Investigation of the reliability of 24h urine excretion as a biomarker of isoflavone exposure over time and over a wide range of isoflavone intakes

Objective: To study the variation in genistein + daidzein intake over a 6-month period and test the reliability of 24 h urinary isoflavones as a biomarker of exposure over time. Design: Dietary genistein + daidzein intake was assessed at various time points throughout six months in 15 healthy subjects. Group 1 (n = 8) followed nonsupplemented diets and Group 2 (n = 7) took a 35 mg/d isoflavone supplement for 3 months and each subject provided a 24 h urine collection, validated with para-aminobenzoic acid, during weeks 7, 15 and 19. Urine was analysed for genistein and daidzein using LC-MS. Results: Isoflavone intake in Groups 1 and 2 ranged from 0.00 to 1.1 mg/d and 0.1 to 53.1 mg/d, respectively. Urine excretion for both groups ranged from 0.20 to 9.56 mg/d. The relationship between 24 h excretion and isoflavone intake is y = 0.44 x +/- 0.03(standard deviation) + 1.57; r = 0.89, P < 0.001. Conclusion: The 24 h urinary isoflavones can be used as biomarkers of isoflavone exposure over time.