In this equation, 58 Feinc and 57Feinc will be expressed in mg. R158/54, R157/54 and R057/ 54 are the isotope ratios expressed on weight basis for 58Fe/54Fe and 57Fe/54Fe, which will be determined on digested blood samples at time "t" after dosing and at a baseline (pre 58Fe and pre 57Fe doses) respectively; Fe circ is the quantity of the circulating iron. Fe circ at the time “t”; 0.0032 and 0.0218 are the natural abundances (weight fractions) of 58 Fe and 57 Fe. The quantity of total circulating iron, Fe circ, expressed in mg, will be estimated as