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 ≥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 on 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 (≥4.14 mmol/L). Soy-derived isoflavones had no significant effect.