An essential amino acid
is an amino acid
cannot be synthesized de
by the organism (usually referring to humans), and
therefore must be supplied in the diet.
Essentiality vs. conditional essentiality in humans
(*) Essential only in certain cases.
(**) Truly unclassified. Added to sustain the 21 Numbers of
Essential Amino Acids.
Eight amino acids are generally regarded as essential for humans:
, and lysine
sulphur-containing amino acids), tyrosine
(or aromatic amino acids), histidine
are required by infants and
growing children. Essential amino acids are so called not because
they are more important to life than the others, but because the
body does not synthesize them, making it essential to include them
in one's diet in order to obtain them. In addition, the amino acids
are considered conditionally essential
they are not normally required in the diet, but must be supplied
exogenously to specific populations that do not synthesize it in
adequate amounts. An example would be with the disease phenylketonuria
(PKU). Individuals living
with PKU must keep their intake of phenylalanine extremely low to
prevent mental retardation and other metabolic complications.
However, phenylalanine is the precursor for tyrosine synthesis.
Without phenylalanine, tyrosine cannot be made and so tyrosine
becomes essential in the diet of PKU patients.
The distinction between essential and non-essential amino acids is
somewhat unclear, as some amino acids can be produced from others.
-containing amino acids, methionine
, can be converted into each other
but neither can be synthesized de novo
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
. Likewise arginine
, which are interconvertible by
the urea cycle
, are considered a single
"Newer" Common Amino Acids
The above list of 20 common amino acids is the traditional one, but
there are more than 20 amino acids that are routinely found in
proteins. The common 22 amino acids, referred to as the proteinogenic
amino acids, include the
less-well known pyrrolysine
. Selenocysteine thus far is
the only one that is conditionally essential. In addition to the
22+ alpha-amino acids, scientists are finding far more beta-amino
acids in bacteria with a variety of natural functions, such as
contributing to antibiotic resistance.
Recommended daily amounts
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 WHO
recommended daily amounts
currently in use for essential amino acids in adult
, together with their standard one-letter
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.
Use of essential amino acids
Foodstuffs that lack essential amino acids are poor sources of
equivalents, as the body tends to
the amino acids obtained,
converting proteins into fats
. Therefore, a balance of
essential amino acids is necessary for a high degree of net protein utilization
, which is
the mass ratio of amino acids converted to proteins to amino acids
balanced set of essential amino acids for humans. Animal sources
such as meat, poultry, eggs, fish, milk, and cheese provide all of
the essential amino acids. Near-complete proteins are also found in
some plant sources such as quinoa
, among others. Soya
appears as lower in sulfur-containing amino acids
(methionine and cysteine), which instead are abundant in many other
plant protein sources. It is not necessary to consume plant foods
containing complete proteins as long as a reasonably varied diet is
maintained. By consuming a wide variety of plant foods, a full set
of essential amino acids will be supplied and the human body can
convert the amino acids into proteins.
The net protein utilization of a human eating only one protein
source (only wheat, for instance) is affected by the
limiting amino acid
content (the essential amino
acid found in the smallest quantity in the foodstuff) of that
source. Adequate protein utilization, however, will readily be
obtained if a balanced variety of protein sources is eaten within a
reasonable time (say, 4 hours) , and/or the total (limited) protein
consumed is greater than the requirement.
Essential Amino Acid Deficiency
A chronic deficiency in the essential amino acids will lead to
either a form of childhood oedema
known as kwashiorkor
known as marasmus
Using the one letter designation shown above, mnemonic devices
have been developed for
students wanting or needing to memorize the essential amino acids.
Previous devices have utilized the first letter of the amino acids'
names, and in general did not include arginine which is not always
essential. One mnemonic device that has been used in the past is
PVT TIM HALL
Another method uses the first letter of each essential amino acid
to begin each word in a phrase, such as: "A
aluable." This method begins with the two amino acids
that need some qualifications as to their requirements.
Note that these devices work by using the first letter of the
actual amino acids name. Due to repetition of letters, several
amino acids have one letter abbreviations that are different than
their first letter (e.g. lysine is K). Thus the complete list of
essential amino acids utilizing one-letter codes is
,T,V,W. It would help college
students to have a one letter code mnemonic. One being: I
ednesday, and F
could be I L
ednesday, and F
riday. Yet another is M
I L K F
. And another is I'M L
e! A slightly easier
mnemonic, due to its topical relevance, is "Must Ingest These,
Vitamin-Like, From Wheat, Kale." One of the shortest and easiest
seems to be "RH+ MILK FTW
Val!" with the "+"
implying conditional essentiality.