84 mg of protein per calorie (71 calories total).[20] For comparison, there are 2.8 grams of protein in a serving of raw broccoli (100 grams) or 82 mg of protein per calorie (34 calories total), or theDaily Value of 47.67g of protein after eating 1,690g of raw broccoli a day at 574 cal.[21] An egg contains 12.5g of protein per 100g, but 4 mg more protein per calorie, or the protein DV after 381g of egg, which is 545 cal.[22] The ratio of essential amino acids (the quality of protein) is not taken into account, one would actually need to eat more than 3 kg of broccoli a day to have a healthy protein profile, and almost 6 kg to get enough calories.[21] It is recommended that adult humans obtain between 10–35% of their 2000 calories a day as protein.[23]
Scientists had known since the early 20th century that rats could not survive on a diet whose only protein source waszein, which comes frommaize (corn), but recovered if they were fedcasein from cow's milk. This ledWilliam Cumming Rose to the discovery of the essential amino acidthreonine.[24] Through manipulation of rodent diets, Rose was able to show that ten amino acids are essential for rats:lysine,tryptophan,histidine,phenylalanine,leucine,isoleucine,methionine,valine, andarginine, in addition to threonine. Rose's later work showed that eight amino acids are essential for adult human beings, with histidine also being essential for infants. Longer-term studies established histidine as also essential for adult humans.[25]
The distinction between essential and non-essential amino acids is somewhat unclear, as some amino acids can be produced from others. Thesulfur-containing amino acids,methionine andhomocysteine, can be converted into each other but neither can be synthesizedde novo in humans. Likewise, cysteine can be made from homocysteine but cannot be synthesized on its own. So, for convenience, sulfur-containing amino acids are sometimes considered a single pool of nutritionally equivalent amino acids as are thearomatic amino acid pair,phenylalanine andtyrosine. Likewisearginine,ornithine, andcitrulline, which are interconvertible by theurea cycle, are considered a single group.[citation needed]
If one of the essential amino acids is not available in the required quantities, protein synthesis will be inhibited, irrespective of the availability of the other amino acids.[2] Protein deficiency has been shown to affect all of the body's organs and many of its systems, for example affecting brain development in infants and young children; inhibiting upkeep of the immune system, increasing risk of infection; affecting gutmucosal function and permeability, thereby reducing absorption and increasing vulnerability tosystemic disease; and impacting kidney function.[2] The physical signs of protein deficiency includeedema,failure to thrive in infants and children, poor musculature, dull skin, and thin and fragile hair. Biochemical changes reflecting protein deficiency include lowserum albumin and lowserum transferrin.[2]
The amino acids that are essential in the human diet were established in a series of experiments led byWilliam Cumming Rose. The experiments involved elemental diets to healthy male graduate students. These diets consisted ofcorn starch,sucrose,butterfat without protein,corn oil, inorganic salts, the knownvitamins, a large brown "candy" made of liver extract flavored withpeppermint oil (to supply any unknown vitamins), and mixtures of highly purified individual amino acids. The main outcome measure wasnitrogen balance. 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 aminoacid deficiency should be distinguished fromprotein-energy malnutrition, which can manifest asmarasmus orkwashiorkor.Kwashiorkor was once attributed to pure protein deficiency in individuals who were consuming enoughcalories ("sugar baby syndrome"). However, this theory has been challenged by the finding that there is no difference in the diets of children developingmarasmus as opposed to kwashiorkor.[26] Still, for instance inDietary Reference Intakes (DRI) maintained by theUSDA, lack of one or more of the essential amino acids is described asprotein-energy malnutrition.[2]
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