Lipoprotein response to diets high in soy or animal protein with and without isoflavones in moderately hypercholesterolemic subjects

Objective-The objective of this study was to assess the independent effect of soy relative to common sources of animal protein and soy-derived isoflavones on blood lipids. Methods and Results-Forty-two subjects with LDL cholesterol levels greater than or equal to 3.36 mmol/L were fed each of four diets in randomized order for 6 weeks per phase. Diets contained a minimum of 25 g animal protein or isolated soy protein/4.2 MJ, with each containing trace amounts or 50 mg of isoflavones/4.2 MJ. Soy protein had a modest effect on total, LDL and HDL cholesterol, and triglyceride concentrations (-2%, P = 0.017; -2%, P = 0.042; +3%; P = 0.034, -11%, P < 0.001, respectively). Soy protein had no significant effect oil plasma lipids in individuals with LDL cholesterol < 4.14 mmol/L and significantly reduced total and LDL cholesterol and triglyceride concentrations in individuals with LDL cholesterol :4.14 mmol/L (-4%, P = 0.001; -5%, P = 0.003; -15%, P < 0.001, respectively). No significant effect of isoflavones on plasma lipid levels was observed either constituent to the soy protein or supplemental to the animal protein. Conclusions-Although potentially helpful when used to displace products containing animal fat from the diet, the regular intake of relatively high levels of soy protein (> 50 g/day) had only a modest effect on blood cholesterol levels and only in subjects with elevated LDL cholesterol levels (greater than or equal to 4.14 mmol/L). Soy-derived isoflavones had no significant effect.