Subjects of this study consisted of 333 women (aged 45-75 years) drawn from a large United Kingdom prospective study of diet and
cancer, the European Prospective Investigation of
Cancer and Nutrition-Norfolk study. Using newly developed gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry methods incorporating triply (13)C-labeled standards, seven
phytoestrogens (
daidzein,
genistein,
glycitein,
O-desmethylangolensin,
equol,
enterodiol, and
enterolactone) were measured in 114 spot urines and 97 available serum samples from women who later developed
breast cancer. Results were compared with those from 219 urines and 187 serum samples from healthy controls matched by age and date of recruitment. Dietary levels were low, but even so, mean serum levels of
phytoestrogens were up to 600 times greater than postmenopausal
estradiol levels.
Phytoestrogen concentrations in spot urine (adjusted for urinary
creatinine) correlated strongly with that in serum, with Pearson correlation coefficients > 0.8. There were significant relationships (P < 0.02) between both urinary and serum concentrations of
isoflavones across increasing tertiles of dietary intakes. Urinary
enterodiol and
enterolactone and serum
enterolactone were significantly correlated with
dietary fiber intake (r = 0.13-0.29). Exposure to all
isoflavones was associated with increased
breast cancer risk, significantly so for
equol and
daidzein. For a doubling of levels, odds ratios increased by 20-45% [log(2) odds ratio = 1.34 (1.06-1.70; P = 0.013) for urine
equol, 1.46 (1.05-2.02; P = 0.024) for serum
equol, and 1.22 (1.01-1.48; P = 0.044) for serum
daidzein]. These estimates of risk are similar to those established for
estrogens and
androgens in postmenopausal
breast cancer but need confirmation in larger studies.