The University of Maryland Medical Center’s website on Complementary Medicine (a clearly written, up-to-date resource, by the way) provides this review of folic acid (also known as folate or Vitamin B9) as a supplement protecting against the development of cancers. The strongest evidence appears to be for folic acid’s ability to protect against colorectal and breast cancers. In our excerpt below, we have highlighted two significant passages.
For recommendations on how to take this supplement (you may be getting it in a multivitamin or a B-complex supplement), see the NYBC entry on Folic Acid.
Folic acid appears to protect against the development of some forms of cancer, particularly cancer of the colon, as well as breast, esophagus, and stomach, although the information regarding stomach cancer is more mixed. It is not clear exactly how folate might help prevent cancer. Some researchers speculate that folic acid keeps DNA (the genetic material in cells) healthy and prevents mutations that can lead to cancer.
Population-based studies have found that colorectal cancer is less common among individuals with very high dietary intakes of folic acid. The reverse appears to be true as well: low folic acid intake increases risk of colorectal tumors. To have a significant effect on reducing the risk of colorectal cancer, it appears that at least 400 mcg of folic acid per day over the course of at least 15 years is required. Similarly, many clinicians recommend folic acid supplementation to people who are at high risk for colon cancer (for example, people with a strong family history of colon cancer).
Similarly, one population-based study also found that cancers of the stomach and esophagus are less common among individuals with high intakes of folic acid. Researchers interviewed 1095 patients with cancer of the esophagus or stomach as well as 687 individuals who were free of cancer in three health centers across the United States. They found that patients who consumed high amounts of fiber, beta-carotene, folic acid, and vitamin C (all found primarily in plant-based foods) were significantly less likely to develop cancer of the esophagus or stomach than those who consumed low amounts of these nutrients. Another important, good-sized study, however, did not find any connection between folic acid intake and stomach cancer. The possibility of some protection from folate against stomach cancer in particular needs clarification and, therefore, more research is warranted.
Low dietary intake of folate may increase the risk of developing breast cancer, particularly for women who drink alcohol. Regular use of alcohol (more than 1 ½ to 2 glasses per day) is associated with increased risk of breast cancer. One extremely large study, involving over 50,000 women who were followed over time, suggests that adequate intake of folate may lessen the risk of breast cancer associated with alcohol.