Urinary excretion of isoflavonoids and the risk of breast cancer

Isoflavonoids are a group of biologically active phytochemicals that humans are exposed to mainly through soy food intake, Because of the similar chem ical structure of these compounds and estradiol, it has been hypothesized t hat isoflavonoids may be related to the risk of breast cancer. Overnight ur ine samples from 60 incident breast cancer cases and their individually mat ched controls were assayed for urinary excretion rates of five major isofla vonoids (daidzein, genistein, glycitein, equol, and O-desmethylangolensin) and total phenols, These subjects were from a large population-based case-c ontrol study conducted in Shanghai, and urine samples from breast cancer ca ses were collected before any cancer therapy to minimize the potential infl uence of the disease and its sequelae on study results. Urinary excretion o f total phenols and all individual isoflavonoids, particularly glycitein, w as substantially lower in breast cancer cases than controls. For total isof lavonoids, the mean excretion was 13.95 mmol/mg creatinine (SD, 20.76 nmol/ mg creatinine) for cases and 19.52 mmol/mg creatinine (SD, 25.36 nmol/mg cr eatinine) for controls (P for difference = 0.04). The case-control differen ce was more evident when median levels of these compounds were compared, wi th the median excretion of all major isoflavonoids being 50-65% lower in ca ses than in controls, Individuals in the highest tertile of daidzein, glyci tein, and total isoflavonoids had about half the cancer risk of those in th e lowest tertile. The adjusted odds ratio for breast cancer was 0.14 (95% c onfidence interval, 0.02-0.88) for women whose urinary excretion of both ph enol and total isoflavonoids was in the upper 50% compared with those in th e lower 50%, The results from this study support the hypothesis that a high intake of soy foods may reduce the risk of breast cancer.