Soy in breast cancer: No increase in mortality or recurrence, potentially better outcomes
A 70-month prospective study in Southern Chinese women, sponsored by the World Cancer Research Fund, finds no evidence of pre-diagnosis soy intake negatively affecting mortality or recurrence among breast cancer survivors. On the contrary, the results indicate that moderate soy intake may be associated with better outcomes.
“Soy isoflavones are abundant in traditional Asian diet and are natural oestrogen receptor modulators, with both oestrogen-like and anti-oestrogenic properties,” said the study’s author, Professor Suzanne Ho of the Chinese University of Hong Kong.
“A number of epidemiological studies have found soy intake to be inversely correlated with breast cancer risk,” said Ho. [Lancet 1991;337:1197-1200; Br J Cancer 2001;85:372-378; Carcinogenesis 2002;23:1491-1496; Nutr Cancer 2007;57:20-27]
“However, genistein has been shown to enhance the proliferation of breast cancer cells in vitro and to promote oestrogen-dependent mammary tumour growth in animal models,” she pointed out. [J Natl Cancer Inst 2006;98:1275-1284]
Between 2010 and 2013, the Hong Kong Breast Cancer Survival Study (HKBCSS) recruited 2,811 Chinese women (age, 24 to 75 years) with newly diagnosed breast cancer from Princess Margaret Hospital and Prince of Wales Hospital, 1,460 of whom completed a baseline survey covering dietary, socio-demographic, medical history, reproductive and lifestyle variables, and other prognostic factors. The patients were followed up until November 2018 for a mean of 70.6 months, with the survey conducted at 18-, 36- and 60-month follow-up.
The patients were subdivided into four quartiles on the basis of daily isoflavone intake (Q1: <2.8 mg/d; Q2: 2.84 mg/d to 6.3 mg/d; Q3: 6.34 mg/d to 12.8 mg/d; Q4: ≥12.84 mg/d). “The baseline characteristics were well distributed between all four quartiles, with the exception of patients in the upper quartile having a higher level of education and more commonly not having any comorbidities than patients in other quartiles,” noted Ho.
“After adjusting for potential confounders, patients in Q2 and Q3 were found to have lower hazard ratios [HRs] of 0.61 [95 percent confidence interval (CI), 0.36 to 1.03] and 0.67 [95 percent CI, 0.41 to 1.10] for all-cause mortality than patients in Q1 and Q4 [adjusted HR, 1.0 and 1.06, respectively],” reported Ho.
A similar pattern of HR reduction was seen for breast cancer recurrence. Women in Q2 and Q3 of isoflavone intake had adjusted HRs of 0.65 [95 percent CI, 0.43 to 0.98] and 0.85 [95 percent CI, 0.58 to 1.25] vs HRs of 1.0 and 1.07 in the Q1 and Q4 groups, respectively. “There was no risk reduction in the group with the highest soy intake compared with the lowest intake group,” noted Ho.
“In conclusion, we found no evidence that soy consumption increases the risk of mortality or recurrence among Chinese breast cancer survivors,” said Ho. “Combining the data of Q2 and Q3 patients, we could see that moderate soy intake, equivalent to approximately one glass of soy milk a day, is associated with a 36 percent reduction in all-cause mortality risk and a 25 percent reduction in the risk of breast cancer recurrence.”