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Dietary soy protein and isoflavones have no significant effect on bone and a potentially negative effect on the uterus of sexually mature intact Sprague-Dawley female rats

Nakai, Masaaki PhD1; Cook, Leslie MS1; Pyter, Leah M MS1; Black, Molly VT2; Sibona, Jean PhD3; Turner, Russell T PhD3; Jeffery, Elizabeth H PhD2; Bahr, Janice M PhD1

doi: 10.1097/01.GME.0000146109.50235.DO
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Objective: To evaluate the effect of dietary soy protein and isoflavones on bone and the reproductive tract in premenopausal rats.

Design: Three-month-old intact Sprague-Dawley female rats (N = 50) were fed diets containing casein, soy protein, or casein with isoflavone extract for 12 weeks. The amount of casein, soy protein, and extract (per kilogram diet) in each group was: (1) 200 g casein (control); (2) 100 g casein plus 100 g soy protein (low soy); (3) 200 g soy protein (high soy); 4) 200 g casein plus 17.2g extract (low extract); and (5) 200 g casein plus 34.4 g extract (high extract). Diet consumption, body weight, uterine wet weight, urinary deoxypyridinoline concentration, and bone mineral density of the femur and lumbar vertebrae were measured. Femur rigidity was evaluated by histomorphometry. The uterus and vagina were studied histologically.

Results: Rats in all treatment groups had lower body weights and lower deoxypyridinoline concentrations compared with controls, but none of the differences was statistically significant. There was no significant difference in femur and lumbar bone mineral density, uterine wet weights, or histomorphometry between the control and treatment groups. Histologically, uteri and vaginae were normal in all groups except that 1 of 10 rats in the high-soy group and 2 of 10 rats in the high-extract group showed extensive squamous metaplasia in the uterine gland.

Conclusion: These results suggest that dietary isolated soy protein and isoflavones have no effect on bone and the vagina during premenopausal period, but may have an adverse effect on the uterus.

The effect of dietary soy protein and isoflavones on bone and the reproductive tract was evaluated in adult intact Sprague-Dawley rats. The results suggest that soy protein and isoflavones have no effect on bone and the vagina but may have adverse effects on the uterus of premenopausal women.

From the Departments of 1Animal Sciences and 2Food Science and Human Nutrition, University of Illinois, Urbana, IL; and 3Department of Orthopedics, Mayo Graduate School of Medicine, Rochester, MN.

Received May 26, 2004; revised and accepted September 15, 2004.

This study was supported in part by National Institutes of Health Grant AG17521 (J.M.B./R.T.T./E.H.J.).

Address correspondence to: Janice Bahr, PhD, Professor, Department of Animal Sciences, University of Illinois, 1207 W. Gregory Dr, Urbana, IL 61801. E-mail: jbahr@uiuc.edu.

©2005The North American Menopause Society