There are many potential mechanisms by which soy may improve cardiovascular outcomes, including reduction in total cholesterol, LDL, HDL, triglycerides, lipoprotein a, blood pressure, C-reactive protein, homocysteine, endothelial function, systemic artery compliance, and oxidised LDL (Balk et al 2005). A review by the North American Menopause Society suggests that the most convincing health effects of soy can be attributed to the actions of isoflavones on lipids, with studies finding statistically significant reductions in LDL and triglycerides, as well as increases in HDL (Greenwood et al 2000). It is unclear how soy exerts its beneficial effects on lipid metabolism or Soy 1107
which are the active components of soy, which may include soy protein, bioactive
peptides, interaction of isoflavones within the intact soy matrix, or other compounds (Cassidy & Hooper 2006, Torres et al 2006).
Consumption of soy protein may produce cardiovascular benefits through multiple mechanisms, including the low methionine content reducing serum homocysteine concentration (Nagata et al 2003), reduction of the insulin/glucagon ratio (Sanchez & Hubbard 1991), down regulation of the hepatic transcription factor sterol regulatory element binding protein-1, which in turn reduces lipotoxicity in the liver (Torres et al 2006), regulation of hepatic lipid metabolism through upregulation of LDL receptors and increase in bile acid secretion (Potter 1995), reducing hepatic fatty acid and triglyceride biosynthesis and increasing fatty acid oxidation (Tovar et al 2005), preventing the transfer of fatty acids to extra adipose tissues by increasing the adipocyte hormone adiponectin (Nagasawa et al 2003), and increasing bile acid secretion and bacterial conversion of cholesterol to the non-absorbable coprostanol (Huff & Carroll 1980). Soy protein peptides may also act to decrease intestinal cholesterol absorption and bile acid uptake, reduce aortic accumulation of cholesterol esters and suppress food intake and gastric emptying by increasing cholecystokinin, and inhibiting angiotensin-converting enzyme (Torres et al 2006).
Soy protein is also reported to have beneficial effects on renal function, with suggestions that the isoflavones genistein and daidzein reduce glomerular damage by protecting LDL from oxidation and the high arginine content acts as a precursor for NO thus improving renal flow (Torres et al 2006).
Although it has been suggested that there is no evidence of beneficial effects of phyto-oestrogens on blood pressure, arterial compliance or oxidation of LDL-cholesterol, there may be beneficial effects on endothelial function and homocysteine concentrations in postmenopausal women (Cassidy & Hooper 2006). Soy isoflavones have, however, been found to improve systemic arterial compliance in perimenopausal and menopausal women (Nestel 1997). Soy protein, regardless of isoflavone content, modulates serum lipid ratios in a direction beneficial for cardiovascular disease risk in healthy young men (McVeigh et al 2006).
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