Plasma and urine concentrations of isoflavones as biomarkers of phyto-oestrogen intake following dietary soy supplementation

Background The current study assessed the value of spot urine and plasma samples as biomarkers of phyto-oestrogen exposure. Design Twenty-one subjects, aged 21–76 (9 males and 12 females) completed the five-week intervention study. They provided spot urine and plasma samples at the start of the study, weekly samples for four weeks while eating a daily cereal bar containing 28 mg total genistein and daidzein, and samples one week after discontinuation of the supplement. Dietary phyto-oestrogen intake was estimated using food diaries. Results Phyto-oestrogen intake ranged from 0.05 to 77.3 mg and correlated with urine and plasma concentrations; r = 0.86, p<0.001 and r = 0.88, p<0.001, respectively. The relationship between phyto-oestrogen intake and plasma and urine concentrations is y= 1.34(standard deviation (SD) 0.09) x +1.23 and y = 0.88(SD 0.05) x + 1.02, respectively. Conclusions Phyto-oestrogen concentrations in spot urine and plasma samples can be used as biomarkers of phyto-oestrogen intake.