Twenty-six healthy women ( 40 5 y ) were randomly assigned to receive either 600 mg of elemental Ca/d as CaCO3 ( Ca group , n = 13 ) or a placebo ( P group , n = 13 ) for 34 d. Heme Fe and non-heme Fe bioavailability were determined before and after treatment using ( 55 ) Fe and ( 59 ) Fe radioisotopes .