Reducing effect of ingesting tannic acid on the absorption of iron, but not of zinc, copper and manganese by rats

Interest in the beneficial effects of polyphenols, including tannic acid (TA), is increasing, although, these compounds also have adverse effects; for example, on the absorption of iron (Fe), and possibly other trace minerals. We examined the effect of a graded dose of TA on the absorption of Fe and compared with that of zinc (Zn), copper (Cu) and manganese (Mn) in rats. We also investigated the effect of TA on cecal fermentation which plays a role in absorption. In Experiment 1, to set the optimum dose of Fe, male Sprague-Dawley rats (weighing 70-90 g) after acclimatization were fed with different levels of dietary Fe (5, 10, 20, 30 and 35 mg/kg). We observed that the hematocrit (Ht), serum Fe concentration and transferrin saturation (%) were each reduced in those rats fed less than 20 mg/kg Fe in a dose-dependent manner. In Experiment 2, the rats were fed with test diets containing the minimum required level of Fe, 30 mg/kg diet, with (5, 10, 15 and 20 g/kg diet) or without TA for a period of three weeks. Feeding a diet containing more than 10 g TA/kg diet, but not 5 g TA/kg diet, reduced the hemoglobin concentration (Hb), Ht and serum Fe concentration due to decreased Fe absorption. In contrast, the Zn, Cu and Mn absorption was not affected by TA feeding. It is also demonstrated that liver Fe, but not the Zn, Cu and Mn contents, were lower in the TA groups than in the TA-free control group. Feeding TA slightly decreased the pH value of the cecal contents with an increase in the major short-chain fatty acid pool. About 15% of the ingested TA were recovered in the feces of each TA-fed group. Our results demonstrate that more than 10 g TA/kg diet induced anemia by reducing the Fe absorption, although there was no effect on the absorption of other important trace minerals. Our findings suggest that the usual intake of polyphenols is relatively safe, but that a high intake by supplementation or by dietary habit of tannin affects only the Fe level.