Author
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Hunt, Janet |
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Roughead, Zamzam |
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Submitted to: The American Journal of Clinical Nutrition
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 7/8/1999 Publication Date: N/A Citation: N/A Interpretive Summary: Iron absorption from food can vary 5-10 fold, depending on the ease of iron absorption, which is influenced by the form of iron in the food, and the enhancement or inhibition of iron absorption by other foods in the same meal. This ease of iron absorption is called bioavailability. Although dietary iron bioavailability substantially influences short-term measurements of iron absorption, bioavailability negligibly affects body iron stores in longer, controlled studies. This study investigated whether men fed diets with high or low iron bioavailability would adapt their iron absorption to maintain body iron stores. Iron absorption from whole diets was measured in 31 healthy men before and after 10 wk of consuming diets with high or low iron bioavailability. The high bioavailability diet contained more meat, ascorbic acid, and refined grains, in contrast with the low bioavailability diet, which contained plenty of whole grains, legumes, and tea. Two forms of iron in food are absorbed very differently. Heme iron, which is about 40% of the iron in meat, poultry, and fish, is very well absorbed. Nonheme iron, the rest of the iron in animal foods as well as in plant foods is less well absorbed. Adaptation occurred with nonheme, but not heme iron absorption. Iron absorption from the high bioavailability diet decreased significantly from about 1 to 0.7 mg/d and from the low bioavailability diet increased significantly from 0.12 to 0.17 mg/d in 10 wk. Blood indicators of iron nutrition were not affected, but fecal ferritin, a measure of intestinal adaptation, was significantly affected. In conclusion, normal men partially adapt to dietary iron bioavailability and short-term measurements of absorption overestimate differences in iron bioavailability between diets. Technical Abstract: Short-term measurements of iron absorption are substantially influenced by dietary bioavailability, yet bioavailability negligibly affects serum ferritin in longer, controlled trials. This study tested the hypothesis that men fed diets with high or low iron bioavailability will adapt their iron absorption to homeostatically maintain body iron stores. Heme and nonheme iron absorption from whole diets were measured in 31 healthy adult men at 0 and 10 wk while consuming weighed, 2-d repeating diets with either high or low iron bioavailability for 12 wk. The high and low bioavailability diets contained, respectively, 14.4 and 15.3 mg/d nonheme and 1.8 and 0.1 mg/d heme iron, and differed in content of meat, ascorbic acid, whole grains, legumes, and tea. Adaptation occurred with nonheme, but not heme iron absorption. Total iron absorption from the high bioavailability diet decreased from 0.96 to 0.69 mg/d (p<0.05) and from the low bioavailability diet increased from 0.12 to 0.17 mg/d (p<0.05) in 10 wk, which reduced the difference in iron bioavailability between diets from 8-fold to 4-fold. Serum ferritin was insensitive to diet, but fecal ferritin decreased rapidly and substantially on the low, compared with the high bioavailability diet. Erythrocyte incorporation of absorbed iron was associated directly with red cell distribution width and inversely with serum ferritin. In conclusion, iron-replete men partially adapt to dietary iron bioavailability, and reduce iron absorption from a high bioavailability diet to about 0.7 mg iron daily. Short-term measurements of absorption overestimate differences in iron bioavailability between diets. |
