Why is arginine classified as an essential amino acid in humans when it is synthesized in the urea cycle?
Since in a number of textbooks it is erroneously stated that a significant amount of arginine is formed as a result of its synthesis through the urea cycle. This is far from the case, as studies have demonstrated that there is no net synthesis of arginine by the liver .
Biologically available arginine comes from three sources: (1) recycling of amino acids from normal cellular protein turnover, (2) dietary intake, and (3) de novo synthesis from arginine precursor compounds .
The human body expresses enzymes that are able to synthesize arginine endogenously, and therefore it is not an essential amino acid that needs to be obtained from the diet .
However, there are exceptions, such as during times of stress and rapid growth. Then the need for arginine can be markedly increased, which makes it necessary to provide exogenous arginine. Therefore, arginine is classified as a conditionally essential amino acid.
1. Vance L. Albaugh, Adrian Barbul Arginine. <span style="mso-ansi-language:EN-US" lang="EN-US">Reference Module in Life Sciences</span> (2017)
2. <span style="mso-ansi-language:EN-US" lang="EN-US">Sidney M Morris Jr</span> Arginine: beyond protein. The American Journal of Clinical Nutrition, Volume 83, Issue 2, February 2006, Pages 508S–512S, <span style="mso-ansi-language:EN-US" lang="EN-US">https://doi.org/10.1093/ajcn/83.2.508S</span>