Serum cobalamins and cobalamin-binding proteins during total parenteral nutrition

Levels of serum cobalamin and both saturated and unsaturated serum cobalamin-binding proteins (transcobalamin II and R-binders) were determined prospectively in 43 patients before and after 2 weeks of total parenteral nutrition (TPN). Nine patients showed subnormal serum levels of cobalamin but none of them had clinical signs of cobalamin deficiency. Serum cobalamin levels significantly declined after 2 weeks of TPN as did both saturated and unsaturated R-binder levels whereas transcobalamin II levels remained constant. Since cobalamin in serum is entirely bound to proteins, of which R-binders comprise 80-90%, the changes in R-binder levels will markedly influence serum cobalamin levels, in fact they adequately explain the short time fluctuations of serum cobalamin levels observed. It is argued that the changes in serum cobalamin levels may reflect a changed distribution of cobalamins over the various body compartments rather than decreased or increased body stores. Moreover, only cobalamin bound to transcobalamin II is delivered to the tissues. Therefore, serum cobalamin levels alone may inadequately reflect cobalamin availability in patients receiving TPN.