Measurements of iron absorption from prenatal multivitamin--mineral supplements.

Although prenatal multivitamin--mineral supplements containing 60 to 65 mg of iron, taken once daily, are used widely to assure that pregnant women absorb the approximately 3.5 mg of supplemental iron per day that they require, there have been no studies concerning the absorption of iron from these preparations. Using cross-over studies in groups of normal nonpregnant women of childbearing age, such iron absorption was assessed using a technique in which absorption is calculated from the measured increase in serum iron after the oral ingestion of iron in various forms. With each of 4 different brands of prenatal supplements, mean iron absorption was less than the required 3.5 mg and ranged from 1.8 to 3.0 mg. These values were significantly less (P less than .01) than the 8.1 mg that was absorbed from 65 mg of iron alone. Decreased iron absorption in the prenatal supplements was shown to be due to inhibition by calcium carbonate and magnesium oxide and, in some cases, to poor iron release. When one of the 4 brands was reformulated to contain less calcium carbonate and less magnesium oxide, mean iron absorption increased to 4.5 mg. It is concluded that the amount of iron absorbed from many prenatal multivitamin--mineral supplements is significantly less than with standard forms of iron in nonpregnant women and that bioavailability studies should be performed on pregnant patients to determine whether these commercial preparations provide adequate amounts of iron during pregnancy.