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Dietary Soybean Protein Prevents Bone Loss in an Ovariectomized Rat Model of Osteoporosis^1,2,

Bahram H. Arjmandi³, Lee Alekel, Bruce W. Hollis^*, Daxa Amin, Maria Stacewicz-Sapuntzakis, Peilin Guo and Subhash C. Kukreja

Department of Human Nutrition and Dietetics, University of Illinois at Chicago, Chicago, IL 60612 ^* Department of Pediatrics, Medical University of South Carolina, Charleston, SC 29425 Department of Medicine, West Side Veterans Administration Medical Center, Chicago, IL 60612

The purpose of this study was to examine whether soybean protein isolate prevents bone loss induced by ovarian hormone deficiency. Thirty-two 95-d-old Sprague-Dawley rats were randomly assigned to four treatment groups [sham-operated (sham); ovariectomized (ovx); ovx + soybean; ovx + 17ß-estradiol (E₂)] and killed after 30 d. Rats in the sham, ovx and ovx + 17ß-estradiol groups were fed a casein-based diet, and the soybean group was fed soybean protein isolate instead of casein; the diets were otherwise comparable. Rats in the ovx group had significantly lower densities of the right femur (P < 0.001) and the fourth lumbar vertebra (P < 0.05) than rats in the sham group. These lower bone densities were not observed in animals receiving 17ß-estradiol or fed soybean. The ovx group also had significantly (P < 0.01) greater serum concentrations of 1,25-dihydroxycholecalciferol than the other three groups. Our findings suggest that dietary soybean protein is effective in preventing bone loss due to ovarian hormone deficiency. Because serum activities of both alkaline phosphatase and tartrate-resistant acid phosphatase were significantly greater in the ovx group and in the ovx + soybean group but not in the group receiving 17ß-estradiol, compared with sham animals, this confirms that ovariectomy enhances and 17ß-estradiol suppresses the rate of bone turnover. Despite the higher rate of bone turnover in the soybean-fed animals, the vertebral and femoral bone densities of these rats were significantly greater than those of rats in the ovx group, suggesting that formation exceeded resorption. Further studies are needed to clarify whether this protective effect on bone is due to the protein itself or to the presence of isoflavones in soybean protein.

KEY WORDS: • soybean protein isolate • estradiol • ovariectomy • bone loss • rats

¹ Supported in part by a grant from the Campus Research Board, University of Illinois at Chicago.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence and reprint requests should be addressed.

Manuscript received 11 May 1995. Revision accepted 6 October 1995.

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