An essential amino acid, or indispensable amino acid, is an amino acid
that cannot be synthesized de novo (from scratch) by the organism, and
thus must be supplied in its diet. The nine amino acids humans cannot
synthesize are phenylalanine, valine, threonine, tryptophan,
methionine, leucine, isoleucine, lysine, and histidine (i.e., F V T W
M L I K H).
Six other amino acids are considered conditionally essential in the
human diet, meaning their synthesis can be limited under special
pathophysiological conditions, such as prematurity in the infant or
individuals in severe catabolic distress. These six are arginine,
cysteine, glycine, glutamine, proline, and tyrosine (i.e., R C G Q P
Y). Five amino acids are dispensable in humans, meaning they can be
synthesized in sufficient quantities in the body. These five are
alanine, aspartic acid, asparagine, glutamic acid and serine (i.e., A
D N E S).
1 Essentiality in humans
2 Recommended daily intake
2.1 Relative amino acid composition of protein sources
Protein per calorie
3 Complete proteins in non-human animals
5 Effects of deficiency
6 See also
8 External links
Essentiality in humans
Aspartic acid (D)
Glutamic acid (E)
(*) Pyrrolysine, sometimes considered the "22nd amino acid", is not
used by humans.
Eukaryotes can synthesize some of the amino acids from other
substrates. Consequently, only a subset of the amino acids used in
protein synthesis are essential nutrients.
Recommended daily intake
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
United States recommended daily amounts currently in use for essential
amino acids in adult humans, together with their standard one-letter
WHO mg per kg body weight
WHO mg per 70 kg
US mg per kg body weight
+ C Cysteine
10.4 + 4.1 (15 total)
+ Y Tyrosine
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.
sulfur-containing amino acids), tyrosine (or aromatic amino acids),
and arginine are always required by infants and growing
Relative amino acid composition of protein sources
Various attempts have been made to express the "quality" or "value" of
various kinds of protein. Measures include the biological value, net
protein utilization, protein efficiency ratio, protein
digestibility-corrected amino acid score and complete proteins
concept. These concepts are important in the livestock industry,
because the relative lack of one or more of the essential amino acids
in animal feeds would have a limiting effect on growth and thus on
feed conversion ratio. Thus, various feedstuffs may be fed in
combination to increase net protein utilization, or a supplement of an
individual amino acid (methionine, lysine, threonine, or tryptophan)
can be added to the feed.
Although plants tend to have less protein per weight than animal
sources such as eggs or milk, they are nevertheless "complete" in
that, as a whole, they contain all of the amino acids essential in
human nutrition. The same is true for algae and marine
phytoplankton. Eating various plant foods in combination can provide a
protein of higher biological value. Certain native combinations of
foods, such as corn and beans, soybeans and rice, or red beans and
rice, contain the essential amino acids necessary for humans in
Protein per calorie
It can be shown that common vegetable sources contain adequate
protein, often more protein per Calorie than the standard reference,
whole raw egg, while other plant sources, particularly fruits contain
less. For example, while 100 g of raw broccoli only provides 28 cal
and 3 g of protein, it has over 100 mg of protein per cal. An egg
contains five times as many calories (143 cal) but only four times as
much protein, roughly 90 mg of protein per cal. However, a carrot
has only 23 mg protein per cal or twice the minimum
recommendation, a banana meets the minimum, and an apple is below
recommendation. It is recommended that adult humans obtain
10–35% of their calories as protein, or roughly 11–39 mg of
protein per cal per day (22–78 g for 2000 cal). The US FDA daily
reference value of 50 g protein per 2000 cal is 25 mg/cal per
Complete proteins in non-human animals
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.
William Cumming Rose to the discovery of the essential amino
acid threonine. 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, and arginine, 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.
The distinction between essential and non-essential amino acids is
somewhat unclear, as some amino acids can be produced from others. The
sulfur-containing amino acids, methionine and homocysteine, can be
converted into each other but neither can be synthesized de 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 the aromatic amino acid pair,
phenylalanine and tyrosine. Likewise arginine, ornithine, and
citrulline, which are interconvertible by the urea cycle, are
considered a single group.
Effects of deficiency
Main article: Protein-energy malnutrition
If one of the essential amino acids is less than needed for an
individual the utilization of other amino acids will be hindered and
thus protein synthesis will be less than what it usually is, even in
the presence of adequate total nitrogen intake.
Protein deficiency has been shown to affect all of the body's organs
and many of its systems, including the brain and brain function of
infants and young children; the immune system, thus elevating risk of
infection; gut mucosal function and permeability, which affects
absorption and vulnerability to systemic disease; and kidney
function. The physical signs of protein deficiency include edema,
failure to thrive in infants and children, poor musculature, dull
skin, and thin and fragile hair. Biochemical changes reflecting
protein deficiency include low serum albumin and low serum
The amino acids that are essential in the human diet were established
in a series of experiments led by William Cumming Rose. The
experiments involved elemental diets to healthy male graduate
students. These diets consisted of cornstarch, 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. The main outcome measure was nitrogen 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.
Essential amino acid
Essential amino acid deficiency should be distinguished from
protein-energy malnutrition, which can manifest as marasmus or
Kwashiorkor was once attributed to pure protein
deficiency in individuals who were consuming enough calories ("sugar
baby syndrome"). However, this theory has been challenged by the
finding that there is no difference in the diets of children
developing marasmus as opposed to kwashiorkor. 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
Biological Value (BV)
Edible protein per unit area of land
Essential fatty acid
List of standard amino acids
Protein Digestibility Corrected Amino Acid Score
Ketogenic amino acid
Glucogenic amino acid
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^ Richard Cammack. "Newsletter 2009, Biochemical Nomenclature
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HUMAN NUTRITION" (PDF).
WHO Press. , page 150
Institute of Medicine
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Protein and Amino Acids". Dietary
Reference Intakes for Energy, Carbohydrates, Fiber, Fat, Fatty Acids,
Cholesterol, Protein, and Amino Acids. Washington, DC: The National
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^ McDougall J.
Plant foods have a complete amino acid composition.
^ Woolf, P. J.; Fu, L. L.; Basu, A. (2011). Haslam, Niall James, ed.
"VProtein: Identifying Optimal Amino Acid Complements from Plant-Based
Foods". PLoS ONE. 6 (4): e18836. doi:10.1371/journal.pone.0018836.
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^ Essential Amino Acids. phy-astr.gsu.edu: "Tillery points out that a
number of popular ethnic foods involve such a combination, so that in
a single dish, one might hope to get the ten essential amino acids.
Mexican corn and beans, Japanese rice and soybeans, and Cajun red
beans and rice are examples of such fortuitous combinations."
USDA National Nutrient Database". USDA.gov. 2015-03-31. Retrieved
^ "Web MD Protein: Are You Getting Enough?". webmd.com. 2014-09-05.
^ "Information for Consumers (Drugs)". Fda.gov. 2008-10-29. Retrieved
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requirements of man. II. The role of threonine and histidine". The
Journal of Biological Chemistry. 188 (1): 49–58.
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essential amino acid in normal and chronically uremic man". J Clin
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requirements of man. III. The role of isoleucine; additional evidence
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Amino acid content of some vegetarian foods at veganhealth.org.
Amino Acid Profiles of Some Common Feeds at Virginia Tech.
Molecular Expressions: The Amino Acid Collection at Florida State
University. Features detailed information and crystal photographs of
each amino acid.
vProtein, an online software tool to analyze the essential amino acid
profiles of single and pairs of plant based foods based on human
The encoded amino acid
Branched-chain amino acids (Valine
Positive charge (pKa)
Negative charge (pKa)
Aspartic acid (≈3.9)
Glutamic acid (≈4.1)
Amino acids types: Encoded (proteins)