Metabolism of 14C-Phytate in Rats: Effect of Low and High Dietary Calcium Intakes

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Abstract
The purpose of this study was to explore in rats the in vivo metabolism of phytate, using [U-14C]phytate and to assess the effects of calcium intake on phytate and myo-inositol metabolism. Labeled phytate was prepared from wheat following injection of the plants at the milk stage with [U-14C]myo-inositol. Groups of 30-day-old male rats were adapted for at least 6 weeks either to a low-calcium (2.9 mM/100 g dry diet) or a high-calcium (30.6 mM/100 g dry diet) cereal-based diet. They were then transferred to metabolic cages, continued on their respective diets and given an oral dose of [14C]phytate or [14C]myo-inositol. Appearance of radioactivity in expired air, feces and urine was monitored for 48 hours; the rats were then killed for determination of 14C-activity in liver, kidneys, blood and femur. The high-calcium diet significantly increased the output of radioactivity in feces (54% of dose) and reduced the appearance of radioactivity in expired air (25% dose) and in body tissues following 14C-phytate administration. The high-calcium intake did not affect the fate of [14C]myo-inositol (96% of dose). These results suggest that phytate or a derivative(s) is almost quantitatively absorbed (94% of dose) when calcium intake is low (Ca/P molar ratio; 0.21) and it is extensively oxidized to CO2 (60% of dose). Thus, a high-calcium intake (Ca/P molar ratio; 2.24) inhibits the utilization of [14C]phytate. Hence, it is suggested that the impact of dietary phytate on trace mineral bioavailability will depend upon the presence of factors, including excess calcium, that alter the absorption and utilization of phytate.