0 th ▁century ▁that ▁r ats ▁could ▁not ▁surv ive ▁on ▁a ▁di et ▁whose ▁only ▁protein ▁source ▁was ▁ze in , ▁which ▁comes ▁from ▁ma ize ▁( c orn ), ▁but ▁recovered ▁if ▁they ▁were ▁fed ▁case in ▁from ▁cow ' s ▁milk . ▁This ▁led ▁William ▁C um ming ▁Rose ▁to ▁the ▁discovery ▁of ▁the ▁essential ▁am ino ▁acid ▁thre on ine . ▁Through ▁manip ulation ▁of ▁rod ent ▁di ets , ▁Rose ▁was ▁able ▁to ▁show ▁that ▁ten ▁am ino ▁ac ids ▁are ▁essential ▁for ▁r ats : ▁l ys ine , ▁try pt oph an , ▁hist id ine , ▁phen yl al an ine , ▁le uc ine , ▁is ole uc ine , ▁m eth ion ine , ▁val ine , ▁and ▁arg in ine , ▁in ▁addition ▁to ▁thre on ine . ▁Rose ' s ▁later ▁work ▁showed ▁that ▁eight ▁am ino ▁ac ids ▁are ▁essential ▁for ▁adult ▁human ▁be ings , ▁with ▁hist id ine ▁also ▁being ▁essential ▁for ▁inf ants . ▁Long er - term ▁studies ▁established ▁hist id ine ▁as ▁also ▁essential ▁for ▁adult ▁humans . ▁ ▁Inter change ability ▁The ▁distinction ▁between ▁essential ▁and ▁non - ess ential ▁am ino ▁ac ids ▁is ▁somewhat ▁unclear , ▁as ▁some ▁am ino ▁ac ids ▁can ▁be ▁produced ▁from ▁others . ▁The ▁sul fur - cont aining ▁am ino ▁ac ids , ▁m eth ion ine ▁and ▁hom oc yst e ine , ▁can ▁be ▁converted ▁into ▁each ▁other ▁but ▁neither ▁can ▁be ▁synth es