Is the recommended daily iron intake for women too low?

To the editor:

The recommended daily allowance/intake (RDA/RDI) for iron in menstruating and nonmenstruating women in the United Kingdom is 14.8 mg and 8.7 mg, respectively,¹  whereas in the United States, 18 mg and 8 mg are the general recommendations.²  The RDA/RDI levels are therefore considered sufficient to maintain the female iron balance; however, with 90% of menstruating and 34% of nonmenstruating United Kingdom women failing to attain these levels from their diet,¹  supplementation is required.

Serum ferritin concentrations, in the absence of inflammation, provide a reliable estimate of a person's iron status.³  Although a value of 69 μg/L indicates a 99% level of confidence for stainable bone marrow iron,⁴  Guyatt et al suggest that a serum ferritin concentration of 100 μg/L or higher ensures that a person is iron replete without the need to undertake bone marrow aspirates.⁵  In a study of more than 500 menstruating United Kingdom wartime servicewomen receiving a daily food ration providing 35 mg of iron, all had significant increases in hemoglobin concentrations 10 months later.⁶  In men and women given a daily iron supplement of 100 mg, increases in hemoglobin concentrations occurred only in females and the mean values between the sexes were very similar.⁷  Serum ferritin estimations were not available to these earlier researchers nor did they know about hepcidin and iron regulation.⁸ 

We observed 52 iron-replete (serum ferritin ≥ 100 μg/L, erythrocyte sedimentation rate [ESR] ≤ 10 mm/h) women who for 6 months received either a daily iron (Fe) supplement providing Fe 24 mg or Fe 48 mg, or nothing. There were no vegetarians in group I, 1 in groups II and IV, and 2 in groups III and V. No person was vegan.

Nonmenstruating and menstruating women who received no iron (groups I and III) had significant decreases in the mean serum ferritin level after 6 months, as did menstruating women receiving Fe 24 mg/d (group IV; Table 1). In contrast, in nonmenstruating women who received Fe 24 mg/d (group II) and menstruating women receiving Fe 48 mg/d (group V), no significant change was observed.

If the RDA/RDI values are adequate, based on dietary data¹  we would have expected 66% of nonmenstruating women in the United Kingdom to be capable of maintaining their baseline serum ferritin level; however, only 1 woman (11%) in group I (no iron) was able to do so. In contrast, 10 nonmenstruating women (90%) taking Fe 24 mg/d (group II) maintained their ferritin concentrations. It is noteworthy that this level of iron supplementation is 3 times higher than the United States/United Kingdom RDA/RDI recommendations. In menstruating women taking no iron (group III), we expected 10% capable of maintaining their serum ferritin concentration by diet alone; none were able to do so. In menstruating women taking Fe 24 mg/d, only 3 (25%) maintained their ferritin status. In those taking Fe 48 mg/d (group V), 80% maintained their serum ferritin concentrations; however, 20% remained in a negative iron balance despite taking 3 times the RDA/RDI levels.

The RDA/RDI recommendation for iron in women appears inadequate, and further study in a larger cohort of iron-replete women is urgently required.