Biochemistry, Essential Amino Acids

Essential amino acids, also known as indispensable amino acids, are amino acids that humans and other vertebrates cannot synthesize from metabolic intermediates. These amino acids must be supplied from an exogenous diet because the human body lacks the metabolic pathways required to synthesize these amino acids. In nutrition, amino acids are classified as either essential or non-essential. These classifications resulted from early studies on human nutrition, which showed that specific amino acids were required for growth or nitrogen balance even when there is an adequate amount of alternative amino acids. Although variations are possible depending on the metabolic state of an individual, the general held thought is that there are nine essential amino acids, including phenylalanine, valine, tryptophan, threonine, isoleucine, methionine, histidine, leucine, and lysine. The mnemonic PVT TIM HaLL (private Tim Hall) is a commonly used device to remember these amino acids as it includes the first letter of all the essential amino acids. In terms of nutrition, the nine essential amino acids are obtainable by a single complete protein. A complete protein, by definition, contains all the essential amino acids. Complete proteins usually derive from animal-based sources of nutrition, except for soy. The essential amino acids are also available from incomplete proteins, which are usually plant-based foods. The term limiting amino is used to describe the essential amino acid present in the lowest quantity in a food protein relative to a reference food protein like egg whites. The term limiting amino may also refer to an essential amino acid that does not meet the minimal requirements for humans.