A cohort of Chinese women who are breast cancer survivors had no increased mortality from soy intake, according to a new study.

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The work adds to the existing body of evidence that women with breast cancer, or risk for breast cancer, don’t need to modify their soy intake to mitigate risk, said the study’s first author, Suzanne C. Ho, PhD.

Speaking at the annual meeting of the North American Menopause Society, Dr. Ho noted that the combination of increasing breast cancer incidence and improved outcome has resulted in larger numbers of breast cancer survivors in Hong Kong, where she is professor emerita at the Chinese University of Hong Kong.

The prospective, ongoing study examines the association between soy intake pre- and postdiagnosis and total mortality for Chinese women who are breast cancer survivors. Dr. Ho said that she and her colleagues hypothesized that they would not see higher mortality among women who had higher soy intake – and this was the case.

Of 1,497 breast cancer survivors drawn from two facilities in Hong Kong, those who consumed higher quantities of dietary soy did not have increased risk of all-cause mortality, compared with those in the lowest tertile of soy consumption.

There are theoretical underpinnings for thinking that soy could be a player in cancer risk, but the biochemistry and epidemiology behind the studies are complicated. Estrogen plays a role in human breast cancer, and many modern breast cancer treatments actually dampen endogenous estrogens.

However, epidemiologic data have shown that consumption of soy-based foods – which contain phytoestrogens, primarily in the form of isoflavones – is inversely associated with developing breast cancer.

This is all part of why soy-based foods have been thought of as a mixed bag with regard to breast cancer: Soy isoflavones are, said Dr. Ho, “Natural estrogen receptor modulators that possess both estrogenlike and antiestrogenic properties.”

Other chemicals contained in soy may fight cancer, with effects that are antioxidative and strengthen immune response. Soy constituents also inhibit DNA topoisomerase I and II, proteases, tyrosine kinases, and inositol phosphate, effects that can slow tumor growth. Still, one soy isoflavone, genistein, actually can promote growth of estrogen-dependent tumors in rats, said Dr. Ho

Dr. Ho and her colleagues enrolled Hong Kong residents for the study of mortality among breast cancer survivors. Participants were included if they were Chinese, female, aged 24-77 years, and had their first primary breast cancer histologically confirmed within 12 months of entering the study. Cancer had to be graded below stage III.

Using a 109-item validated food questionnaire, investigators gathered information about participants’ soy intake and general diet for the year prior to breast cancer diagnosis. Other patient characteristics, relevant prognostic information from medical records, and anthropometric data were collected at baseline, and repeated at 18, 36, and 60 months.

The primary outcome measure – all-cause mortality during the follow-up period – was tracked for a mean 50.9 months, with a 78% retention rate for study participants, said Dr. Ho. In total, 96 patients died during follow-up, making up 5.9% of the premenopausal and 7% of the postmenopausal participants.

Statistical analysis corrected for potential confounders, including patient and disease characteristics and treatment modalities, as well as overall energy consumption.

Patients were evenly divided into tertiles of soy isoflavone intake, with cutpoints of 3.77 mg/1,000 kcal and 10.05 mg/1,000 kcal for the lower limit of the two higher tertiles. For the highest tertile, though, mean isoflavone intake was actually 20.87 mg/1,000 kcal.

Patient, disease, and treatment characteristics did not differ significantly among the tertiles.

An adjusted statistical analysis looked at pre- and postmenopausal women separately by tertile of soy isoflavone consumption, setting the hazard ratio for all-cause mortality at 1.00 for women in the lowest tertile of soy consumption.

For premenopausal women in the middle tertile, the HR was 0.45 (95% confidence interval, 0.20-1.00), and 0.86 for those in the highest tertile (95% CI, 0.43-1.72); 782 participants, in all, were premenopausal.

For the 715 postmenopausal women, the HR for those in the middle tertile of soy consumption was 0.94 (95% CI, 0.43-2.05), and 1.11 in the highest (95% CI, 0.54-2.29).

Taking all pre- and postmenopausal participants together, those in the middle tertile of soy isoflavone intake had an all-cause mortality HR of 0.63 (95% CI, 0.37-1.09). For the highest tertile of the full cohort, the HR was 0.95 (95% CI, 0.58-1.55).

Confidence intervals were wide in these findings, but Dr. Ho noted that “moderate soy food intake might be associated with better survival.”

“Prediagnosis soy intake did not increase the risk of all-cause mortality in breast cancer survivors,” said Dr. Ho, findings she called “consistent with the literature that soy consumption does not adversely effect breast cancer survival.”

The study is ongoing, she explained, and “longer follow-up will provide further evidence on the effect of pre- and postdiagnosis soy intake on breast cancer outcomes.”

The study had a homogeneous population of southern Chinese women, with fairly good retention and robust statistical adjustment for confounders. However, it wasn’t possible to assess bioavailability of isoflavones and their metabolites, which can vary according to individual microbiota. Also, researchers did not track whether patients used traditional Chinese medicine.

The World Cancer Research Fund International supported the study. Dr. Ho reported no conflicts of interest.

SOURCE: Ho S et al. NAMS 2018, Abstract S-23.