Title:
Metabolic and nutritional significances of nonessential amino acids

Ryosei Sakai, PhD. DVM, Institute for Innovation, Ajinomoto Co., Inc., Japan

Abstract:
Nonessential amino acids (NEAAs), such as glutamate (Glu), aspartate (Asp), alanine etc., account for more than half of amino acids in food proteins. Despite quantitative impacts of NEAAs in food proteins, their nutritional significance seems unclear and their requirements for human have not been established by public organizations including those of US/Canada and WHO/FAO. One of the reasons for this neglect would be their high productions in the body. According to the estimation from published data, daily productions of NEAAs exceeded their intake levels by many degrees. For example, the productions were at least hundreds times higher than intakes in cases of Glu and Asp. Such high rates of productions would suggest some crucial importance of their synthesis and degeneration for biological activities and for survival.
Despite of their high production rates in the body, there are many studies indicating impacts of dietary NEAAs. We have studied the effects of dietary Glu removal from the diet, since its production is higher than any other NEAAs and since Glu is one of the most abundant amino acids in food proteins. Feeding of Glu-free diet reduced growth and caused mild atrophy in small intestine in weaning rats. These effects were accompanied by decrease in lean body mass and increase in body fat mass. Removal of dietary Glu delayed wound healing in mice wound model. All of these effects resemble to those of protein deficiency. Indeed, nitrogen (N) balance study indicated certain amount of Glu requirement in rats. This was confirmed by indicator amino acid oxidation method, which has been widely used to determine requirements of essential amino acids in animals and human. All of these results indicate significance of dietary Glu.
It has been proposed that dietary supply of alpha amino-N would be essential for mammals, since it does not seem to be synthesized physiologically from inorganic N. We have shown that dietary Glu was a good N precursor for the synthesis of the other amino acids. Based on our recent data and published data, functions of dietary Glu and the other NEAAs will be discussed further.