Evidence suggesting that isoflavones could reduce the risk of breast has been conflicting. Supplements of isoflavones are already taken to reduce menopause symptoms and therefore is an alternative to hormone replacement therapy.
The plant compounds, found in large quantities in soyfoods, have also been attributed to the lower rates of breast cancer among women in Asia. However not all trials have proved the link between soy intake and lower breast cancer risk. This association is also difficult to measure in Western populations where women do not tend to consume much soy-based produce.
The researchers from the University Medical Center in Utrecht used a Dutch cohort of 15,555 women aged 49-70 years taking part in the European Prospective Investigation into Cancer and Nutriton (1993-1997).
Data on habitual dietary intake in the preceding year showed that participants typically had a low phytoestrogen intake, of 0.4mg isoflavones and 0.7 mg lignans daily on average, note the researchers in this month's American Journal of Clinical Nutrition (vol 79, no 2, 282-288).
The researchers analysed data on isoflavone and lignan intake, estimated through a literature search, the use of food-composition tables, and contact with experts, among the 280 women diagnosed with breast cancer during follow-up.
After adjusting for known breast cancer risk factors and daily energy intake, they found no significant trends between breast cancer risk and phytoestrogens in the diet.
In a related editorial Dr Regina G. Ziegler of the National Cancer Institute in Bethesda, Maryland, said that research on the effects of phytoestrogens is "complicated, inconsistent and inconclusive".
"At present, scientific research does not support increasing phytoestrogen intake among US women to Asian levels, nor does it suggest that the typical phytoestrogen intake is problematic for healthy women," she concludes.
Breast cancer is the second leading cause of cancer deaths in women (after lung cancer) and is the most common cancer among women, excluding nonmelanoma skin cancers. According to the World Health Organization, more than 1.2 million people will be diagnosed with breast cancer this year worldwide.