▁transition ▁metal ▁in ▁organ isms " ▁after ▁iron ▁and ▁it ▁is ▁the ▁only ▁metal ▁which ▁appears ▁in ▁all ▁en zym e ▁classes . ▁There ▁are ▁ 2 – 4   gram s ▁of ▁z inc ▁distributed ▁throughout ▁the ▁human ▁body , ▁and ▁it ▁plays ▁" ub iqu it ous ▁bi ological ▁roles ". ▁A ▁ 2 0 0 6 ▁study ▁estimated ▁that ▁about ▁ 1 0 % ▁of ▁human ▁prote ins ▁( 2 8 0 0 ) ▁potentially ▁bind ▁z inc , ▁in ▁addition ▁to ▁hundreds ▁which ▁transport ▁and ▁traffic ▁z inc . ▁In ▁the ▁U . S ., ▁the ▁Re comm ended ▁Diet ary ▁Allow ance ▁( R DA ) ▁is ▁ 8   m g / day ▁for ▁women ▁and ▁ 1 1   m g / day ▁for ▁men . ▁Harm ful ▁excess ive ▁supp lement ation ▁may ▁be ▁a ▁problem ▁and ▁should ▁probably ▁not ▁exceed ▁ 2 0   m g / day ▁in ▁health y ▁people , ▁although ▁the ▁U . S . ▁National ▁Research ▁Council ▁set ▁a ▁Tol er able ▁Upper ▁In take ▁of ▁ 4 0   m g / day . ▁ ▁Mercur y ▁and ▁cad m ium ▁are ▁to xic ▁and ▁may ▁cause ▁environmental ▁damage ▁if ▁they ▁enter ▁rivers ▁or ▁rain ▁water . ▁This ▁may ▁result ▁in ▁cont amin ated ▁cro ps ▁as ▁well ▁as ▁the ▁bio acc um ulation ▁of ▁mer cur y ▁in ▁a ▁food ▁chain ▁leading ▁to ▁an ▁increase ▁in ▁ill ness es ▁caused ▁by ▁mer cur y ▁and ▁cad m