The study found that women who had not yet reached menopause and who had the highest average intake of folate had a 40 percent reduced risk of developing breast cancer.
Folate is a water-soluble B vitamin that occurs naturally in foods such as leafy green vegetables, fruits and dried beans and peas.
“In our study, it appears that folate is most protective of hormone negative, or what we call double negative, breast cancer,” said Shrubsole.
“We don’t have evidence that an extremely high intake of folate protects against breast cancer, but it appears that low folate levels may increase a premenopausal woman’s risk of developing breast cancer.”
Previous studies on folate intake had produced conflicting evidence about the vitamin’s possible influence on breast cancer risk.
Using data from the Shanghai Women’s Health Study in China, the investigators assessed the records of 72,861 women who enrolled in the study between 1997 and 2008. They identified 718 women who developed breast cancer during the course of the study.
When they enrolled, all of the women answered a food frequency questionnaire indicating which foods they ate on a regular basis and the quantities of the foods consumed. Since the food supply in Shanghai is not fortified with vitamins, including folic acid, and few women in this population take supplements, the researchers were able to estimate daily B vitamin intake levels of the raw foods consumed.
After studying the data, the researchers found no link between vitamins B6 or B12 and breast cancer risk, but there was a clear protective effect with folate consumption among premenopausal women.
Shrubsole said the study indicates that sufficient folate intake is helpful as a prevention measure but the data don’t quantify the amount needed for this protective effect.
Since the Asian women in the study were getting their folate from food, there is a lesson for those in Western societies, Shrubsole said.
“The good news for American women is that we don’t necessarily need to use a vitamin supplement in order to benefit from folate. While the women in this study may not eat the same kind of diet, it is possible that naturally derived folates from food are equally, or more important than, the synthetic folic acid that is a part of our American diet.
“I think this is an important message because physiologically, the way our bodies use nutrients is similar whether you are in Tennessee or in Shanghai, China,” Shrubsole said.
For the first time, the researchers also found evidence that too much niacin in the diet may be linked to an increased risk of hormone positive breast cancer. No other studies have found this effect and the authors of the study recommend further research to validate these findings.
The principal investigator for the Shanghai Women’s Health Study is Wei Zheng, M.D., Ph.D., chief of the Division of Epidemiology and director of the Vanderbilt Epidemiology Center.
The study was funded by grants from the National Cancer Institute and the U.S. Department of Defense Breast Cancer Research Program.