Dietary soy protein and isoflavones: no effect on the reproductive tract and minimal positive effect on bone resorption in the intact female Fischer 344 rat

Our previous study evaluating 3 months of feeding soy protein or isoflavones (IF) to intact adult Sprague-Dawley (SD) rats showed no change in bone and the vagina but occasional extensive squamous metaplasia of the uterine glandular epithelium was observed. The current study was designed to characterize further these effects of soy protein or IF on the uterus using the Fischer 344 (F344) rat, a known high responder strain to estrogenic stimuli. Three-month-old intact F344 rats were divided into five groups and fed diets containing either casein, low or high amount of soy protein or casein with low or high amount of IF extract. Body weight, urinary deoxypyridinoline (Dpyr), bone mineral density (BMD) of the femur and lumbar, uterine wet weight and histology of the reproductive tract were evaluated. No significant difference was seen in bone parameters between control and treatment groups except for a lower Dpyr in the high soy and a higher lumbar BMD in the low soy groups. No alteration was seen in the reproductive tract of all treatment groups. Contrary to our hypothesis, the present results suggest that the uterus of the F344 strain is less sensitive to dietary soy protein and IF than that of the SD strain.