Usual dietary consumption of soy foods and its correlation with the excretion rate of isoflavonoids in overnight urine samples among Chinese women in Shanghai

Soy foods and certain soy constituents, particularly isoflavones, have been suggested to have potential cancer-inhibitory effects in laboratory and epidemiological studies. Chinese women in Shanghai consume high levels of soy foods and have low incidence rates of breast and other hormone-related cancers. To assess the usual dietary consumption of soy foods and evaluate the correlation of soy food consumption with the urinary excretion of isoflavonoids in overnight urine samples in this population, we analyzed data from 60 healthy women included in an ongoing population-based case-control study of breast cancer in Shanghai. Usual consumption of soy foods in the previous five-year period was assessed using a food-frequency questionnaire, and urinary excretion of daidzein, genistein, glycitein, equol, and O-desmethylangolensin was measured from overnight urine samples collected at the time of dietary assessment. Virtually all women (96.7%) in Shanghai consumed soy foods at least once a week. The median intake of soy food was 100.6 g/day, with 25th and 75th percentiles of 36.8 and 238.2 g, respectively. The median intake of isoflavones was 39.26 mg/day, and there was a nearly fourfold difference between the 25th and 75th percentiles of this measurement. With the increasing intake of soy foods, urinary excretion rates of total isoflavonoids and all individual major isoflavonoids were increased in a dose-response manner (trend test p < or = 0.05). At individual levels the urinary excretion rate of total isoflavonoids was correlated closely with dietary soy food intake, with a correlation coefficient of around 0.5 (p < 0.001). These results indicate that the urinary excretion rate of total isoflavonoids measured from overnight urine samples may reflect reasonably well the usual intake of soy foods in a population with a high level of soy food consumption.