A randomized trial comparing the effect of casein with that of soy proteincontaining varying amounts of isoflavones on plasma concentrations of lipids and lipoproteins

Context: Isolated soy protein reduces plasma concentrations of total and lo w-density lipoprotein (LDL) cholesterol. Objective: To identify the agent(s) responsible for the cholesterol-lowerin g effect of soy in mildly hypercholesterolemic volunteers: isoflavones isol ated together with soy protein or soy protein itself. Design: Double-blind randomized parallel trial. Setting: Single-center study. Participants: A total of 156 healthy men and women with LDL cholesterol lev els between 3.62 mmol/L (140 mg/dL) and 5.17 mmol/L (200 mg/dL) after instr uction in a National Cholesterol Education Program Step I diet and recruite d by advertisement from the community. Intervention: One of 5 daily diets (25 g of casein [for isoflavone-free com parison] or 25 g of isolated soy protein containing 3, 27, 37, or 62 mg of isoflavones). Main Outcome Measures: Change and percent change from baseline in plasma co ncentrations of triglycerides and total, LDL, and high-density lipoprotein cholesterol after 9 weeks. Results: Compared with casein, isolated soy protein with 62 mg of isoflavon es lowered total and LDL cholesterol levels by 4% (P = .04) and 6% (P = .01 ), respectively. In patients with LDL cholesterol levels in the top half of the population studied (>4.24 mmol/L [>164 mg/dL]), comparable reductions were 9% (P<.001) and 10% (P = 001), respectively; in this group, isolated s oy protein with 37 mg of isoflavones reduced total (P = .007) and LDL (P = .02) cholesterol levels by 8%, and there was a dose-response effect of incr easing amounts of isoflavones on total and LDL cholesterol levels. Plasma c oncentrations of triglycerides and high-density lipoprotein cholesterol wer e unaffected. Ethanol-extracted isolated soy protein containing 3 mg of iso flavones did not significantly reduce plasma concentrations of total or LDL cholesterol. Conclusions: Naturally occurring isoflavones isolated with soy protein redu ce the plasma concentrations of total and LDL cholesterol without affecting concentrations of triglycerides or high-density lipoprotein cholesterol in mildly hypercholesterolemic volunteers consuming a National Cholesterol Ed ucation Program Step I diet. Ethanol-extracted isolated soy protein did not significantly reduce plasma concentrations of total or LDL. cholesterol.