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Effect of Phytate:Zinc Molar Ratio and Isolated Soybean Protein on Zinc Bioavailability¹

Central Research, Ralston Purina Company, Checkerboard Square, St. Louis, MO 63188

The effect of phytate to zinc (Zn) molar ratio on zinc bioavailability was evaluated in rats. The bioavailability was determined by giving an oral dose of ⁶⁵Zn and measuring the liver uptake and disappearance from the gastrointestinal tract after 4 hours and fecal and urinary ⁶⁵Zn excretion from 24 to 168 hours. Rats were fed a diet containing 12 ppm zinc from zinc sulfate with and without phytic acid from sodium phytate for 14 days. At the end of 14 days feeding, the rats were intubated with a homogenized diet containing 12 ppm zinc, one µCi ⁶⁵Zn and graded levels of phytic acid so that the phytate:Zn molar ratio varied from 0 to 100. Zinc bioavailability was significantly reduced only in the rats fed diets containing phytate and intubated diet containing phytate:zinc molar ratio of 12.5 and above. The results from this study also indicated that measuring ⁶⁵Zn disappearance and uptake was a valid bioassay for determining zinc bioavailability. The effect of isolated soybean protein on extrinsic zinc bioavailability was also evaluated in rats fed zinc adequate and zinc deficient diets. The results showed that zinc bioavailability was significantly reduced by replacing egg white protein with isolated soybean protein only when the rats were fed zinc deficient diet prior to single oral dose of ⁶⁵Zn mixed with test proteins. The effect of isolated soybean protein on intrinsic zinc bioavailability for the growing rats was also evaluated. The results demonstrated that the zinc bioavailability in isolated soybean protein can be improved by fortifying with zinc so that the phytate:Zn molar ratio is less than 10.

KEY WORDS: • phytate • zinc bioavailability • soybean protein

¹ Presented in part at the 65th annual meeting of the Federation of American Societies for Experimental Biology, Atlanta, April, 1981. Fed. Proc. 40, 938 (Abs. 4017).

Manuscript received 28 April 1981.