Soy and Cancer

by | Jun 5, 2012 | Naturopathic health, Nutrition

There is a great deal of misconception and misunderstanding about the effects soy products have or can have in regards to cancer. Much of this confusion is due to the influx of personal belief and/or hearsay without looking at what is actually known. Here is a summary of what is known about the use of soy products and cancer.

Diet Matters

The effect of diet and food choices on the occurrence, progression, risk of recurrence and overall survival in individuals who have been treated for cancer is well established. (1-4) Many studies have shown that a dietary pattern characterized by a high intake of vegetables and whole grains reduces the incidence and mortality associated with most types of cancer. However, judging from the number of questions I receive about soy and the positive or negative effects of soy-based foods in regards to cancer, there is still much confusion on this subject.

Soy Foods & Cancer

Soy foods (and flaxseeds) are rich sources of phytoestrogens, including isoflavones which are biologically active compounds that can exhibit both anti-estrogenic and estrogen-like properties. High circulating estrogen levels are a documented risk factor for breast cancer recurrence. (5) In addition, older studies have shown soy isoflavones to promote in vitro (in a petri dish) growth of breast cancer cells and mammary tumor growth in laboratory animals. This has raised some concern that consuming soy may have adverse effects on woman with breast cancer.

However, three large studies in the recent past have found no adverse effects of soy food intake on breast cancer recurrence or total mortality either alone or in combination with tamoxifen; once more, these studies suggest the potential for soy foods to exert a positive synergistic effect with tamoxifen. (6-8) In addition, several studies have shown that consuming soy foods throughout life can decrease the risk of several types of cancer, including breast and prostate cancer and there have been no human studies performed to date that have shown a negative impact of soy or soy products on cancer progression, recurrence or survival.

Therefore, current evidence does not suggest that consuming soy foods is likely to have adverse effects on the risk of occurrence, recurrence or survival for those with cancer and may provide some benefit. To further this point, the American Cancer Society recently published a report stating that “soy contains several phytochemicals, some of which have weak estrogenic activity and seem to protect against hormone-dependent cancers”; that report goes on to say that “for the breast cancer survivor, current evidence suggests no adverse effects on recurrence or survival from consuming soy and soy foods, and there is the potential for these foods to exert a positive synergistic effect with tamoxifen.” (9)

Keep in mind that these studies looked at soy and soy foods, not supplements containing high amounts of isoflavones. Therefore, although current evidence suggests that consuming soy foods is safe and may be beneficial in regards to cancer, the same cannot be said to be true for soy- or isoflavone-containing supplements. More research is necessary in this area to determine the effects of these supplements.

 

 

References

  1. Jones LW, Demark-Wahnefried W. Diet, exercise, and complementary therapies after primary treatment for cancer. Lancet Oncol. 2006;7:1017-1026.
  2. Pekmezi DW, Demark-Wahnefried W. Updated evidence in support of diet and exercise interventions in cancer survivors. Acta Oncol. 2011;50:167-178.
  3. Rock CL, Demark-Wahnefried W. Nutrition and survival after the diagnosis of breast cancer: a review of the evidence. J Clin Oncol. 2002;20:3302-3316.
  4. Norman SA, Potashnik SL, GalantinoML, De Michele AM, House L, Localio AR. Modifiable risk factors for breast cancer recurrence: what can we tell survivors? J Womens Health (Larchmt). 2007;16:177-190.
  5. Rock CL, Flatt SW, Laughlin GA, et al; Women’s Healthy Eating and Living Study Group. Reproductive steroid hormones and recurrence-free survival in women with a history of breast cancer. Cancer Epidemiol Biomarkers Prev. 2008;17:614-620.
  6. Caan BJ, Natarajan L, Parker B, et al. Soy food consumption and breast cancer prognosis. Cancer Epidemiol Biomarkers Prev. 2011;20:854-858.
  7. Shu XO, Zheng Y, Cai H, et al. Soy food intake and breast cancer survival. JAMA. 2009;302:2437-2443.
  8. Guha N, Kwan ML, Quesenberry CP Jr, Weltzien EK, Castillo AL, Caan BJ. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res Treat. 2009;118:395-405.
  9. Rock, CL, Doyle C, Demark-Wahnerfried, W, et al. Nutrition and Physical Activity Guidelines for Cancer Survivors. CA Cancer J Clin 2012.