A study reported in the October Cancer Research (Vol. 6, No. 19: 7136-7141) adds some weight to the theory that high amounts of folate in the diet may help prevent breast cancer. The best way for women to get this folate, however, is still a subject of debate.

Folate is a B vitamin found naturally in many foods, including dry beans, peas, and leafy green vegetables, such as spinach. In the US, folate also has been added to all grain products, such as breads, pastas, and cereals in the form of folic acid.

The body is able to absorb folic acid (used in fortifying foods and in supplements) about twice as easily as the folate found naturally in foods.

The recommended daily intake of folate for adults is at least 400 micrograms (mcg). Since its addition to grain products in 1998, most people in the US now get this much needed folate in their daily diet.

It has long been known that folate can help prevent certain spinal cord birth defects, which is why it is recommended as a supplement for women of child-bearing age. Evidence is also mounting that folate may help prevent heart disease.

Its relationship to cancer, however, is less clear. Several studies have shown a link between high folate intake and a decreased risk of colon and rectum cancer. But the possible link with breast cancer has not been researched as thoroughly.

In the present study, researchers at Vanderbilt-Ingram Cancer Center at Vanderbilt University in Nashville, Tenn., and colleagues interviewed 1,321 women diagnosed with breast cancer and an additional 1,382 women without breast cancer in Shanghai, China, to determine their dietary habits over the previous five years.

They found that the 20% of women who consumed the most folate had a significantly lower risk of developing breast cancer when compared to the 20% with the lowest folate intake.

The risk was reduced even further in those women who consumed larger amounts of "folate cofactors" — other nutrients such as methionine, vitamin B6, and vitamin B12 — that help the body use folate properly.

The few previous studies that have looked at the relationship between dietary folate and the risk of breast cancer have been largely inconclusive. These studies were, for the most part, conducted in Western countries, where the use of alcohol (which is known to increase folate requirements) or vitamin supplements (which often contain folate) is common.

One of the strengths of the new study is that none of the women consumed alcohol or took vitamin supplements, so any change could more easily be attributed to folate levels in the diet.

But while this may be important from a scientific standpoint, it may not mean as much from a practical one because many American women already get enough folate in their diet or by taking vitamin supplements regularly, explains Marji McCullough, ScD, RD, nutritional epidemiologist with the American Cancer Society.

And many women drink at least occasionally.

"Previous studies haven't found a relationship between folate and breast cancer, although it may provide some benefit among women who consume alcohol, especially if it's more than one drink a day," she says.

McCullough also points out some of the potential limitations of the study design. For one, it relied on women's memories of what they had eaten in the past five years, which may not be entirely accurate.

The authors calculated folate content of the foods eaten by comparing them to known folate values of foods eaten in the US, another potential source of error. But overall, she adds, the study is still important.

McCullough cautions that there is still no evidence that consuming large amounts of folic acid in the form of supplements can reduce the risk of cancer.

Studies are now underway to investigate whether increasing folate intake from foods or folic acid supplements may reduce the risk of cancer, but until results are available, folic acid supplements are not recommended specifically to reduce cancer risk.

One reason for caution with regard to folic acid supplements is that while folate does not appear to be harmful by itself in high doses, daily intakes over 1,000 mcg (the tolerable upper intake level) can cover up B12 vitamin deficiency which would normally show up in the form of anemia (low red blood cell counts). If untreated, long-term B12 deficiency can lead to permanent nerve damage.

Experts say, if you are 50 years of age or older, talk with your doctor about your B12 status before you start taking a supplement that contains folic acid.