Biomarkers of isoflavone intake : validity at high intakes

Abstract

Isoflavones are biologically active plant chemicals (phytoestrogens) which are ordinarily present in human diets. There is considerable research interest in their potential to prevent or treat several chronic diseases. Biomarkers can demonstrate compliance during dietary interventions and validate associations between intake of isoflavones and health outcomes. The objectives of this study were to validate 24-hour urine collections, timed spot urine samples and timed plasma samples as biomarkers of isoflavone intake up to 165mg/day. Healthy volunteers (20 women and 11 men) consumed 55mg/d, 110mg/d or 165mg/d soy isoflavones or placebo for seven consecutive days in a randomised, double-blind, crossover study. Timed blood samples, timed spot urine samples (taken in the afternoon, 5-7 hours after consuming the isoflavone supplement) and 24-hour urines were obtained at baseline and during each intervention. Isoflavone content of the samples was assayed by liquid chromatography and mass spectrometry. 24-hour urines were validated by percentage PABA recovery. The relationship between daily isoflavone intake and 24-hour urinary isoflavone excretion was: y = 6.63132xº•⁷⁴²¹ ≡ x = (y ÷ 6.63132)¹•³⁴⁷⁵ where y = isoflavone excretion in μg/24h and x = isoflavone intake in μg/24h r² = 0.86; p < 0.001; n = 109 samples from 31 volunteers. The relationship between daily isoflavone intake and plasma isoflavone concentration was: y = (3.3543x10⁻³)xº•⁴⁸⁸⁹ ≡ x = (y ÷ 3.3543x10⁻³)²•º⁴⁵⁴ where y = plasma isoflavone in μg/ml and x = isoflavone intake in μg/24h r² = 0.61; p < 0.001; n = 100 samples from 30 volunteers. The relationship between daily isoflavone intake and spot urine isoflavone concentration was: y = (2.0324x10⁻³)xº•⁸ºº⁹ ≡ x = (y ÷ 2.0324x10⁻³)¹•²⁴⁸⁶ where y = isoflavone excretion in μg/ml and x = isoflavone intake in μg/24h r² = 0.69; p < 0.001; n = 143 samples from 31 volunteers. It was concluded that 24-hour urine collections, timed plasma samples and timed spot urine samples are valid biomarkers of isoflavone intakes up to 165mg/day. A curvilinear relationship was defined between a) isoflavone dose and bioavailability in plasma and b) isoflavone dose and 24-hour urinary excretion.