Dietary phytoestrogens -- plant substances that have weak estrogen-like activity -- have little impact on the risks of developing hormone-sensitive cancers like breast and prostate cancer or colorectal cancers, new research suggests.

In a large study of some 25,000 British adults, researchers failed to find any significant differences in cancer risk related to dietary intake of these compounds.

Phytoestrogens are found in a wide range of foods including dairy products, soy foods, cereals, fruits, vegetables, nuts, seeds, coffee and tea. Previous studies have suggested dietary phytoestrogen intake is associated with increased breast cancer risk and reduced colorectal cancer risk in women. The results from earlier studies were hampered, however, by limited data about phytoestrogen content in food.

No previous research has examined the association between phytoestrogen intake and prostate cancer risk.

In the current study, reported in the American Journal of Clinical Nutrition, researchers assigned phytoestrogen values to nearly 11,000 foods following chemical analyses. For the first time, phytoestrogen values were assigned to animal products.

Unlike plants, which themselves contain phytoestrogens, phytoestrogens are generated by the digestion of animal products like meat and dairy products by microbes in the gut, the researchers explain.

Phytoestrogen consumption was estimated for cancer-free adult participants in the European Prospective Investigation into Cancer and Nutrition - Norfolk (EPIC-Norfolk). EPIC-Norfolk participants, recruited between 1993 and 1997, filled out a diet diary for a week and provided information about age, height, weight, smoking, aspirin use, menopausal status, and family history of cancer among other things.

Cancers that developed within 12 months of study recruitment were identified from a cancer registry totaling 244 breast cancers, 221 colorectal cancers, and 204 prostate cancers. The diets and other relevant information from those who developed cancer were compared to information from other participants (controls) who did not develop cancer.

While acknowledging more study is needed, the authors concluded that there is little evidence that phytoestrogen intake is associated with subsequent risk of breast or prostate cancer.

However, phytoestrogens found in eggs and dairy products may influence the risk of prostate cancer and colon cancer in women, they report.

The associations are weak and without further study do not warrant changes in diet, lead investigator Heather Ward, of the MRC Center for Nutrition and Cancer in the Department of Public Health and Primary Care at Strangeways Research Laboratory in Cambridge, England, told Reuters Health.

The results of the present study do not suggest that anyone should alter their phytoestrogen intake, in part because the majority of the associations between phytoestrogen intake and cancer risk were not significant, the doctoral candidate wrote in an email.

It is worth noting that phytoestrogen intake within an Asian-style diet is more than ten-fold greater than in Western diets, without evidence of an increase in cancer risk, she added.

Because phytoestrogen consumption is on the rise in Britain, the authors urge further monitoring because the relation between phytoestrogen and cancer may change over time.

SOURCE: American Journal of Clinical Nutrition, December 2009.