The Chemical Forms of Iron in Commercial Prenatal Supplements Are Not Always the Same as Those Tested in Clinical Trials

In the US, 70% of pregnant women use an iron-containing prenatal supplement product; however, only 2.6% of pregnant women have iron-deficiency anemia and 16.3% are iron deficient. Yet, published data on the amounts and chemical forms of iron used in formulating these products are not available, although they are known to affect bioavailability. This information is especially important in comparing commercially available products with those that were tested in clinical trials. Our examination of nonprescription and prescription iron-containing prenatal supplement products in NIH's Dietary Supplement Label Database (DSLD) and DailyMed found the labeled amount of elemental iron ranged between 9 and 60 mg/serving in 148 nonprescription supplements and between 4.5 and 106 mg/serving in 101 prescription supplements. Ferrous fumarate was the preferred chemical form used in these products. In contrast, ferrous sulfate was the preferred chemical form of iron reported in the clinical trials summarized in a 2015 Cochrane Systematic review assessing the effects of daily oral iron supplements for pregnant women. Ferrous sulfate was not found on any prenatal supplement product label in the DSLD or DailyMed. The chemical forms of products on the market and those tested in clinical trials are dissimilar, and we believe this may have clinical implications. The findings raise several questions. Do outcomes in clinical trials correlate with the benefits and risks that might adhere to iron supplements with different iron formulations? Should the differences in chemical forms, their bioavailability, and safety profiles, be considered in greater depth when evaluating the effect of the various formulations on maternal iron nutriture? Should new clinical trials for pregnant and lactating women in the US use a form of iron not found in prenatal supplements sold in the US or should a more common form be used?