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.

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