Background: Inositol hexaphosphate (IP(6)) is a well-known inhibitor of iron absorption, whereas the effects of the less-phosphorylated derivatives of IP(6) are less known.
Objectives: The objective was to investigate the effects of inositol tri-, tetra-, and pentaphosphates (IP(3), IP(4), and IP(5), respectively) on iron absorption in humans.
Design: Iron absorption was measured in 5 experiments from single meals by extrinsic labeling with (55)Fe and (59)Fe and determination of whole-body retention and the erythrocyte uptake of isotopes. In experiments 1-3 the meals contained white-wheat rolls to which 10 mg P as IP(5), IP(4), or IP(3), respectively, was added. Inositol 1,2,6-triphosphate [Ins(1,2, 6)P(3)] and a mixture of isomers of IP(4) and IP(5) were studied. White-wheat rolls contained 10 mg P as IP(3) + IP(4) and 2 mg P as IP(5) + IP(6) in experiment 4 and 20 mg P as IP(3) + IP(4) and 3 mg P as IP(5) + IP(6) in experiment 5; inositol phosphates were obtained 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 experiments 4 and 5, iron absorption was reduced by 54% and 64%, respectively, suggesting that IP(3) and IP(4) contributed to the inhibitory effect.
Conclusions: IP(5) has an inhibitory effect on iron absorption, whereas IP(3) and IP(4) in isolated form have no such effect. IP(3) and IP(4) in processed food contribute to the negative effect on iron absorption, presumably by binding iron between different inositol phosphates. To improve iron absorption from cereals and legumes, degradation of inositol phosphates needs to be to less-phosphorylated inositol phosphates than IP(3).