Why Are Certain Amino Acids Considered Essential Amino Acids?

Amino acids are nitrogen-containing molecules used by your cells to build proteins and other important compounds. Only 20 amino acids are required to manufacture the hundreds of thousands of enzymes, structural proteins, antibodies and hormones in your body, even though each of these molecules possesses its own unique amino acid sequence. According to experts at the University of Arizona, your body has the capacity to synthesize 10 of the amino acids you need for optimal health, but the other 10 must be acquired from dietary sources.

Essential Amino Acids

Your cells do not possess the metabolic machinery to produce half of the amino acids they require for normal function. Therefore, these "essential" amino acids must be obtained from your food. If even one of the essential amino acids is lacking from your diet, your body will break down other functional proteins -- muscle, for example -- to obtain that single nutrient for other, more critical uses. The essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. An August 2004 review in "The Journal of Nutritional Biochemistry" reports that arginine also is an essential amino acid in newborn infants, due to their limited ability to synthesize this nutrient.

Nonessential Amino Acids

Those amino acids that can be synthesized from materials already present in your body are called nonessential amino acids. Although these molecules still are needed for normal cellular function, they do not have to be acquired from dietary sources. Alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine are considered nonessential amino acids in both adults and infants. In older children and adults, arginine also is nonessential.

Conditionally Essential

The synthesis of nonessential amino acids is dependent on your cells having access to the necessary precursors and nutrients. If a needed element is missing, the amino acid cannot be synthesized, and it then becomes "conditionally essential." It must either be obtained from dietary sources or your body must break down other proteins to acquire it. For example, serine normally can be made in your tissues from glycine or threonine, but the conversion of these two amino acids to serine requires niacin, pyridoxine and folic acid. Deficiencies of any of these three B vitamins leads to a drop in serine synthesis, and you then need to obtain serine from dietary sources.

Considerations

All 20 standard amino acids are required for proper cellular function. The terms "essential" and "nonessential" refer to whether your body can synthesize a given amino acid if it has access to the required precursors and supporting nutrients. A nonessential amino acid can become conditionally essential if you are deficient in any of its precursors. A well-balanced, calorically adequate diet is the best way to ensure that you are getting both essential amino acids and the raw materials for the nonessential amino acids.