High daily levels of vitamin B6 may reduce the risk of getting colon cancer by 58 percent, claims a new study from Harvard Medical School.

The research, published in the May 4 issue of the Journal of the National Cancer Institute, builds on other studies that have already indicated a strong preventive effect from the vitamin.

"There are several smaller studies that have found a protective effect from dietary intakes of B6," said lead researcher Esther K. Wei, an instructor in medicine at Harvard Medical School and Brigham and Women's Hospital. However, "this is the first large study of women to look at blood levels of B6" and find a protective effect, she added.

Wei and her colleagues evaluated nearly 33,000 women who were participants in the Nurses' Health study, a long-running study that began in 1976. Since then, researchers have focused on subsets of the original 121,700 participants, all nurses between 30 and 55 years of age when they enrolled, to study various health issues.

Among this subset, Wei and her team analyzed blood samples collected in 1989, looking for levels of a substance called PLP (pyridoxal 5'-phosphate) -- the main active form of vitamin B6 in the blood. Then they looked at medical records to determine who contracted cancer of the colon or cancer of the colon and rectum (colorectal).

They found a total of 194 colorectal cancer cases and 410 cases of colon polyps, which often precede colorectal cancer. After dividing the women into four groups, from lowest to highest blood levels of PLP, the researchers found that the highest quartile group had a 44 percent lower risk of colorectal cancer and a 58 percent lower risk of colon cancer. The same associations held for the development of polyps.

The associations between high blood levels and lower risk held even after Wei's group controlled for other colon cancer risk factors, such as family history of the disease.

Women in the group with the highest blood levels of B6 had about six times the blood levels of the nutrient as those in the lowest group, Wei said.

Dietary intake of Vitamin B6 was also associated with reduced risk. Women in the lowest intake group got a median of 1.6 milligrams a day compared to 8.6 milligrams in the highest intake group. The recommended daily intake of B6 for most adults varies from 1.3 to 1.7 milligrams a day, depending on age and gender. Those in the lowest group were getting close to the recommended amount of daily B6, Wei said, while those in the highest group, which got the most protective effect from colon cancer, were getting about five times the recommended daily intake, but still at a safe level, she said.

According to the Institute of Medicine, 100 milligrams per day of B6 is the upper threshold of a safe intake level.

It's possible, said Wei, to take in 8.6 milligrams of Vitamin B6 a day by taking a multivitamin and eating vitamin B6-rich food. But she stressed that "you don't have to take in 8.6 milligrams a day to get a reduced risk," since reductions in cancer risk started showing up at levels of just 3.3 milligrams a day.

Vitamin B6 is involved in around 100 physiologic reactions and functions in the body, according to Wei, including protein metabolism, red blood cell function and proper functioning of the nervous and immune systems.

The vitamin can be sourced from multivitamins, fortified cereals, beans, meat, poultry, fish and some vegetables and fruits.

Scientists aren't sure why high blood levels of vitamin B6 protect against colon and colorectal cancer, Wei said, but she noted that "individuals who have high levels of B6 have less chance of having damaged DNA, which can lead to cancer."

Dr. Durado Brooks, director of colorectal cancer for the American Cancer Society, called the new study "interesting work." If the finding about blood levels of the vitamin and reduced cancer risk is replicated, he said, "it will be a useful bit of information to give people about lowering their risk of colon cancer."

Meanwhile, there are a number of measures people can take to lower risk, he said. "Multiple vitamins with folate have been previously shown to reduce risk." Keeping a healthy body weight and maintaining an exercise program can reduce risk, too, he said. For those at average risk of colon cancer, the Cancer Society recommends formal screening beginning at age 50.

The screening tests include a fecal occult blood test, barium enema or examinations of the colon via a test called a sigmoidoscopy or a colonoscopy.

Colorectal cancer is the third most common cancer in the United States, according to the American Cancer Society. The Society estimates that 104,950 new cases of colon cancer and 40,340 new cases of rectal cancer will be diagnosed in 2005, with about 56,290 deaths linked to the disease. The death rate from colorectal cancer has been declining, however, probably because of early detection and improved treatments.