**1. Introduction**

The first amino acid to be isolated is credited to L.N. Vauquelin and P.J. Robiquet, who discovered asparagine in 1806 [1]. Isolation of the amino acids continued throughout the nineteenth and early twentieth century, with the last significant discovery occurring in 1935 with the identification of threonine as both a new and essential amino acid [2, 3]. During the time of which threonine was discovered, W.C. Rose conducted a series of experiments identifying the amino acids required for growth. At the conclusion of these experiments, Rose reported that 10 amino acids were indispensable and must be included in diets in order to support growth [4]. The evolution of indispensable amino acids based on the experimentation of Rose is presented in **Table 1**.

Post World War II, interest in animal nutrition increased exponentially, reaching a peak in the 1970s, as more importance was placed on food production [7]. Early poultry amino acid nutrition research mirrored the amino acid studies of the time, with the determination of essential amino acids in poultry [8–12]. These original studies were conducted in commercial-type type diets, with the first reports of the use of


*1 Adapted from Rose [4, 5] and Rose et al. [6].*

*2 Removed from list of dispensable amino acids.*

*3 Prior to identification.*

#### **Table 1.**

*Status of amino acids as indispensable or dispensable as reported by W.C. Rose<sup>1</sup> .*

purified-type diets, with crystalline amino acids serving as the only source of protein, being that of Hegsted in 1944 [13]. The use of purified-type diet laid the foundation for accurate determination of amino acid requirements, as they allowed for researchers to know the exact dietary amino acid contents.

In the early phase of purified diets, researchers attempted to create reference diets that could produce similar growth performance to chicks fed commercial-type diets. Extensive work was done at the University of Illinois to construct the aforementioned reference diet [14–18]. In 1965, Dean and Scott [19] published a report detailing a reexamination of the findings of their previous work at the University of Illinois. In this set of experiments, it was found that after refinements had been made to the amino acid levels in the reference diet, earlier determined amino acid requirements were over estimated and could be reduced without negatively affecting performance. These findings indicated the interplay among the amino acids when determining requirement estimates.
