For the study, the researchers analyzed information from more than 45,000 female nurses who began the study when they were ages 27 to 44, and were followed for 22 years.
Researchers from the UCLA Fielding School of Public Health examined the potential effect of a diet associated with chronic inflammation on breast cancer. Thus, some studies claimed that soy might affect the action of hormone therapies that are used in the treatment of breast cancer.
"Isoflavones - the component of soy that has estrogen-like properties - have been shown to slow the growth of breast cancer cells in laboratory studies, and epidemiological analyses in East Asian women with breast cancer found links between higher isoflavone intake and reduced mortality", Dr. Zhang says.
However, the study did not include the impact of alcohol, normally part of a traditional Mediterranean diet, due to its known links to breast cancer.
The study was published today (March 1) in the journal Cancer Research.
The team found a 21 percent decrease in all-cause mortality among women in the highest quartile of intake, when compared to those in the lowest quartile.
The findings could be particularly valuable as women with this form of cancer cannot be treated with hormone therapy and usually have a worse outcome than those with other types of breast cancer. "Because of this disparity, it remains unknown whether isoflavone consumption should be encouraged or avoided for breast cancer patients", Dr. Zhang explains.
The researchers gave each woman's diet an "inflammatory score", with higher scores indicating diets that, in previous studies, have been linked with higher levels of inflammation in the body.
"Based on our results, we do not see a detrimental effect of soy food intake among women who were treated with endocrine therapy", said Zhang. Also, soy had a protective effect in those women with hormone receptor-negative tumors. "However, each woman's breast cancer risk is different based on numerous factors, including genetic predisposition, demographics, and lifestyle".
"Women who did not receive endocrine therapy as a treatment for their breast cancer had a weaker, but still statistically significant, association", she added. Women who were in the group with the highest score in terms of their diet in adolescence was estimated that there were 35% more likely to develop breast cancer before menopause than women who were in the group with the lowest score.
"Our findings suggest that survival may be better in patients with a higher consumption of isoflavones".