Pregnant women are routinely advised to add folic acid as a part of their daily dietary intake in order to promote the healthy growth of the fetus, but are these supplements also harming the mother? A study out of St. Michael’s Hospital in Canada found that when rats were given high doses of folic acid, similar in the amount given to patients and survivors of breast cancer, this essential B vitamin increased the development of preexisting cancerous cells.

“This is a critically important issue because breast cancer patients and survivors in North America are exposed to high levels of folic acid through folic acid fortification in food and widespread use of vitamin supplements after a cancer diagnosis,” lead researcher of this study and physician at St. Michael’s, Dr. Young-In Kim, said in a statement. “Cancer patients and survivors in North America have a high prevalence of multivitamin and supplement use, with breast cancer patients and survivors having the highest prevalence.”

According to the Centers for Disease Control and Prevention, 400 micrograms of folic acid is recommended for a woman every day, starting a month before she intends on becoming pregnant. This essential B vitamin can help grow new cells in our body and prevent certain brain and spinal birth defects such as anencephaly and spina bifida. The Canadian and U.S. governments began adding folic acid to flour, pasta, and cornmeal products in 1998 to ensure women were getting enough.

Kim and his colleagues from the hospital in Toronto tested the effects of folic acid supplementation on preexisting mammary tumors in female mice. Each rat was either given a diet containing 2.5, four or five times the daily recommendation of folic acid for women, or a control diet without the B vitamin. Folic acid doses over 2.5 times the amount given to women “significantly promotes” the growth of pre-cancerous and cancerous cells in the mammary glands of rats.

In recent years, the role of folic acid in the development and progression of breast cancer has become highly controversial. While many health care professionals regard the synthetic form of folate as an essential part of our diet, others say it only increases a woman’s risk of developing breast cancer. The research team urges all women to seek the advice of their physician before starting a daily supplement regimen when becoming or after becoming pregnant.

The American Cancer Society considers BRCA1 and BRCA2 mutations to be the most common cause of hereditary breast cancer. Women and men, with a BRCA1 mutation in their genes stand a 55 to 65 percent lifetime risk of developing breast cancer, while people with BRCA2 mutations face a risk of 45 percent. Approximately 12 percent of women in the United States will develop breast cancer at some point in their life, making it the most common cancer among American women.

Source: Manshadi S, Ishiguro L, Sohn K, Medline A, Renlund R, Croxford R, Kim Y. Folic Acid Supplementation Promotes Mammary Tumor Progression in a Rat Model. PLoS One. 2014.