Folic acid is a well-known supplement to many women, particularly those who are or plan to be pregnant. But a new study suggests that taking large amounts of folate - a B vitamin - and its synthetic form, folic acid, might actually increase the risk of developing breast cancer.

Publishing their results in the journal PLoS ONE, the researchers note that the topic of folic acid and its role in cancer has been controversial.

Some studies have suggested the vitamin may protect against cancer. The team cites, for example, several epidemiologic studies that suggest dietary intake and blood levels of folate cut the risk of colorectal cancer.

However, recent studies have begun to suggest high amounts of folic acid could increase breast cancer risks. This latest study is the first to demonstrate how folic acid supplementation may promote growth in mammary tumors.

Dr. Young-In Kim, study author and researcher at St. Michael's Hospital in Canada, says:

"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."

The researchers note that folate intake has "significantly increased" during the past 10 years as a result of mandatory folic acid fortification in food, which aims to reduce incidence of neural tube defects, such as spina bifida, in newborns.

They add that since 1998, the Canadian and US governments have required food manufacturers to supplement white flour, enriched pasta and cornmeal products with folic acid.

High doses of folic acid 'promotes growth of cancerous cells'

About 30-40% of North Americans take folic acid supplements for potential health benefits, which the researchers note are currently unproven.

The Office of Dietary Supplements, part of the National Institutes of Health, has placed the recommended daily level of folate intake at 400 micrograms (mcg) for men and women over 19 years of age.

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Natural sources of folate can be found in leafy greens, such as spinach, but it can also be found in foods such as broccoli, egg yolks, lentils and oranges.

However, according to the Centers for Disease Control and Prevention, a guideline set in 1991 by the US Public Health Service recommends that some women who plan to get pregnant should consume 4,000 mcg of folic acid daily through the first trimester.

Dr. Kim and his team showed that, in doses two-and-a-half to five times the daily requirement, folic acid supplements promoted the growth of existing pre-cancerous or cancerous cells in the mammary glands of rats.

The researchers say there is cause for concern because patients newly diagnosed with cancer, as well as cancer survivors, use vitamins and supplements much more than the general population.

While only 50% of the general population take supplements, 64-81% of cancer patients do.

Folic acid and cancer progression: different factors at play

The role of folic acid in cancer progression seems to be dependent on a number of different factors.

For example, while the researchers showed that folic acid supplementation may promote growth of established lesions, they say other studies have shown that it may actually prevent the development of cancer in normal tissues.

Folate also appears to interact with alcohol, the researchers note. Low folate intake has been shown to increase breast cancer risk in women who regularly consume moderate or high amounts of alcohol, while high folate intake minimizes risks in these women.

These effects do not apply to women with low or no alcohol consumption, they add.

Though their latest study was carried out in rats, the researchers say that their findings "suggest that there is sufficient cause for concern about the potentially deleterious effect of folic acid supplementation on breast cancer progression."

Although women planning to get pregnant have been targeted to increase folate intake, Medical News Today recently reported on a study that suggested low folate in the father's diet could also be linked to offspring birth defects.