ential . ▁ ▁The ▁limit ing ▁am ino ▁acid ▁is ▁the ▁essential ▁am ino ▁acid ▁found ▁in ▁the ▁smallest ▁quantity ▁in ▁the ▁food st uff , ▁most ▁plant - based ▁food s ▁have ▁a ▁limit ing ▁am ino ▁acid . ▁This ▁concept ▁is ▁important ▁when ▁calculating ▁animal ▁fe eds . ▁A ▁complete ▁protein ▁contains ▁all ▁the ▁essential ▁am ino ▁ac ids ▁in ▁the ▁right ▁balance ; ▁meat , ▁milk ▁and ▁eggs ▁are ▁complete ▁protein ▁sources ▁for ▁humans . ▁ ▁Ess ential ity ▁in ▁humans ▁ ▁(* ) ▁P yr ro ly s ine , ▁sometimes ▁considered ▁the ▁" 2 2 nd ▁am ino ▁acid ", ▁is ▁not ▁used ▁by ▁humans . ▁ ▁E uk ary otes ▁can ▁synth es ize ▁some ▁of ▁the ▁am ino ▁ac ids ▁from ▁other ▁substr ates . ▁Con sequently , ▁only ▁a ▁subset ▁of ▁the ▁am ino ▁ac ids ▁used ▁in ▁protein ▁synth esis ▁are ▁essential ▁nut ri ents . ▁ ▁Re comm ended ▁daily ▁int ake ▁ ▁Est im ating ▁the ▁daily ▁requirement ▁for ▁the ▁ind isp ens able ▁am ino ▁ac ids ▁has ▁proven ▁to ▁be ▁difficult ; ▁these ▁numbers ▁have ▁under g one ▁considerable ▁revision ▁over ▁the ▁last ▁ 2 0 ▁years . ▁The ▁following ▁table ▁lists ▁the ▁W HO ▁and ▁United ▁States ▁recommended ▁daily ▁amounts ▁currently ▁in ▁use ▁for ▁essential ▁am ino ▁ac ids ▁in ▁adult ▁humans , ▁together ▁with ▁their ▁standard ▁one - letter ▁abbre vi ations . ▁ ▁The ▁recommended ▁daily ▁int akes ▁for ▁children ▁aged ▁three ▁years ▁and ▁older ▁is ▁