Essential vs. Nonessential Amino Acids
The building blocks of human proteins are 22 amino acids.
These may come from both animal and plant sources.
Nine amino acids are essential, or indispensable, because they
cannot be synthesized by humans.
The 22 amino acids are listed in
Table 7
and tabulated according
to essentiality.
Nonessential amino acids can be biosynthesized by the transfer
of amino groups to carbon compounds that are formed as intermediates of glucose (glu
cogenic amino acids) and lipid
(ketogenic amino acids) metabolism.
Essential
Conditionally
Essential
Nonessential
Histidine
Arginine
Alanine
Isoleucine
Asparagine
Aspartic acid
Leucine
Cysteine*
Cystine
Lysine
Glutamine
Glutamic acid
Methionine
Serine
Glycine
Phenylalanine
Taurine
Proline
Threonine
Tyrosine*
Tryptophan
Valine
Table 7. Essential, conditionally
essential, and nonessential amino
acids. *Cysteine is sometimes considered essential together with methionine; as is tyrosine with phenylalanine.
Methionine and phenylalanine are required as precursors for the
synthesis of the amino acids cysteine and tyrosine respectively.
For this reason, methionine and cystine are sometimes
considered together as essential, as are phenylalanine and tyrosine.
Nonessential amino acids may become conditionally essential. Ordinarily nonessential amino acids may not be synthesized in
large enough amounts to meet the body’s needs if the metabolic pathways for their synthesis are immature or impaired.
For example, cysteine, tyrosine, and possibly taurine may be
required for premature infants. After trauma or surgery, glutamine may not be synthesized in adequate amounts.
Biological Quality
The biological quality of a dietary protein is determined by:
1)
The amount
2)
The proportion of essential amino acids it provides
3)
Digestibility If a protein contains a disproportionately low amount of one or
more amino acids or is not completely digested, the amount needed to meet protein requirements will be greater than that of a protein that has a well-balanced pattern of amino acids and is highly digestible.
If any of the essential amino acids is not available in sufficient
amounts relative to other essential amino acids, protein synthesis will be limited.
If any of the essential amino acids is present in excessive
amounts relative to other essentia
l amino acids, it will be wasted
relative to protein synthesis.
Other body proteins, such as albumin, may be broken down to
provide limiting amino acids so that protein synthesis may continue. Animal Protein
Protein from most animal sources (meat, fish, dairy, and eggs) is
considered to have high biological value or to be
complete
because
all nine essential amino acids are present in good proportion in these proteins.
Gelatin, an exception to high-quality animal protein, is of low-
quality; it lacks tryptophan.
Nutrition for Sports, Essentials of 58
