Submitted to: American Journal of Clinical Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2009
Publication Date: 3/1/2009
Citation: Hunt, J.R., Zito, C.A., Johnson, L.K. 2009. Body Iron Excretion by Healthy Men and Women. American Journal of Clinical Nutrition. 89(6):1792-1798.
Interpretive Summary: Iron is excreted from the body so gradually that accurate measurements of human iron excretion have required measurement of the gradual dilution of the blood concentrations of an administered iron isotope over several months or years. A limited number of such measurements in men have been a primary basis for determining recommendations for iron intake of men, and by extrapolation, women. The objective of this study was to reproduce iron excretion measurements in healthy men and extend them to women. The turnover rate of an iron isotope that had been administered at least one year earlier was determined from blood sampled twice a year for up to 3 years from 53 volunteers. The median iron excretion in mg/d was 1.18 (0.11 to 2.07) for 29 men, 1.58 (0.65 to 4.88) for 19 menstruating women, and 0.99 (0.86 to 1.57) for 5 postmenopausal women. The distribution of iron excretion highly skewed for the menstruating women, with menstrual iron accounting for 90% of the differences between women. The body iron of the menstruating women decreased by 4.6%, and for the men, tended to increase by 1.5% during the study. These results extend limited iron excretion measurements in men to include direct measurements in women, which can be applied to setting dietary iron recommendations. A 40-fold range in human iron excretion is reflected in human requirements for absorbed iron, which in turn can be met by the body adjusting the efficiency of iron absorption, provided that the diet contains sufficient and absorbable iron.
Technical Abstract: Background: Iron excretion measured by isotope dilution has been a primary basis for factorial derivation of recommendations for iron intake, but results have been available for men only. Objective: The objective of this study was to reproduce iron excretion measurements in healthy men and extend them to women. Design: The turnover rate of 55 Fe that had been administered at least one year earlier was determined from blood sampled semi-annually for up to 3 years from 53 subjects in the community. Body iron was determined from hemoglobin, serum ferritin, and transferrin receptor. Complete menstrual collections were obtained from 13 women. Results: Total iron excretion in mg/d was (median (range)) 1.18 (0.11-2.07) for 29 men, 1.58 (0.65-4.88) for 19 menstruating women, and 0.99 (0.86-1.57) for 5 postmenopausal women. If hormonal contraceptive users were omitted, the median for 15 menstruating women increased to 1.66 mg/d. The distribution of iron excretion was normal for the men and postmenopausal women, and highly skewed for the menstruating women, with menstrual iron accounting for 90% of the variation. Iron excretion was not strongly related to body weight. The body iron of the menstruating women decreased somewhat, by 4.6%, and for the men, tended to increase by 1.5% during the study. Conclusions: The results extend direct iron excretion measurements in men to include similar measurements in women. The results emphasize the wide range of iron excretion in humans, resulting in a 40-fold range of requirements for absorbed iron.