Relation between soy-associated isoflavones and LDL and HDL cholesterol concentrations in humans: a meta-analysis

Background: Differences in isoflavone content of soy protein may explain the absence of a dose-response relation between soy protein intake and blood cholesterol concentrations. Objective: To study specifically the effect of soy-associated isoflavones on cholesterol concentrations in well-controlled trials substituting soy protein with dairy or animal protein. Design: Studies were identified by MEDLINE searches (1995-6 June 2002) and reviewing reference lists. Studies were included if they had a control group or treatment, experimental diets only differed in the amounts of soy protein and isoflavones and were each fed for at least 14 days. A total of 10 studies met these criteria, providing 21 dietary comparisons. Subjects: Studies comprised 959 subjects ( 336 men and 623 women), average age ranged from 41 to 67 y and baseline cholesterol concentration from 5.42 to 6.60 mmol/l. Interventions: The intake of soy-associated isoflavones increased by 1-95 mg/day and the intake of soy protein increased by 19 60 g/day. Results: Feeding daily 36 g soy protein with 52 mg soy-associated isoflavones on average decreased low-density lipoprotein (LDL) cholesterol by -0.17 +/- 0.04 mmol/l (mean +/- s.e.) and increased high-density lipoprotein (HDL) cholesterol by 0.03 +/- 0.01 mmol/l. There was no dose-response relation between soy-associated isoflavones and changes in LDL cholesterol (R = -0.33, P = 0.14) (Pearson correlation coefficient) or HDL cholesterol (R = -0.07, P = 0.76) or their ratio. Conclusions: Consumption of soy-associated isoflavones is not related to changes in LDL or HDL cholesterol.