The risk of pre-menopausal breast cancer were also reduced by 59 per cent for adults with the highest soy protein intake, and by 56 per cent for adults with the highest average isoflavone intakes, according to findings from a study with 73,223 Chinese women participating in the Shanghai Women's Health Study.
Population studies have shown that a diet rich in soy is associated with fewer cases of breast cancer, linked to the presence of soy isoflavones. China has the world's lowest incidence and mortality from breast cancer - a disease that has over one million new cases every year worldwide.
“This large, population-based, prospective cohort study provides strong evidence of a protective effect of soy food intake against pre-menopausal breast cancer,” wrote the researchers, led by Wei Zheng Vanderbilt University School of Medicine.
The findings are published in the current issue of the American Journal of Clinical Nutrition.
Soy isoflavones are naturally occurring oestrogen-like compounds, and supplements are currently marketed as a way of reducing symptoms of the menopause and offer an alternative to hormone replacement therapy.
Conflicting reports however have clouded the picture about the beneficial effects of soy isoflavones, with some studies indicating that breast cancer cells in mice were stimulated by the isoflavones. Population studies have shown that women with a high-soy diet generally have lower rates of breast cancer.
Previously, studies have reported that, while the underlying mechanism is not known, it is hypothesised that the oestrogenic effects of soy isoflavones cause changes in breast tissue during childhood that may decrease sensitivity to carcinogens later in life. A similar protective effect has been found in studies of overweight girls, perhaps because fat tissue also secretes oestrogen.
Zheng and co-workers investigated if intakes of soy food, measured using a validated food-frequency questionnaire, during adolescence and adulthood were associated with breast cancer risk. Over almost seven and a half years, the researchers documented 592 incident cases of breast cancer.
Adolescent intakes of soy foods were associated with a 43 per cent reduction in pre-menopausal breast cancer risk, while high intakes of soy protein and isoflavones were associated with 59 and 56 per cent reductions in the risk of breast cancer before the menopause.
“Women who consumed a high amount of soy foods consistently during adolescence and adulthood had a substantially reduced risk of breast cancer,” wrote the researchers. “No significant association with soy food consumption was found for postmenopausal breast cancer.”
The other researchers were affiliated with Vanderbilt University, the Shanghai Cancer Institute (China), The US National Cancer Institute, and Kangwon National University (China).
Source: American Journal of Clinical Nutrition June 2009, Volume 89, Number 6, Pages 1920-1926, doi:10.3945/ajcn.2008.27361“Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women's Health Study” Authors: S.-A. Lee, X.-O. Shu, H. Li, G. Yang, H. Cai, W. Wen, B.-T. Ji, J. Gao, Y.-T. Gao, W. Zheng