▁is ▁greater ▁than ▁ 1 . 8   pp m . ▁When ▁this ▁content ▁exceed s ▁ 2 . 0   pp m , ▁few ▁plants ▁will ▁perform ▁well ▁and ▁some ▁may ▁not ▁surv ive . ▁ ▁It ▁is ▁thought ▁that ▁bor on ▁plays ▁several ▁essential ▁roles ▁in ▁animals , ▁including ▁humans , ▁but ▁the ▁exact ▁phys i ological ▁role ▁is ▁poor ly ▁understood . ▁A ▁small ▁human ▁trial ▁published ▁in ▁ 1 9 8 7 ▁reported ▁on ▁post men op aus al ▁women ▁first ▁made ▁bor on ▁def ic ient ▁and ▁then ▁re ple ted ▁with ▁ 3   m g / day . ▁Bor on ▁supp lement ation ▁marked ly ▁reduced ▁ur inary ▁calci um ▁ex cret ion ▁and ▁elev ated ▁the ▁ser um ▁concentr ations ▁of ▁ 1 7 ▁beta - est rad i ol ▁and ▁test oster one . ▁ ▁The ▁U . S . ▁Institute ▁of ▁Medicine ▁has ▁not ▁confirmed ▁that ▁bor on ▁is ▁an ▁essential ▁nut ri ent ▁for ▁humans , ▁so ▁neither ▁a ▁Re comm ended ▁Diet ary ▁Allow ance ▁( R DA ) ▁nor ▁an ▁A de qu ate ▁In take ▁have ▁been ▁established . ▁Ad ult ▁di et ary ▁int ake ▁is ▁estimated ▁at ▁ 0 . 9 ▁to ▁ 1 . 4   m g / day , ▁with ▁about ▁ 9 0 % ▁absor bed . ▁What ▁is ▁absor bed ▁is ▁mostly ▁ex cret ed ▁in ▁ur ine . ▁The ▁Tol er able ▁Upper ▁In take ▁Level ▁for ▁adult s ▁is ▁