Soy cuts risk of breast cancer death, recurrence

By David Liu

Moderate intake of soy protein may help reduce the risk of breast cancer death and recurrence, according to a new study published in the Dec 9, 2009 issue of Journal of American Medical Association.

Soy foods including soy milk, tofu, soybeans, soy protein, and soy nuts are high in isoflavones, a group of phyto-estrogens that are believed to reduce the risk of breast cancer.  But there are some concerns about the safety of soy food consumption among breast cancer survivors.

Xiao Ou Shu MD, Ph.D. of Vanderbilt University Medical Center in Nashville Tenn. and colleagues examined data from 5,042 female breast cancer survivors aged 20 to 75 in China and found high intake of soy food products was correlated with lower risk of breast cancer death and recurrence.

The subjects were diagnosed with breast cancer between March 2002 and April 2006 and followed up through June 2009.  The participants were surveyed for information on cancer diagnosis and treatment, lifestyle exposure after diagnosis, and disease progression at six, 18, 36 and 60 months after diagnosis.

During the average 3.9 year-follow-up, 444 deaths and 534 cases of breast cancer recurrence were identified among 5033 women who had received surgery treatment.

The researchers found that intake of soy foods, measured as soy protein or soy isoflavones, was inversely associated with breast cancer mortality and recurrence.

Women who had highest intake of soy protein were 29 percent less likely to die during the follow-up and 32 percent less likely to suffer breast cancer recurrence compared to those who had the lowest intake of soy protein.

Specifically, the 4-year mortality rates among women with the lowest intake and women with highest intake of soy protein were 10.3 and 7.4 percent, respectively. The risks of having recurrence were 11.2 and 8.0 percent respectively.

Many breast cancer survivors and medical professionals alike are concerned about the safety of using soy food products because they fear that isoflavones may interact with tamoxifen, an anti-cancer drug doctors recommend for breast cancer patients to use for up to five years to prevent cancer from developing in another breast.  Tamoxifen has been questioned for its worthiness.

Another concern is that isoflavones may also promote estrogen-receptor positive breast cancer even though there is no solid scientific evidence to prove it.

In the current study, the inverse association was found among women who had either breast cancer, estrogen-receptor-positive or estrogen-receptor-negative and among both tamoxifen users and non-users.

The researchers found the correlation between intake of soy protein and breast cancer death and recurrence was linear when intake of soy protein was less than 11 grams per day. Actually the apparent protective effect did not increase after soy protein intake exceeded 11 grams per day.

The researchers concluded "This study suggests that moderate soy food intake is safe and potentially beneficial for women with breast cancer."

Rachel Ballard-Barbash, M.D., M.P.H., of the National Cancer Institute, Bethesda, Md., and Marian L. Neuhouser, Ph.D., of the Fred Hutchinson Cancer Research Center, Seattle cautioned in an accompanying editorial that the findings may not apply to the women in the United States because the diet, lifestyle, medical diagnosis and treatment in the U.S. differ from what are used in china.

But they suggest that clinicians can tell their patients that use of soy foods is safe, but the potential benefits are restricted to soy foods and may not be extended to soy or isoflavone supplements.

The study merely found an association between intake of soy foods and risk of breast cancer death and recurrence. It means that use of soy protein does not definitely reduce the risk although there is such a possibility. Further studies need to be done to confirm the potential.

Other similar studies that show soy consumption may reduce risk of development of breast cancer.

Dr. Motoki Iwasaki of the National Cancer Center in Tokyo and colleagues reported in March 2008 in the Journal of Clinical Oncology that women who consumed soy foods and had highest levels of genistein in their blood were only one-third as likely as others to develop breast cancer over a period of 10 years.

The study involved more than 24,000 middle-aged and older Japanese women. Participants were surveyed for their dietary habits and followed by an average of 10 years during which 144 women were diagnosed with breast cancer.

Women who consumed the highest amounts of soy foods indicated by the level of genistein in the blood were 65 percent less likely to develop breast cancer than the women who ate the lowest amounts of soy foods.

But the scientists discovered moderate levels of the isoflavone were not linked to reduced risk of breast cancer.

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Wu AH and colleagues of the University of Southern California Keck School of Medicine in Los Angeles, California reported a study in Dec 2009 in the British Journal of Cancer suggesting that eating soy food regularly may help prevent breast cancer in postmenopausal women.

The risk of acquiring breast cancer was reduced by 18 percent among those who had high intake of soy foods compared to those who had low intake.

Thirty five thousand Singapore Chinese women were surveyed between April 1993 and Dec 1998 for their dietary habits using a validated semi-quantitative food frequency questionnaire covering 165 food and beverage items. Six hundred and twenty eight cases of breast cancer were identified during the follow-up through Dec 25, 2005.

Compared to those with below-median consumption of soy, women with above-media consumption of soy food products were at 18 percent reduced risk of breast cancer. In menopausal women, the risk was cut by 28 percent.

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LA Korde at the National Cancer Institute and colleagues published a study in the March 24, 2009 issue of Cancer Epidemiology Biomarkers & Prevention suggesting that eating soy food in childhood may help reduce the risk of developing breast cancer.

They found those who ate soy food in childhood were up to 60 percent less likely to suffer breast cancer compared to those who did not eat soy foods.

This case-control study involved 597 women with breast cancer with 73 percent pre-menopausal and 966 women with the disease at age 20 to 55  from three ethnic groups: Chinese, Japanese, and Filipino.

The researchers found the seemingly protective effect was more significant among women who ate soy bean foods in childhood.  The risk of developing breast cancer was cut by 60 percent, 20 percent and 24 percent when soy foods were consumed in childhood, adolescence and adulthood, respectively.

Evidence suggests that soy foods may indeed have some protective effect against breast cancer.

In March 2009, Elvira de Mejia and colleagues of the University of Illinois published a study in the Molecular Nutrition and Food Research saying that one soy peptide called lunasin processes some anti-inflammatory effect which is linked to development of various cancers.

Erin Hsu at the University of California, Los Angeles presented a study at the 2007 Annual Meeting of the American Association for Cancer Research saying that genistein, a major isoflavone in soybeans may help prevent the spreading of breast cancer.

Hsu and colleagues found genistein has an effect on the "CXCR4/CXCL12 axis" which is known to play a central role in the spreading of breast cancer.

CXCR4 chemokine receptors are produced by primary cancer cells on their surface while CXCL 12 chemkine ligands are produced by the organs to which the cancer cells may spread.    The attraction between the two stimulates the invasive properties of cancer cells and draws the cancer cells to the organs.

When genistein was applied to breast cancer cell lines, it reduced levels of CXCR4 and CXCL12 messenger RNAs and proteins in a dose-dependent manner, Hsu and colleagues found.

The article may contain some content from articles published early by foodconsumer.org