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Humans and other vertebrates cannot generate essential amino acids from metabolic intermediates, also known as necessary amino acids. The human body lacks the metabolic pathways necessary to produce these amino acids, thus they must be obtained from an external diet. Amino acids are categorised in nutrition as either essential or nonessential. Early research on human nutrition revealed that some amino acids were necessary for development or nitrogen balance even when there was an adequate supply of alternative amino acids, leading to these categories.