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Availability to Rats of Zinc from Soybean Seeds as Affected by Maturity of Seed, Source of Dietary Protein, and Soluble Phytate

Agricultural Research Service, U. S. Plant, Soil and Nutrition Laboratory, Tower Road, Ithaca, New York 14853

Male rats were used to assess the availability of zinc in soybean seeds (Glycine max, var. ‘Amsoy’) as affected by maturity of seed, dietary protein source, and exogenous dietary sodium phytate. Seeds were harvested from either 84-(immature) or 120-day-old (mature) plants grown in ⁶⁵Zn-labeled nutrient solutions that contained either 0.066 or 0.262 ppm zinc as zinc sulfate. Increased zinc supply to plants increased the zinc content of seeds; immature seeds had higher concentrations of zinc than mature seeds. Phytic acid concentration averaged about 0.6 and 1.7% dry weight in immature and mature seeds, respectively. Seeds of each type were incorporated into fed to zinc-depleted rats. In experiment 1, rats fed mature, high-phytate seeds absorbed about 60% of the ⁶⁵Zn in the seeds, whereas rats fed immature, low-phytate seeds absorbed about 89% of the ⁶⁵Zn (P < 0.05). In experiment 2, the source of dietary protein (egg albumen or soy protein) did not markedly affect ⁶⁵Zn absorption by rats fed mature soybean seeds. In experiment 3, dietary sodium phytate (1.76% of diet as phytic acid) depressed (P < 0.05) ⁶⁵Zn absorption by about 16 and 44% from mature soybean seeds and zinc suflate, respectively. Zinc in soybean seeds apparently was as readily available to rats as that reported for other legume and cereal seeds. Moreover, the nutritional quality of soybean seeds with respect to zinc apparently can be increased by increasing zinc supplies to plants.

KEY WORDS: • zinc • zinc availability • phytic acid • Phytin • soybeans

Manuscript received 7 December 1981.