Soy foods are not detrimental to — and may even be beneficial for — women with breast cancer, according to a new study published online today in Cancer.

The research adds some clarity to controversies surrounding soy consumption by patients with breast cancer.

On the one hand, consumption of soy has previously been associated with a reduced risk for breast cancer mortality or recurrence in Asian populations, say the authors, led by Fang Fang Zhang, MD, PhD, from the Friedman School of Nutrition Science and Policy at Tufts University, Boston, Massachusetts.

Furthermore, isoflavone, a phytoestrogen abundant in soy, is thought to inhibit the production of estrogen, which in turn may spur the growth of hormone-sensitive breast tumors.

On the other hand, there is "limited" evidence of a beneficial effect of soy on breast cancer in Western countries, where consumption is much lower, the authors say.

Furthermore, isoflavone is also known for binding and activating estrogen receptors in breast tumors, which may interfere with tamoxifen therapy in patients with breast cancer.

Thus, before the revelations of the current study, the authors say that it has been "controversial whether women should be advised to avoid or increase their intake of food products or supplements that contain isoflavone to reduce breast cancer risk or progression."

To shed light on this debate, the investigators looked at the relationship between dietary intake of isoflavones and all-cause mortality in a multiethnic cohort of 6235 North American and Australian women with breast cancer.

The authors used data collected from the women about dietary intake within 5 years of their enrollment (before and after) in the Breast Cancer Family Registry.

The team now reports that there was an inverse association between soy intake and death.

Specifically, after a median follow-up of 9.4 years, 1224 deaths were documented. And women in the highest quartile of dietary isoflavone intake (≥1.5 mg/day) had a 21% decrease in all-cause mortality compared with women in the lowest quartile (<0.3 mg/day; fourth quartile vs first quartile: hazard ratio [HR], 0.79; 95% confidence interval [CI], 0.64 - 0.97; P for trend < .01).

In stratified analyses, a reduced risk for all-cause mortality associated with high (highest vs lowest quartile) dietary isoflavone intake was statistically significant for women with estrogen receptor–negative/progesterone receptor–negative tumors (HR, 0.49; 95% CI, 0.29 - 0.83; P for trend = .006) and for women who did not receive hormone therapy as a component of their treatment for breast cancer (fourth quartile vs first quartile: HR, 0.68; 95% CI, 0.51 - 0.91; P for trend = .02).

No associations were observed for women who had hormone receptor–positive tumors and those who received hormone therapy.

In summary, the decrease in all-cause mortality was largely confined to women with hormone receptor–negative tumors and women who were not treated with antiestrogens, such as tamoxifen.

“Based on our results, we do not see a detrimental effect of soy food intake among women who were treated with endocrine therapy," said Dr Zhang in a press statement, addressing the above-mentioned theoretical concern.

"For women with hormone receptor–negative breast cancer, soy food products may potentially have a protective effect," she added, explaining that there was a weaker but still statistically significant association for these women.

An expert not involved with the study was enthusiastic about the new results in patients with breast cancer.

"We now have evidence that soy foods not only prevent breast cancer but also benefit women who have breast cancer. Therefore, we can recommend women to consume soy foods because of soy's many health benefits," writes Omer Kucuk, MD, from the Winship Cancer Institute of Emory University, Atlanta, Georgia, in an accompanying editorial. He adds that eating soy foods has been shown to prevent other diseases, such as diabetes and hypertension.

We can recommend women to consume soy foods because of soy’s many health benefits. Dr Omer Kucuk We can recommend women to consume soy foods because of soy’s many health benefits.

The new study results are limited to soy foods — and do not refer to supplements, he says.

"Although dietary intake of soy foods is healthy and safe, the use of soy isoflavone supplements is another matter, because it has not been evaluated in large, randomized clinical trials," writes Dr Kucuk.

Supplements cannot be recommend for use among patients with breast cancer at this time, he says.

This work was supported by grant from the US National Cancer Institute. The study authors and editorialist have disclosed no relevant financial relationships.

Cancer. Published online March 6, 2017. Abstract, Editorial

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