ino ▁ac ids ▁common ▁to ▁all ▁life ▁forms , ▁the ▁nine ▁am ino ▁ac ids ▁humans ▁cannot ▁synth es ize ▁are ▁phen yl al an ine , ▁val ine , ▁thre on ine , ▁try pt oph an , ▁m eth ion ine , ▁le uc ine , ▁is ole uc ine , ▁l ys ine , ▁and ▁hist id ine . ▁ ▁Six ▁other ▁am ino ▁ac ids ▁are ▁considered ▁condition ally ▁essential ▁in ▁the ▁human ▁di et , ▁meaning ▁their ▁synth esis ▁can ▁be ▁limited ▁under ▁special ▁path oph ys i ological ▁conditions , ▁such ▁as ▁prem atur ity ▁in ▁the ▁infant ▁or ▁individuals ▁in ▁severe ▁cat abol ic ▁dist ress . ▁These ▁six ▁are ▁arg in ine , ▁c yst e ine , ▁g ly c ine , ▁gl ut am ine , ▁pro line , ▁and ▁ty ros ine . ▁Six ▁am ino ▁ac ids ▁are ▁non - ess ential ▁( dis p ens able ) ▁in ▁humans , ▁meaning ▁they ▁can ▁be ▁synth es ized ▁in ▁sufficient ▁quantities ▁in ▁the ▁body . ▁These ▁six ▁are ▁al an ine , ▁as part ic ▁acid , ▁as par agine , ▁gl ut am ic ▁acid , ▁ser ine , ▁and ▁s elen oc yst e ine ▁( cons ider ed ▁the ▁ 2 1 st ▁am ino ▁acid ). ▁P yr ro ly s ine ▁( cons ider ed ▁the ▁ 2 2 nd ▁am ino ▁acid ) ▁is ▁not ▁used ▁by ▁humans ; ▁thus , ▁it ▁is ▁non ess