Effect of ascorbic acid intake on nonheme-iron absorption from a complete diet

Background: Ascorbic acid has a pronounced enhancing effect on the absorpti on of dietary nonheme iron when assessed by feeding single meals to fasting subjects. This contrasts with the negligible effect on iron balance of lon g-term supplementation with vitamin C. Objective: Our goal was to examine the effect of vitamin C on nonheme-iron absorption from a complete diet rather than from single meals. Design: Iron absorption from a complete diet was measured during 3 separate dietary periods in 12 subjects by having the subjects ingest a labeled whe at roll with every meal for 5 d. The diet was freely chosen for the first d ietary period and was then altered to maximally decrease or increase the di etary intake of vitamin C during the second and third periods. Results: There was no significant difference in mean iron absorption among the 3 dietary periods despite a range of mean daily intakes of dietary vita min C of 51-247 mg/d. When absorption values were adjusted for differences in iron status and the 3 absorption periods were pooled, multiple regressio n analysis indicated that iron absorption correlated negatively with dietar y phosphate (P = 0.0005) and positively with ascorbic acid (P = 0.0069) and animal tissue (P = 0.0285). Conclusions: The facilitating effect of vitamin C on iron absorption from a complete diet is far less pronounced than that from single meals. These fi ndings may explain why several prior studies did not show a significant eff ect on iron status of prolonged supplementation with vitamin C.