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Inhibitory effects of dietary calcium on the initial uptake and subsequent retention of heme and nonheme iron in humans: comparisons using an intestinal lavage method^1,2,3,4

From the US Department of Agriculture, Agricultural Research Service, Grand Forks Human Nutrition Research Center, Grand Forks, ND

Background: Calcium is the only reported dietary inhibitor of both heme- and nonheme-iron absorption. It has been proposed that the 2 forms of iron enter a common pool in the enterocyte and that calcium inhibits the serosal transfer of iron into blood.

Objectives: We aimed to ascertain whether the inhibitory effect of calcium occurs during initial mucosal uptake or during serosal transfer and to compare the serosal transfer of heme and nonheme iron, which should not differ if the 2 forms have entered a common mucosal iron pool.

Design: Whole-gut lavage and whole-body counting were used to measure the initial uptake (8 h) and retention (2 wk) of heme and nonheme iron with and without a calcium supplement (450 mg). Two experiments tested basal meals with low iron bioavailability and 360 mg Ca (n = 15) or with high iron bioavailability and 60 mg Ca (n = 12).

Results: Added calcium reduced the initial uptake of heme iron by 20%, from 49% to 40% from both meals (P = 0.02), and reduced the total iron absorbed from the low- and high-bioavailability meals by 25% [from 0.033 to 0.025 mg (P = 0.06) and from 0.55 to 0.40 mg (P < 0.01), respectively]. Calcium did not affect the serosal transfer of either form of iron.

Conclusions: Calcium supplementation reduced heme and total iron without significantly affecting nonheme-iron absorption, regardless of meal bioavailability. Calcium inhibited the initial mucosal uptake rather than the serosal transfer of heme iron. Differences in serosal transfer indicate that heme and nonheme iron did not enter a common absorptive pool within 8 h after a meal.

Key Words: Heme iron • nonheme iron • whole-body counting • whole-gut lavage • mucosal uptake • absorption • retention • serosal transfer • humans • calcium • bioavailability