The Essential Amino Acid Reader

It is generally agreed that 20 amino acids are necessary for protein synthesis, with 11 classified as essential amino acids (also called indispensable) and nine as nonessential (also called dispensable). Essential amino acids cannot be produced by the body and must be obtained from food. Nonessential amino acids can be produced by the body and are not required in the diet. However, scientific evidence suggests the addition of a new subcategory: conditionally essential. This changes the classifications to nine essential, five nonessential and six conditionally essential amino acids.¹

The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. There was once speculation that histidine could be synthesized by the body, but after conducting scientific studies, the World Health Organization in 1975 decided this could not be proven and declared histidine essential.

The five nonessential amino acids are capable of being produced by the human body from amino acids already circulating in its tissues and cells or from non-amino acid metabolic intermediates. The matter becomes confusing, however, because only three of the five are consistently listed in reference books as nonessential: alanine, aspartate, and glutamate. Vying for the other two, depending on your reference book, are asparagine, citrulline, glutamine, homocysteine, norleucine and taurine. Some studies show that citrulline, homocysteine, norleucine and taurine are not inserted into proteins during protein synthesis, thus leading to the conclusion that asparagine and glutamine should be the final two nonessential amino acids.^2,3

Finally, given the right metabolic situation and condition, the body can make six more amino acids: arginine, cysteine, glycine, proline, serine and tyrosine. These are the conditionally essential amino acids. According to systems of classifications that don't include this category, only cysteine and tyrosine are considered essential, while arginine, glycine, proline and serine are nonessential. It is important to note that the concept of conditionally essential amino acids is not widely accepted. The reason cysteine and tyrosine are generally considered essential, for example, is because they can be synthesized only from the two essential amino acids methionine and phenylalanine, respectively.

References

1. Mahan K, Escott-Stump S. Krause's food, nutrition and diet therapy. Saint Louis (MO): Saunders;1996. p 66.

2. Laidlaw SA, Kopple JD. Newer concepts of the dispensable amino acids. Am J Clin Nutr 1987;46:593-605.

3. Jackson AA. Amino acids, essential and non-essential. Lancet 1983;(I):1034-7.