Much controversy surrounds the use of soy in the prevention of breast cancer among healthy individuals and its effect on breast cancer survivors with respect to the recurrence of the disease as well as increasing the risk of a new breast cancer.

Lower breast cancer incidence among women in Asian countries is attributed to the consumption of traditional soy food early in life. The Shanghai Breast Cancer Study with 1,459 breast cancer cases estimated a 34% lower risk when comparing the highest soy intake with the lowest, and an even stronger association for estrogen-receptor-positive breast cancer. Similarly, a study among Asian Americans showed that women reporting a high soy intake during both adolescence and adulthood had a 33% lower risk than women with no previous soy consumption. A meta-analysis of 14 studies yielded a significant odds ratio of 0.78, i.e. a possible 20% risk reduction. Although these data support the premise that soy foods prevent breast cancer, it is not known how they would affect breast cancer survivors.

In 3 studies with small number of subjects, increased volume of nipple aspirate fluid was noted in women who were fed a diet supplemented with soy for 6 months. Epithelial hyperplasia was also observed in 7 out of 24 subjects, indicating an estrogenic stimulation from the isoflavones contained in the soy supplemented diet. This finding is a source of concern for many women especially breast cancer survivors seeking alternatives to conventional hormone replacement therapy.

Animal studies have described potential cancer-causing effects of isoflavones, but most of them were conducted with genistein only. In other studies, it seemed that preparations of soy flour induced less cell proliferation than purified isoflavones. It is thought that the complex mixtures of bioactive compounds in soy may interact with one another and lead to a reduction in breast cell growth instead of the stimulatory effects of single isoflavone.

The American Cancer Society recommends that “breast cancer survivors should consume only moderate amounts of soy foods as part of a healthy plant-based diet and should not intentionally ingest very high levels of soy products”.

Omega-3 fatty acids
MacLean’s systematic review of 38 prospective cohort studies on the effect of omega-3 fatty acids consumption on cancer risk did not find a significant association between the two. This lack of association between omega-3 fatty acids and human cancer incidence contrasts with the findings of laboratory studies which generally report small but significant suppressive effects of dietary n-3 fatty acids on the incidence and growth rate of breast, prostate, colon and pancreatic tumours. These lab results cannot be used to predict the effect of omega-3 fatty acids on the development of human cancers for obvious limitations. However, interpretation of these human studies is limited by differences in the characteristics of the populations and the methods used to ascertain exposure to omega-3 fatty acids and tumour incidence.

Folate
A prospective cohort study of 17,447 Anglo-Australian women found no direct association between dietary folate intake and risk of breast cancer but an adequate dietary intake of folate might protect against the increased risk of breast cancer associated with alcohol consumption

Chang Yok Ying
Senior Pharmacist
Specialist Outpatient Clinic Pharmacy

References:
1.	Journal of the American Dietetic Assoc vol 105 Issue 10, Oct 2005 p1524-1528 Soy foods for breast cancer survivors and women at high risk for breast cancer?
2.	Journal of the American Dietetic Assoc vol 106 Issue 3 Mar 2006 p 364 Resolving the soy-breast cancer controversy
3.	Cancer Epidemiology, Biomarkers & Prevention vol 11 844-851 Sept 2002 Soy Intake and Other Lifestyle Determinants of Serum Estrogen Levels among Postmenopausal Chinese Women in Singapore
4.	JAMA Jan 25 2006 vol 295 No.4 Effects of Omega-3 fatty Acids on Cancer Risk A Systematic Review
5.	BMJ 8 Aug 2005 Does dietary folate intake modify effect of alcohol consumption on breast cancer risk? Prospective cohort study