BackgroundA substantial fall in high-density lipoprotein cholesterol (HDL-C) during puberty in boys, but not girls, has been reported in Western populations. The fall in boys is believed to be due to hormonal changes – androgens have been shown to be associated with lower HDL-C, whereas oestrogens are associated with higher HDL-C. The fall in HDL-C during puberty was not observed, however, in a study of Moslem boys in Israel, nor in a group of Japanese boys. A diet high in phyto-oestrogens may account for the lack of a fall in HDL-C in these populations.ObjectiveTo examine the effect of dietary supplementation with phyto-oestrogens on the HDL-C concentration of adolescent boys from a Western population. We hypothesised that dietary supplementation of 50 mg of the isoflavones daidzein and genistein would produce a 12% higher HDL-C concentration than in controls at the end of a 6-week intervention period.DesignA randomised controlled trial.SettingHellyer College in Burnie (Tasmania, Australia).SubjectsAdolescent boys (aged 16–18 years) were recruited through a letter sent to parents. A total of 132 eligible participants enrolled and five subjects withdrew from the trial.ResultsNo significant increase in HDL-C was observed in the treatment group (–0.02 mmol l–1, standard error (SE)=0.03, P = 0.53) or the placebo group (0.05 mmol l–1, SE = 0.03, P = 0.11).ConclusionsFactors other than isolated dietary isoflavones may be responsible for the lack of fall in HDL-C during puberty in Japanese and Moslem boys.