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Green light for soy

New nutrition advice for breast cancer sufferers

We almost all know of someone who has been diagnosed with breast cancer. It is one of the most common causes of cancer related death in women. The incidence is growing, with data from the Australian Institute of Health and Welfare (2010) indicating 1 in 9 women will be diagnosed with breast cancer before the age of 85, and projections to 2015 suggesting the number of new breast cancer cases among women is likely to be 22% higher than in 2006. Early test tube studies conducted in the laboratory caused some concern that soy could promote breast tumour growth. A new study has added to the growing body of evidence that soy foods, like tofu and soy milk, have no adverse affects on breast cancer prognosis.

Most of the earlier studies investigating soy foods and breast cancer examined isoflavones; an antioxidant component found in legumes such as soy beans, in isolation in a test tube and found that they may enhance the growth of breast cancer cells. Based on this early evidence and along with the potential for isoflavones to interfere with the effectiveness of Tamoxifen – a drug that has been used treat breast cancer for over 30 years, some medical professionals treating women with breast cancer advised them to limit their consumption of soy foods. Two recent epidemiological studies conducted with breast cancer survivors have suggested that soy intake does not affect breast cancer prognosis and may even reduce recurrence. New data from the Women’s Healthy Eating and Living study (WHEL), with a larger number of participants further supports the existing research, suggesting there are no adverse affects of soy foods on breast cancer prognosis.

A secondary analysis of data from the WHEL study was used for this paper. The WHEL was a randomised controlled trial of a high fibre and high fruit and vegetable intervention in over 3,000 early stage breast cancer survivors in the USA. The participants aged from 18 – 70 years, recently surviving breast cancer, were recruited for the dietary intervention with an average follow up of 7.3 years from the time of enrolment. Soy intake measured by milligrams of isoflavones, (not total soy protein) was measured post diagnosis by the Arizona semi quantitative Food Frequency Questionnaire which included specific soy items and an option to include other soy foods not on the list.

The women in the highest quintile of isoflavone intake (16.3mg/day) had a 54% reduced risk of death compared with the lowest quintle of intake, (effectively non-soy consumers, 0-0.7mg/day) however this was not statistically significant. The highest quintile of isoflavone intake consumed on average the equivalent to around 1 cup of soy milk or approximately 3 ounces (85g) of tofu each day – which is not particularly high and probably reflects a similar amount to common levels of intakes in western consumers who eat soy. Younger women, Asians and those with a college degree or higher were most likely to be in the highest quartile.

Isoflavone intake was unrelated to the risk of a second breast cancer event, and for overall mortality, risk of death tended to be lower as isoflavone intake increased. Although the interaction between Tamoxifen use and mortality was not statistically significant, the observed effect toward lower mortality with increased soy seemed stronger in women who used Tamoxifen.

This study is now the third epidemiological study that shows lack of harm from soy consumption, and provides sufficient evidence that women with breast cancer no longer need to be advised to avoid soy products .

References:

  • Soy Food Consumption and Breast Cancer Prognosis. Caan BJ, Natarajan L, Parker B, Gold EB, Thomson C, Newman V, Rock CL, Pu M, Al-Delaimy W, Pierce P. Cancer Epidemiology, Biomarkers & Prevention 2011;20(5):854-8.
  • National breast and ovarian cancer centre – www.nbocc.org.au

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