Soy products contain high amounts of isoflavonoids, which have been shown to exhibit possible
cancer-protective properties. Chinese populations in Asia, in particular, have a high level of soy intake and a relatively low risk of
hormone-dependent
cancers. In this study, we assessed the distributions of dietary soy isoflavonoids (
daidzein,
genistein, and
glycitein) and urinary soy isoflavonoids and their metabolites (
daidzein,
genistein,
glycitein,
equol, and
O-desmethylangolensin) among 147 Singapore Chinese (76 men and 71 women) ages 45-74 years, who are participants of the Singapore Cohort Study on diet and
cancer. Urinary values were measured from spot samples collected 10-20 months following recruitment, when usual dietary habits were assessed by a structured food frequency/portion size questionnaire administered in person. Dietary levels of
daidzein and
genistein were comparable within individuals and about seven times higher than the level of dietary
glycitein. All three dietary isoflavonoids showed an approximately 3.5-fold difference between the 25th and 75th percentile values. Similarly,
daidzein was the most abundant and
glycitein the least abundant of the five isoflavonoid compounds in urine. There was a 4.9-fold difference between the 25th and 75th percentile values for the sum of the five urinary isoflavonoids. Among study subjects, there were statistically significant, dose-dependent associations between frequency of overall soy intake and levels of urinary
daidzein (two-sided P = 0.03) and sum of urinary
daidzein,
genistein, and
glycitein (two-sided P = 0.04). In contrast, there were no associations between frequency of overall soy intake and levels of the two
daidzein metabolites (
equol and
O-desmethylangolensin) in urine (two-sided P = 0.85 and 0.34, respectively). We suggest that within the range of exposures experienced by Singapore Chinese, urinary level of
daidzein or the sum of
daidzein,
genistein, and
glycitein obtained from a spot sample can serve as a
biomarker of current soy consumption in epidemiological studies of diet-disease associations.