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Objective: To determine whether the two major isoflavones in red clover differ in their effect on low-density lipoprotein cholesterol (LDL-C)., Design: A randomised, placebo-controlled, double-blind trial; two parallel groups taking one of the two isoflavones within which treatment and placebo were administered in a crossover design., Setting: Free-living volunteers., Subjects: A total of 46 middle-aged men and 34 postmenopausal women., Intervention: Two mixtures of red clover isoflavones enriched in either biochanin (n=40) or formononetin (n=40) were compared. Placebo and active treatment (40 mg/day) were administered for 6 weeks each in a crossover design within the two parallel groups., Main Outcome Measures: Plasma lipids were measured twice at the end of each period., Results: Baseline LDL-C concentrations did not differ significantly between men (n=46) and women (n=34), nor between those randomised to biochanin or formononetin. Interaction between time and treatments, biochanin, formononetin and corresponding placebos (two-way ANOVA) on LDL-C showed a significant effect of biochanin treatment alone. The biochanin effect was confined to men; median LDL-C was 3.61 (3.05-4.14) mmol/l with biochanin and 3.99 (3.16-4.29) mmol/l with the corresponding placebo (RM ANOVA with Dunnett's adjustment P<0.05). The difference between placebo and biochanin effects on LDL-C was 9.5%. No other lipid was affected and women failed to respond significantly to treatment., Conclusion: Isolated isoflavones from red clover enriched in biochanin (genistein precursor) but not in formononetin (daidzein precursor), lowered LDL-C in men. This may partly explain the previous failure to demonstrate cholesterol-lowering effects with mixed isoflavones studied predominantly in women., Sponsorship: Novogen Ltd, North Ryde NSW, Australia, provided partial support including provision of tablets and outside monitoring.