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Bioavailability of Zinc to Rats from Defatted Soy Flour, Acid-Precipitated Soy Concentrate and Neutralized Soy Concentrate as Determined by Intrinsic and Extrinsic Labeling Techniques¹

Sandra Mills Ketelsen, Mary A. Stuart^*, Connie M. Weaver^*, Richard M. Forbes and John W. Erdman, Jr.

Departments of Food Science Animal Science, University of Illinois, Urbana, IL 61801 ^* Department of Foods and Nutrition, Purdue University, West Lafayette, IN 47907

The bioavailability of ⁶⁵Zn from intrinsically and extrinsically labeled soy flour, acid-precipitated soy concentrate and neutralized soy concentrate was evaluated in rats. Weanling rats were fed marginally zinc-deficient diets, providing 8 ppm zinc from one of these three soy products, for 7 days. The rats then received a radioactively labeled test meal, identical in composition to the previous diet except that the soy product was either intrinsically or extrinsically labeled with ⁶⁵Zn. After the test meal the rats were again fed diets the same as those consumed prior to the test meal. Whole-body retention of ⁶⁵Zn at 24 hours and 12 days as well as ⁶⁵Zn retained in tibias of rats given meals containing neutralized concentrate-based meals was significantly lower than for rats given meals containing the soy flour or acid-precipitated concentrate. In addition, retention of ⁶⁵Zn from the extrinsically labeled acid-precipitated concentrate-based meal was significantly higher than from the same product intrinsically labeled. These findings confirm the results of previous feeding studies from which it was suggested that neutralization of soy protein concentrates reduces zinc bioavailability to the rat. In addition, the results are taken to suggest that experimental conditions may influence the validity of the extrinsic labeling technique for zinc.

KEY WORDS: • zinc • bioavailability • soy • intrinsic • extrinsic • phytic acid

¹ Funding for this work was provided in part by the American Soybean Association (ASARP No. 81602).

Manuscript received 14 November 1983.