Adaptation in iron absorption: iron supplementation reduces nonheme-iron but not heme-iron absorption from food

Background: Results of cross-sectional studies suggest that in healthy peop le, iron absorption adapts to meet physiologic needs and stabilize iron sto res, but this has nor been adequately tested in longitudinal studies. Objective: We tested whether heme- and nonheme-iron absorption decrease in response to increased iron intake and whether iron stores reach a steady st ate. Design: In a randomized, placebo-controlled trial, heme- and nonheme-iron a bsorption by healthy men and women (n = 57) were measured before and after 12 wk of supplementation with 50 mg Fe/d as ferrous sulfate. Serum and feca l ferritin were measured during supplementation and for 6 mo thereafter. Results: Initially, both heme- and nonheme-iron absorption were inversely a ssociated with serum ferritin concentration. Volunteers who took iron suppl ements. even those with serum ferritin < 21 mu g/L (n = 5), adapted to abso rb less nonheme iron (3.2% at week 12 compared with 5.0% at week 0, P < 0.0 01) but not less heme iron from a beef-based meal. Serum ferritin concentra tion :vas slightly but significantly higher after iron supplementation than after placebo (difference = 13 mu g/L). This higher ferritin concentration persisted for 26 mo after supplementation, except in subjects with low iro n stores, whose serum ferritin returned to baseline within 3 mo. Fecal ferr itin excretion increased 2.5-fold (P < 0.04) during supplementation. Conclusions: Healthy individuals, even those with low iron tores. had reduc ed nonheme-iron absorption from food in response to iron supplementation. D espite this partial adaptation, iron stores were greater after iron supplem entation than after placebo and this difference was sustained, except in in dividuals with the lowest iron stores.