Folate is a B vitamin found in fruits such as bananas and oranges, leafy green vegetables, asparagus, broccoli, liver, and many types of beans and peas.

Outside experts called the findings intriguing but preliminary, stressing that caution needs to be exercised when interpreting the conclusions.

"In a lot of ways, it's counterintuitive, but it may have validity," said Dr. Howard Manten, an associate professor of medicine and pediatrics at the University of Miami Miller School of Medicine. "We need confirmatory studies."

"It's an interesting study, but with relatively small numbers of patients," added Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La.

This advice does not necessarily pertain to pregnant women, he added, since there is good evidence that extra folate in the diet greatly cuts the risk for having children with neural tube defects such as spina bifida.

Indeed, folic acid has long been known for its effect on reducing certain birth defects when taken in sufficient quantities by pregnant women. That was the rationale behind the U.S. Food and Drug Administration's 1998 order for folic-acid fortification of enriched grain products such as cereals and breads. Canada made fortification mandatory that same year.

According to the study, which appears in the April 25 online issue of Gut, there are also initiatives now in Europe to fortify food with folate.

Previous research had found that folate might protect against colorectal cancer. But many of those studies had looked at dietary intake rather than how much folate was circulating in the body, the authors stated.

In the current study, the biggest-ever prospective look at circulating levels of folate and colorectal cancer risk, researchers at Umea University, Sweden, looked at 226 people with colon cancer and 437 controls from the Northern Sweden Health and Disease Cohort.

Participants completed questionnaires on lifestyle, including diet, and also submitted blood samples for analysis.

People with either the lowest or highest levels of circulating folate were the least likely to develop bowel cancer, the researchers found. Those in the middle were almost twice as likely to develop the disease.

People with a common mutation in the MTHFR gene, which lowers a person's circulating folate levels, also had a lower risk of developing the cancer.

There was no apparent link between homocysteine, an amino acid which may play a role in atherosclerosis, and folate. B vitamins, including folate, tend to keep homocysteine levels down.

If nothing else, the findings should make people think twice before they supplement their diet with large amounts of any one nutrient.

"The study shows us that before we start adding extra things into our diet, we may want to really study them carefully, as we may be doing more harm than good," Brooks said.