Submitted to: The American Journal of Clinical Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/13/2001
Publication Date: 8/1/2002
Citation: Roughead, Z.K., Zito, C.A., Hunt, J.R. 2002. Initial uptake and absorption of nonheme iron and absorption of heme iron in humans are unaffected by the addition of calcium as cheese to a meal with high iron bioavailability. American Journal of Clinical Nutrition. 76:419-425.
Interpretive Summary: We designed this study to compare how the intestinal cell processes the two different forms of iron found in food (heme and nonheme). We were also interested in whether adding calcium to food in the form of cheese would affect iron absorption. Healthy men and women ate a hamburger meal and a cheeseburger meal in random order. Each time, we allowed 8 hours for the intestinal cells to take up the two forms of iron and then the volunteers drank a special solution called GoLytely which completely cleansed the intestinal tract of its contents. We compared the amount of heme and nonheme iron which remained in the body after 8 hours to the amounts remaining in the body 2 weeks later. We found that adding a slice of cheese to a hamburger meal did not change the absorption of either form of iron. Also, as a percent of what was present in the meal, more heme than nonheme iron was taken up by the intestinal cell (35% vs. 11%), and also retained by the body (15% vs. 7%). However, compared to what was initially taken up into the cell, more nonheme iron is transferred to the blood stream than heme iron (63% vs. 43%). We have concluded that the initial uptake into the intestinal cell is an important step in the regulation of nonheme iron absorption. Also, we concluded that heme iron is more bioavailable than nonheme iron at least partially because of its higher initial uptake by the intestinal cell. Although adding a slice of cheese to the meal did not affect the iron absorption, higher amounts of calcium as could be consumed from supplements or fortified foods may inhibit iron absorption.
Technical Abstract: Quantitative data on the mucosal uptake and serosal transfer components of iron absorption in humans are limited for nonheme iron and are not available for heme iron. Our objective was to concurrently measure and compare the initial mucosal uptake and subsequent serosal transfer of heme and nonheme iron in humans. Whole gut lavage and whole body scintillation methodology were applied to compare the 8 h uptake and 2 wk retention (absorption) of heme and nonheme iron by healthy adults (n = 17) after consuming radiolabeled meals. The initial uptake and 2-wk retention of heme iron (36% and 15%, respectively) were significantly greater than for nonheme iron (11% and 7%, respectively, p<0.001). Expressed as a fraction of initial iron uptake, heme iron was less efficiently retained than nonheme iron (0.43 vs. 0.63, p<0.0001). The initial uptake and absorption of nonheme, but not heme iron, correlated inversely with serum ferritin. A modest addition of calcium (127 mg as cheese) to the test meal did not affect the iron absorption measurements. In summary, for nonheme iron, the inverse correlation between the initial mucosal uptake and serum ferritin was stronger than that between absorption and serum ferritin. This indicates that the initial uptake may be the primary control point for regulation of nonheme iron absorption. The higher bioavailability of heme iron compared to nonheme iron was explained primarily by its higher initial uptake into the mucosal cell. Heme and nonheme iron were not similarly released by the mucosal cell indicating that the two forms of iron had not formed a common iron pool inside the mucosal cell.