The mechanisms whereby soy protein may exert its beneficial effects on obesity are not completely clear. Several lines of evidence suggest that soy protein may favorably affect lipid absorption, insulin resistance, fatty acid metabolism, and other hormonal, cellular, or molecular changes associated with adiposity.
It is well established that soy protein consumption reduces serum total cholesterol, LDL cholesterol, and triglycerides as well as hepatic cholesterol and triglycerides. Studies in animals indicate that soy protein ingestion exerts its lipid-lowering effect by reducing intestinal cholesterol absorption and increasing fecal bile acid excretion, thereby reducing hepatic cholesterol content and enhancing removal of LDL (49,50). Dietary soy protein has also been shown to directly affect hepatic cholesterol metabolism and LDL receptor activity. For example, Lovati and co-workers demonstrated an increased binding of VLDL to liver membranes of hypercholesterolemic rats fed a diet containing soy protein, suggesting altered hepatic metabolism with increased LDL and beta-VLDL removal by hepatocytes. Another study by Lovati et al have shown that soy protein diet consistently increased degradation of LDL by mononuclear cells from patients with hypercholesterolemia, even in the presence of an elevated cholesterol intake. Additional support for an effect of soy protein on LDL receptor activity was provided by Kirk et al. in their studies using the LDL-receptor deficient (LDLr-null) mouse. In this study, significant reductions in plasma concentrations of total cholesterol, LDL-C, and VLDL-C were observed in C57BL/6J (wild type) mice fed soy protein isolate. By contrast, no significant effect of the soy protein isolate on plasma lipids was observed in LDLr-null mice, suggesting that soy isoflavones might reduce lipid levels by increasing LDL receptor activity. Earlier work in humans with normal and elevated serum cholesterol has also shown that dietary soy protein reduces insulin/glucagon ratio, which may contribute to the hypocholesterolemic effect of soy protein. More recently, Gudbrandsen et al have shown that feeding obese Zucker rats with soy protein concentrate enriched with isoflavones (HDI) for 6 weeks reduced fatty liver and decreased the plasma levels of alanine transaminase and aspartate transaminase. These effects were accompanied by increased activities of mitochondrial and peroxisomal beta-oxidation, acetyl-CoA carboxylase, fatty acid synthase and glycerol-3-phosphate acyltransferase in liver, increased plasma triacylglycerol level, and decreased hepatic mRNA level of VLDL receptor. However, the decreased gene expression of VLDL receptor found in the liver was not observed in epididymal fat and skeletal muscle of rats fed HDI, indicating that the liver may be the primary organ responsible for the reduced clearance of triacylglycerol-rich lipoproteins from plasma after HDI feeding. Thus, soy protein appears to exert its cholesterol-lowering action through different mechanisms that modulate cholesterol absorption and metabolism.
There is in-vivo evidence that soy protein may influence lipogenesis in the liver. In studies of rats, Iritani et al have shown that dietary soybean protein reduced the concentrations of triglycerides in plasma and especially in liver. These effects were associated with marked reductions in the activities of hepatic lipogenic enzymes, particularly glucose-6-phosphate dehydrogenase, malic enzyme, fatty acid synthetase, as well as acetyl-CoA carboxylase (ACC), suggesting that soy protein reduces liver triglycerides or fat by partly inhibiting hepatic fatty acid synthesis in the liver. ACC, the rate-limiting enzyme that catalyzes the carboxylation of acetyl-Co A to form malonyl-CoA, is the pivotal enzyme in the biosynthesis of long-chain fatty acids. Recently, dietary SPI has also been shown to reduce the expression of ACCa and ACCb isoforms mRNA and protein contents in the liver of rats. This action of SPI appears to be tissue-specific since the suppressive effect on ACC isoform gene expression was observed only in the liver but not in the heart or kidney. Furthermore, the ratios of phospho-ACCa/ACCa and phopho-ACCb/ACCb were unchanged by SPI, suggesting that regulation of ACC by SPI was primarily mediated through alteration of its gene expression rather than phosphorylation or dephosphorylation. A similar reduction of hepatic ACCa mRNA expression by soy protein was also found in another study by Aoki et al in which rats were fed SPI diet. In this study, SPI also reduced the expression of promoter I (PI) specific gene expression of ACCa, suggesting that SPI feeding suppresses ACCa gene expression mainly by regulating PI promoter.
There is also experimental evidence that suggests that soy protein improves insulin resistance and lipid levels by activating peroxisome-proliferator activated receptors (PPARs), which are nuclear transcription factors that regulate the expression of genes involved in glucose homeostasis, lipid metabolism, and fatty acid oxidation. Mezei et al showed that consumption of high-isoflavone soy protein diet improves glucose tolerance, insulin resistance, and hepatic cholesterol and triglyceride concentrations in obese Zucker rats. In cell culture studies, these investigators further showed that isoflavone-containing soy extracts and individual soy isoflavones increased the gene expression of PPARs, suggesting that the beneficial effects of soy protein on glucose and lipid metabolism may be mediated through PPAR activation. More recently, Morifuji et al demonstrated that soy protein feeding in rats decreased hepatic triacylglycerol levels and epididymal adipose tissue weight. These changes were associated with increased activity and mRNA levels of several skeletal muscle enzymes involved in fatty acid oxidation, including carnitine palmitoyltransferase (CPT1) activity and CPT1, beta-hydroxyacyl-CoA dehydrogenase (HAD), acyl-CoA oxidase, and medium-chain acyl-CoA dehydrogenase. Moreover, PPAR gamma coactivator 1 alpha (PGC1 alpha) PGC1 alpha and PPAR alpha mRNA levels were also found to be elevated, suggesting that soy protein intake stimulates skeletal muscle fatty acid oxidation by activating PPAR pathways leading to reduced accumulation of body fat.
Soy protein may reduce adiposity by modulating the expression of sterol regulatory element binding proteins (SREBPs), a family of transcription factors that controls multiple genes involved in fatty acid and cholesterol synthesis. In obese Zucker fa/fa rats, soy protein feeding was shown to reduce the expression of the hepatic SREBP-1 (the principal regulator of hepatic fatty acid biosynthesis) and its target genes – fatty acid synthase (FAS), steroyl-CoA-desaturase-1, and delta-5 and delta-6 desaturases. In addition, the soy protein diet also ameliorated fatty liver and markedly reduced hepatic cholesterol and triglyceride content, despite the fact that the rats were severely hyperinsulinemic. These findings suggest that soy protein consumption downregulates hepatic SREBP-1 expression through an insulin-independent mechanism. In contrast to the changes in the liver, PPAR gamma (nuclear hormone receptor involved in normal adipocyte differentiation) mRNA expression in adipose tissue was increased in obese rats fed soy protein. Histological analysis of epididymal adipose tissue from rats fed the soy protein revealed that there were more adipocytes per area but they were smaller in size than those fed casein. Taken together, these findings suggest that soy protein intake may limit adiposity by reducing the number of dysfunctional adipocytes possibly as a result of low lipogenesis. Soy protein may also reduce hepatic lipotoxicity by maintaining the number of functional adipocytes, preventing the transfer of fatty acids to extra adipose tissues.
Another possible mechanism of action of soy protein is via stimulation of adiponectin, a cytokine produced by fat cells that plays a key role in regulating in adipocyte differentiation and secretory function, and in enhancing insulin sensitivity. Plasma levels of this hormone are reduced in obesity. There is one report showing that dietary SPI intake is associated with increased plasma concentration of adiponectin in Wistar rats, suggesting that soy protein may modulate adiponectin production.
Which component(s) in soy protein is (are) responsible for its hypolipidemic and antiobesity effects is not entirely clear. Because soy protein contains many bioactive compounds or nutrients that may have multiple mechanisms of actions, it is difficult in nutritional intervention trials to disentangle the effect of any one constituent on lipid or body fat reduction. There are, however, in-vivo and in-vitro studies in which the effects of an isolated component or a single compound of soy protein on lipids have been examined.
Certain polypeptides or subunits of soy protein have been shown to mimic some of the effects of dietary soy protein on food intake and lipid metabolism. For example, in Sprague Dawley rats, oral administration of the soybean β-conglycinin peptone suppresses food intake and gastric emptying. These effects were attributed in part to an increase in circulating levels of cholecystokinin. Similarly, in rats fed a hypercholesterolemic diet, ingestion of the alpha subunit of the soy 7S globulin (conglycinin) produced substantial reductions in plasma lipids as well as a marked upregulation of liver beta-VLDL receptors. A soybean β-conglycinin diet was also shown to lower serum triglyceride, glucose, and insulin levels in normal and genetically obese (KK-Ay) mice. These effects were accompanied by reduced hepatic fatty acid synthase activity and increased activities of two enzymes related to fatty acid beta-oxidation and mRNA of acyl-CoA oxidase levels, as well as increased fecal excretion of tryglycerides, indicating that soy β-conglycinin reduces serum TG levels by suppression of hepatic fatty acid synthesis, acceleration of beta-oxidation, and/or increased TG fecal excretion.
Soyasaponins have been reported to reduce serum cholesterol but their role in fatty acid metabolism is unknown. In a recent study of golden Syrian hamsters, a diet containing group B soyasaponins (with no isoflavones) was shown lower plasma total cholesterol, non-HDL cholesterol, triglycerides, and the ratio of total cholesterol to HDL-cholesterol. These changes were associated with increased fecal excretion of bile acids and neutral sterols, suggesting that group B soyasaponins reduces plasma lipids by a mechanism involving greater excretion of fecal bile acids and neutral sterols. Interestingly, an earlier report showed that oral administration of total soyasaponins was also found to prevent the development of obesity and hyperinsulinemia induced by gold thioglucose injection in mice.
Phospholipids present in soy protein may be partly responsible for its antilipidemic effects. Short-term feeding with a diet containing soybean phospholipids for 3 days was shown to markedly reduce the activities of hepatic fatty acid synthetase, malic enzyme, glucose 6-phosphate dehydrogenase and pyruvate kinase in rats. Compared to a fat-free diet or a diet containing soybean oil, the diet containing soybean phospholipids also markedly decreased the hepatic mRNA levels of enzymes in fatty acid synthesis. A greater reduction of serum cholesterol as well as total lipid and cholesterol concentrations in liver was also observed when rats were fed a soy protein peptic hydrolysate with bound phospholipids, compared to soy protein diet alone or soy protein hydrolysate.
Part of the antiobesity effect of soy protein may be due to the presence of the isoflavones, since soy isoflavones have been shown to decrease fat accumulation in certain fat depots in some animal models of obesity. Additionally, work by Mezei et al has shown that consumption of a high isoflavone-containing soy diet improves glucose tolerance and reduces liver triglyceride and cholesterol concentrations obese Zucker rats. Moreover, cell culture studies showed that isoflavone-containing soy extracts and individual soy isoflavones, genistein and daidzein upregulate PPARalpha and PPARgamme-mediated gene expression. Exposure to soy isoflavones was also shown increase the expressions of the mature form of SREBP-2 and SREBP-regulated genes in HepG2 cells. Furthermore, exposure to soy isoflavones also increased HMG CoA reductase protein levels and HMG CoA synthase mRNA levels and increased both HMG CoA synthase and LDL receptor promoter activity, indicating that isoflavones may also regulate the genes involved in cholesterol biosynthesis and homeostasis.
Interestingly, in a recent study of agouti viable yellow (Avy) mice, a genetic model that develops hyperinsulinemia, obesity, type 2 diabetes, and yellow fur, it was shown that dietary genistein supplementation of female mice during gestation at levels comparable with those received by humans consuming high-soy diets, resulted in a shift in coat color of heterozygous mice and protected offsprings from developing obesity. These marked phenotypic changes induced by dietary genistein appear to be mediated by increased DNA methylation in tissues during early embryonic development that persisted into adulthood.
Thus, certain polypeptides (such as 7S globulin or conglycinin), soyasaponins, phospholipids, and isoflavones (genistein and daidzein) present in soybean appear to have complimentary actions on fatty acid and cholesterol metabolism, which may contribute to the overall beneficial effects of soy protein in obesity and associated lipid abnormalities.