Young rats require 10 essential amino acids.
A remarkable series of nutritional investigations, performed just prior to World War I by Thomas B. Osborne and Lafayette B. Mendel, clarified the role of amino acids in the diet. Osborne and Mendel began by comparing the ability of purified proteins, isolated from a number of different sources, to support the growth of young rats. Some proteins had more nutritional value than others. For example, casein from milk satisfied the protein requirements of young rats, whereas gliadin from wheat seeds did not unless lysine was also included in the diet. Osborne and Mendel correctly interpreted their results to mean that gliadin lacks lysine and that rats require lysine in the diet because they cannot synthesize it. Osborne and Mendel were, therefore, the first to provide irrefutable evidence for the existence of an indispensable or essential amino acid.