Discovered in 1922 by American scientists Herbert McLean Evans and Katherine Scott Bishop, vitamin E was originally referred to as food factor X. The substance, which was initially found to be essential for pregnancy in rats and was soon after discovered to be contained in lettuce and yeast, was designated a vitamin by 1924 and correspondingly renamed according to the alphabetical nomenclature characteristic of that class of nutrients. Vitamin E was first isolated from wheat germ oil in 1936 and two years later was successfully synthesized for the first time. Since then, synthetic vitamin E has become a common component of many dietary supplements because the human body cannot make its own and must receive the nutrient from outside sources.