COMPARATIVE STUDY OF THE EFFECTS OF REPLACING CASEIN BY SOYBEAN FLOUR ON THE CHOLESTEROL METABOLISM OF RATS FED WITH NORMOCALORIC AND HYPERCALORIC DIETS

Although regular soy consumption is mainly in the whole grain form, most of the research on soybean has been conducted on soy protein concentrate or isolate. Therefore, the aim of this investigation was to evaluate the potential benefits of soybean flour on the cholesterol metabolism of rats previously fed with normocaloric and hypercaloric casein-based diets. The study was performed with two groups, one fed with control diet (AIN-93) and one with hypercaloric diet, for 9 weeks. Each group was then divided into two subgroups, and casein was replaced by soybean flour in one of them, for 6 weeks (subgroups: control casein (CC), control soy (CS), hypercaloric casein (HC) and hypercaloric soy (HS)). We measured total cholesterol (Total-C) and high density lipoprotein cholesterol (HDL-C) in serum by the enzymatic method; Low-density lipoprotein (LDL-C) using the Friedewald formula and very low-density lipoprotein-cholesterol (VLDL-C) by TG/5. In liver, we measured: total lipids by dry weight; total cholesterol (TC), free cholesterol (FC) and esterified cholesterol (EC) by spectrophotometry; and the expression of 3-hydroxy-3-methylglutaryl-Coenzyme A reductase (HMG-CoAR), low-density lipoprotein receptor (LDLR), sterol regulatory element binding protein 2 (SREBP2), cholesterol 7 α-hydroxylase (CYP7A1) and apolipoprotein B (APOB) by reverse transcription polymerase chain reaction (RT-PCR). When comparing CS vs. CC, a positive effect of soybean flour was found by decreasing the Total-C (P<.01) and LDL-C (P=.05) in serum. In hypercaloric diet, soybean flour, when compared with casein, decreased total lipids (P<.01), TC (P=.05) and FC (P<.01), and the HMG-CoAR (P<.01) and CYP7A1 (P<.001) expressions in liver. These results suggest that replacing casein with soybean flour in normocaloric diets reduces cholesterol in serum, while in hypercaloric diets it improves cholesterol content in rat liver. Therefore, the consumption soybean flour (whole grain) is recommended as a good resource for improving dietary cholesterol metabolism and achieving beneficial health effects.