ized ▁de ▁nov o ▁in ▁humans . ▁Like wise , ▁c yst e ine ▁can ▁be ▁made ▁from ▁hom oc yst e ine ▁but ▁cannot ▁be ▁synth es ized ▁on ▁its ▁own . ▁So , ▁for ▁convenience , ▁sul fur - cont aining ▁am ino ▁ac ids ▁are ▁sometimes ▁considered ▁a ▁single ▁pool ▁of ▁nut r itionally ▁equivalent ▁am ino ▁ac ids ▁as ▁are ▁the ▁a rom atic ▁am ino ▁acid ▁pair , ▁phen yl al an ine ▁and ▁ty ros ine . ▁Like wise ▁arg in ine , ▁orn ith ine , ▁and ▁cit r ull ine , ▁which ▁are ▁inter convert ible ▁by ▁the ▁u rea ▁cycle , ▁are ▁considered ▁a ▁single ▁group . ▁ ▁Effect s ▁of ▁def ic iency ▁ ▁If ▁one ▁of ▁the ▁essential ▁am ino ▁ac ids ▁is ▁less ▁than ▁needed ▁for ▁an ▁individual ▁the ▁util ization ▁of ▁other ▁am ino ▁ac ids ▁will ▁be ▁h inder ed ▁and ▁thus ▁protein ▁synth esis ▁will ▁be ▁less ▁than ▁ade qu ate . ▁Prote in ▁def ic iency ▁has ▁been ▁shown ▁to ▁affect ▁all ▁of ▁the ▁body ' s ▁org ans ▁and ▁many ▁of ▁its ▁systems , ▁including ▁the ▁brain ▁and ▁brain ▁function ▁of ▁inf ants ▁and ▁young ▁children ; ▁the ▁imm une ▁system , ▁thus ▁elev ating ▁risk ▁of ▁in fection ; ▁gut ▁m uc os al ▁function ▁and ▁per me ability , ▁which ▁affect s ▁absor ption ▁and ▁vulner ability ▁to ▁system ic ▁disease ; ▁and ▁kid ney ▁function . ▁The ▁physical ▁signs ▁of ▁protein ▁def ic