Inositol phosphates with different numbers of phosphate groups influence iron absorption in humans

Background: Inositol hexaphosphate (TP,) is a well-known inhibitor of iron absorption, whereas the effects of the less-phosphorylated derivatives of I P6 are less known. Objectives: The objective was to investigate the effects of inositol tri-, tetra-, and pentaphosphates (IP3, IP4, and IP5, respectively) on iron absor ption in humans. Design: Iron absorption was measured in 5 experiments from single meals by extrinsic labeling with Fe-55 and Fe-59 and determination of whole-body ret ention and the erythrocyte uptake of isotopes. In experiments 1-3 the meals contained white-wheat rolls to which 10 mg P as IP5, IP4, or IP3, respecti vely, was added. Inositol 1,2,6-triphosphate [Ins(1,2,6)P-3] and a mixture of isomers of IP4 and IP5 were studied. White-wheat rolls contained 10 mg P as IP3 + IP4 and 2 mg P as IP5 + IP6 in experiment 4 and 20 mg P as IP3 IP4 and 3 mg P as IP5 + IP6 in experiment 5; inositol phosphates were obtai ned via fermentation of sodium phytate. Each experiment had 8-11 subjects. Results: In experiment 1, iron absorption was reduced by 39%, whereas there was no significant effect on iron absorption in experiments 2 and 3. In ex periments 4 and 5, iron absorption was reduced by 54% and 64%, respectively , suggesting that IP3 and IP4 contributed to the inhibitory effect. Conclusions: IP5 has an inhibitory effect on iron absorption, whereas IP3 a nd IP4 in isolated form have no such effect. IP3 and IP4 in processed food contribute to the negative effect on iron absorption, presumably by binding iron between different inositol phosphates. To improve iron absorption fro m cereals and legumes, degradation of inositol phosphates needs to be to le ss-phosphorylated inositol phosphates than IP3.