Eating foods rich in isoflavones, which are found in soy products, could help to reduce the death rate in women with certain types of breast cancer.

A new study, published in the journal Cancer, found that isoflavones are associated with lower death rates in women with hormone-receptor-negative breast cancer and those who are not receiving endocrine therapy.

Isoflavones are oestrogen-like compounds found in certain foods, mainly soy products such as tofu, soy milk, miso and edamame beans.

The research analysed data from more than 6,000 American and Canadian women with breast cancer and grouped them according to the amount of isoflavone containing food they ate. The researchers found a 21 percent decrease in all-cause mortality among women who had the greatest intake of isoflavones compared with those who had the lowest intake. 

The correlation between isoflavone intake and reduced mortality was strongest in women with tumours that lacked estrogen and progesterone receptors. About a quarter of all breast cancer cases are hormone-receptor-negative.

There was also a possible impact in women who did not receive endocrine therapy, such as Tamoxifen, as a treatment for their breast cancer. The correlation was weaker, but still significant.

No correlations were found for women with hormone-receptor-positive tumours, nor for women who received endocrine therapy.

Tufts University Nutrition and cancer epidemiologist Dr Fang Fang Zhang, says the research may mean women with certain types of breast cancer should look to change their diet.

“Our results suggest, in specific circumstances, there may be a potential benefit to eating more soy foods as part of an overall healthy diet and lifestyle,” she says.

But she warns that women with breast cancer should not move to take isoflavone supplements.

“We only examined naturally occurring dietary isoflavone, so we do not know the effect of isoflavone from supplements. We recommend that readers keep in mind that soy foods can potentially have an impact, but only as a component of an overall healthy diet,” Dr Zhang says.

Isoflavones have been shown to slow the growth of breast cancer cells in laboratory studies, and studies of East Asian women with breast cancer found links between higher isoflavone intake and reduced mortality.

However, other research has suggested that the estrogen-like effects of isoflavones may reduce the effectiveness of endocrine therapies used to treat breast cancer.  

This double effect means that it remains unknown whether isoflavone consumption should be encouraged or avoided by breast cancer patients.

Dr Zhang says: “However, based on our results, we do not see a detrimental effect of soy intake among women who were treated with endocrine therapy, which has been hypothesized to be a concern. Especially for women with hormone-receptor-negative breast cancer, soy food products may potentially have a beneficial effect and increase survival.”

The researchers do warn that there may be some limits to the study, in particular, women who consumed higher levels of dietary isoflavone were more likely to be Asian Americans, young, physically active, more educated, not overweight, never smokers, and drink no alcohol. The team controlled for these factors in the analyses, the possibility of a partial confounding effect on the associations identified in the study cannot be ruled out.

“Whether lifestyle factors can improve survival after diagnosis is an important question for women diagnosed with hormone-receptor negative breast cancer, a more aggressive type of breast cancer. Our findings suggest that survival may be better in patients with a higher consumption of isoflavones from soy food,” the researchers say.

17 March 2017

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