Vitamin B₆ may help prevent colorectal cancer in women

(This article was first printed in the July 2005 issue of the Harvard Women’s Health Watch. For more information or to order, please go to www.health.harvard.edu/womens.)

A dietary component is poised to join a list of strategies that help lower our risk for this largely preventable cancer.

Women with higher intakes of vitamin B₆, whether from food or vitamin supplements, may be less likely to develop cancer of the colon or rectum, according to a study from Harvard Medical School. Further tests are needed to confirm the result, but the findings are promising. For one thing, other public health studies have found a similar association between vitamin B₆ and colorectal cancer. And laboratory research has shown that high levels of vitamin B₆ can suppress cancer in human and animal cells.

If vitamin B₆ intake pans out as a preventive strategy, it will be a welcome addition to the many measures we can already take to reduce our chances of developing colorectal cancer.

The study

Researchers from Brigham and Women’s Hospital in Boston analyzed blood samples taken in 1989 from nearly 33,000 women in the Nurses’ Health Study. They measured levels of pyridoxal 5'-phosphate (PLP), the main active form of vitamin B₆ in the blood, among women who later developed colorectal cancer or polyps and a sample of women who remained free of these conditions. They also collected data from food frequency questionnaires and information about supplement use.

After dividing the women into four groups by PLP level, the investigators found that those with the highest levels had the lowest incidence of colorectal cancer and polyps. (Polyps are growths on the inner surface of the colon and rectum that sometimes progress to cancer.) Total vitamin B₆ intake was also inversely related to the likelihood of developing colorectal cancer: Incidence was lowest in women who consumed the most vitamin B₆ — 8.6 milligrams (mg) per day, about five times the recommended daily amount. Average vitamin B₆ intake in the other three groups ranged from 1.6 mg to 3.3 mg per day. The study’s findings were published in the May 4, 2005, issue of the Journal of the National Cancer Institute.

Why might vitamin B₆ have cancer-fighting properties? According to the study’s lead author, Harvard Medical School researcher Esther Wei, Sc.D., vitamin B₆ is one of several nutrients (including folate, methionine, and vitamin B₁₂) that are critical to “one-carbon metabolism,” a group of biochemical pathways that plays an important role in making and maintaining DNA, the cells’ package of genetic information. Disruptions in this process can nudge a cell toward becoming cancerous. Scientists at the National Cancer Institute are investigating the links between B vitamins and gene variations in one-carbon metabolism and certain colorectal polyps.

Dietary links we know about

Of all cancers, colorectal cancer seems most connected to diet, so it makes sense to investigate dietary components. There have been many studies looking at colorectal cancer in relation to single nutrients or foods, including dietary fats, red meat, fiber, vegetables and fruits, calcium, vitamin D, and antioxidants. But results have been mixed and sometimes disappointing, for several reasons. A nutrient or food usually takes a while to have an effect, yet most trials have been short. Also, studies haven’t taken into account the many interactions among foods in a complete diet. Even if you study a large population for a long time, it’s hard to separate the contributions of diet from other behavior that affects health. Even so, a few consistent messages have emerged:

• Eating a lot of red and processed meats probably increases the risk of colorectal cancer.
• Diets high in saturated fat and cholesterol may increase the risk.
• Supplemental calcium and folic acid may reduce the risk.

Key factors within our control

We can’t do anything about age, our most obvious risk factor for colorectal cancer (more than 90% of colorectal cancers are diagnosed in people over age 50). Having a family history of colon cancer or a genetic predisposition for it is also outside our control. But colorectal cancer is remarkably preventable if we take certain steps. Here are some strategies we already know can make a difference:

Screening. The most important thing you can do to reduce your chances of getting colorectal cancer is to be screened, starting at age 50 (earlier if you have a predisposing family history or genetic syndrome). Most colorectal cancers arise from a particular type of polyp called an adenoma. Although adenomatous polyps are common after age 50, only a small proportion of them become cancerous — a process that takes many years, so there’s usually time to find and get rid of them. A physician can see polyps by examining the colon with flexible sigmoidoscopy or, preferably, colonoscopy. Polyps may also bleed, which makes them detectable with simple fecal occult blood testing. Either way, they can then be removed during a colonoscopy. Screening and treatment for precancerous polyps decreases the incidence of colorectal cancer by 75%–90%.

Exercise. After screening, your best protection against colon cancer is physical activity. In the Nurses’ Health Study, physical activity reduced the risk of both adenomas and colon cancer by 50%. Walking briskly for an hour a day was as effective as more strenuous exercise, such as jogging or cycling for half an hour a day. Exercise may work by speeding the elimination of solid waste, which contains potential carcinogens; it may also lower blood insulin, which, at high levels, can stimulate abnormal cell growth in the colon.

Healthy weight. Both the Iowa Women’s Health Study and the Nurses’ Health Study found that being overweight or obese increases the risk of colon cancer in women, possibly because excess weight raises insulin levels.

Multivitamins with folic acid. Most of us know that folic acid helps reduce the risk of birth defects. But it has many other important functions, and a protective role against colorectal cancer is one. It’s not clear how this works, but it may be that folic acid (the synthetic form of the vitamin folate) protects DNA. In the Nurses’ Health Study, women who used multivitamins containing folic acid for at least 15 years were 75% less likely to develop colon cancer than women who didn’t take such multivitamins.

Caution with alcohol. In both women and men, having two drinks or more per day may increase the risk for colorectal cancer, although the mechanism is unclear, and data from long-term studies have not been consistent. A large pooled analysis published in 2004 in the Annals of Internal Medicine found an association between alcohol use and colorectal cancer risk only in people who didn’t take multivitamins.

(This article was first printed in the July 2005 issue of the Harvard Women’s Health Watch. For more information or to order, please go to www.health.harvard.edu/womens.)

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