Effects of soybean isoflavones, probiotics, and their interactions on lipid metabolism and endocrine system in an animal model of obesity and diabetes

The effects of soybean isoflavones with or without probiotics on tissue fat deposition, plasma cholesterol, and steroid and thyroid hormones were studied in SHR/N-cp rats, an animal model of obesity, and were compared to lean phenotype. We tested the hypothesis that probiotics by promoting the conversion of isoflavone glycosides to their metabolically active aglycone form will have a synergistic effect on body fat, cholesterol metabolism, and the endocrine system. Obese and lean SHR/N-cp rats were fed AIN-93 diets containing 0.1% soy isoflavone mixture, 0.1% probiotic mixture, or both together. Different fat tissues were teased and weighed. Plasma was analyzed for cholesterol and steroid and thyroid hormones. In both phenotypes, isoflavones lowered fat deposition in several fat depots. Probiotics alone had no significant effect on fat depots. Isoflavones lowered total, LDL, and HDL cholesterol in lean rats, but in obese rats isoflavones lowered only total and LDL cholesterol. Isoflavones also lowered many of the steroid hormones involved in lipid metabolism but had no significant effect on thyroid hormones. Probiotics had no significant effect on cholesterol or hormones. Thus, our data show that soy isoflavones also lower plasma cholesterol and that this hypocholesterolemic effect appears to be due in part to the modulation of steroid hormones. Probiotics do not seem to enhance the effect of isoflavones.