Ecase # 6-C
Davonna Cufley, M.D.
March 4-8, 2002

Mrs. B is beginning to suffer from menopausal symptoms, and is curious about taking phytoestrogens such as soy, but is also concerned about increasing breast cancer risk.  Her 36 yo sister has recently developed breast cancer.  What are the available data regarding the safety and effectiveness of soy supplements?

Answer: There has been much enthusiasm for a possible protective effect of soy in the diet, which at least appears to be safe.  I found no clinical research on supplements at menopause, and some in vitro data suggest that they could be harmful.  It has been pointed out that the "lower incidence" of breast cancer in Asian countries may be due to increased parity, cultural factors which make it less likely to be acknowledged or screened for, and poorer access to healthcare in some countries, rather than to the effects of dietary soy.  [Risks and benefits of soy phytoestrogens in cardiovascular diseases, cancer, climacteric symptoms, and osteoporosis.  Drug Safety 2001; 24(9): 665-682.  Article available in clinic.]

Low level dietary intake had no effect on breast cancer risk.
American J of Epidemiology. 154(5): 434-41, 2001 Sep 1

Usual dietary intake of 7 phytoestrogens was assessed via food frequency questionnaire for 1326 breast cancer cases in non-Asian US women compared to 1657 controls.  Dietary phytoestrogens at the level commonly consumed by non-Asian US women (equivalent to less than 1 serving of soy per week) had no effect on breast cancer risk.

Increased dietary soy associated with decreased cancer risk in obese (but not non-obese) Chinese women.  Also with less ER/PR positive cancer.
British J of Cancer. 85(3): 372-8, 2001 Aug 3.

Usual soyfood intake was assessed via food frequency questionnaire for 1459 breast cancer cases and 1556 age-matched controls in Shanghai.  96.6% of women ate soy at least once per week.  A statistically non-significant reduced risk of breast cancer was observed in those eating soy at least once per week.  Women in the highest decile group had a 30% reduction in breast cancer compared to those in the lowest decile group (p for this trend .28, however).  The relationship was strongest (and statistically significant) for obese women who ate soy, with OR of .3 for the highest soy intake group.  The reduction in breast cancer risk was also stronger for ER and PR positive cancer (OR.22, 95%CI .28-.75), than for other breast cancers.

Dietary soy in adolescence (a "sensitive period" for breast tissue) was inversely associated with breast cancer risk.
Cancer Epidemiology, Biomarkers, and Prevention.  10(5): 483-8, 2001 May.

Another analysis of the same Chinese women above: adolescent soyfood intake was inversely associated with breast cancer risk (p for trend <.001).

In vitro data on genestein (a soy isoflavone) mixed.
Cancer Research.  61(13): 5045-50, 2001 Jul 1.

Soy protein diets increased estrogen-dependent tumor growth in a dose dependent manner in the athymic mouse tumor implant model.  These researchers had previously shown that genistein stimulates growth of estrogen-dependent human breast cancer cells in vivo.  In contrast, other research has found that genestein inhibited growth of T-47D and MCF-7 breast cancer cells.