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Inhibitory Effects of Phytic Acid and Other Inositol Phosphates on Zinc and Calcium Absorption in Suckling Rats

Bo Lönnerdal, Ann-Sofi Sandberg, Brittmarie Sandström and Clemens Kunz¹

Department of Nutrition, University of California, Davis, CA 95616 Department of Food Science, Chalmers University of Technology, Göteborg, Sweden Department of Clinical Nutrition, Sahlgrenska Hospital, University of Göteborg, Sweden

While it is known that phytic acid, inositol hexaphosphate, has a negative effect on zinc and calcium absorption, the effects of inositol which is phosphorylated to a lesser extent are less known. We have prepared inositol triphosphate (IP-3), tetraphosphate (IP-4), pentaphosphate (IP-5) and hexaphosphate (IP-6) by hydrolysis of sodium phytate and separation by ion-exchange chromatography and have studied their effect on zinc and calcium absorption. Using a suckling rat pup model, we found that liver uptake of ⁶⁵Zn after 6 h was 5% of the total dose from solutions of IP-6, 19% from IP-5, 28% from IP-4, 29% from IP-3 and 31% from ZnCl₂ (control). Non-absorbed calcium was 17%, 1.4%, 0.5%, 0.5% and 0.5% of the given dose of ⁴⁵Ca, respectively. Thus, at a high degree of phosphorylation (IP-6, IP-5), zinc and calcium uptake was inhibited, while no effect was observed for the other phosphates. Consequently, total "phytate" analysis, which includes inositol phosphates with varying degrees of phosphorylation, can give misleading information with regard to mineral availability. In addition, even limited dephosphorylation of inositol hexaphosphate can have a positive effect on mineral absorption.

KEY WORDS: • phytic acid • inositol phosphate • zinc • calcium • minerals • trace elements • absorption

¹ Supported by a fellowship from Deutsche Forschungsgemeinschaft.

Manuscript received 24 March 1988. Revision accepted 1 November 1988.

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