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Inositol phosphates with different numbers of phosphate groups influence iron absorption in humans^1,2,3

¹ From the Department of Food Science, Chalmers University of Technology, and the Department of Clinical Nutrition and Internal Medicine II, University of Göteborg, Sahlgrenska Hospital, Göteborg, Sweden

Background: Inositol hexaphosphate (IP₆) is a well-known inhibitor of iron absorption, whereas the effects of the less-phosphorylated derivatives of IP₆ are less known.

Objectives: The objective was to investigate the effects of inositol tri-, tetra-, and pentaphosphates (IP₃, IP₄, and IP₅, respectively) on iron absorption in humans.

Design: Iron absorption was measured in 5 experiments from single meals by extrinsic labeling with ⁵⁵Fe and ⁵⁹Fe 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 IP₅, IP₄, or IP₃ , respectively, was added. Inositol 1,2,6-triphosphate [Ins(1,2,6)P₃] 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 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 IP₃ and IP₄ contributed to the inhibitory effect.

Conclusions: IP₅ has an inhibitory effect on iron absorption, whereas IP₃ and 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 from cereals and legumes, degradation of inositol phosphates needs to be to less-phosphorylated inositol phosphates than IP₃.

Key Words: Phytate • inositol phosphates • iron absorption • humans

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