Obesity is the single strongest risk factor for colon cancer in women, U.S. researchers reported on Monday.

They found that women with precancerous polyps in the colon were more likely to be obese than women without these lesions. And obesity more strongly predicted who would have these growths than smoking or having a family history of colon cancer.

“Of all the risk factors like age, family history, smoking, the most potent risk factor was being obese,” Dr. Joseph Anderson of Stony Brook University in New York, who led the study, said in a telephone interview.

“One in five lesions may be attributable to obesity.”

Colorectal cancer is the third-leading cause of cancer death in the United States. It will affect 153,000 Americans in 2007, according to the American Cancer Society, and will kill 52,000.

Family history, smoking and diet are all linked with colorectal cancer but Anderson said experts are still struggling to identify the causes that underlie most cases.

Doctors can use colonoscopy, in which a tiny camera is threaded up into the colon, to not only detect precancerous polyps but to remove them, thus often preventing cancer.

Anderson and colleagues examined the records of 1,252 women who underwent colonoscopy, classifying patients by age, smoking history, family history of colorectal cancer, and body mass index or BMI. Obesity was defined as having a BMI of 30 or higher.

Then they looked to see who had the most polyps, and who was more likely to have them at all.

“BMI was a huge risk factor. And it’s a risk factor that’s increasing,” Anderson said.

BMI was not linked to the risk of colon cancer for men, Anderson and colleagues found.

“We need to counsel people on things like losing weight and staying thin,” said Anderson, who presented his findings to a meeting in Philadelphia of the American College of Gastroenterology.

“Given the increasing number of obese patients in the United States, identifying them as high risk may have important screening implications,” he added.

Why obesity might be linked with colon cancer is unclear, said Anderson.

“Probably the leading factors are going to be insulin and insulin-like growth factor,” he said. People who have more visceral fat, the fat around the internal organs that is associated with the worst effects of being overweight, also have higher levels of insulin and insulin-like growth factor.

New research suggests that a nutrient in red meat, poultry and dairy products may contribute to the development of intestinal polyps, which can lead to colon cancer.

The study, which involved women only, was preliminary, and no one is yet suggesting a change in diet as a result.

However, the research into the nutrient, called choline, could ultimately lead to new dietary recommendations, said Eunyoung Cho, an epidemiologist at Brigham and Women’s Hospital in Boston.

“There may be some impact,” Cho said. “But this is one study, and it’s hard to make any conclusion based on this study.”

The role played by choline, a nutrient required by the body, has been unclear. Some researchers had thought it might provide protection against colon cancer, which kills an estimated 52,000 people in the United States each year, according to the American Cancer Society. The disease is the second biggest cancer killer in the United States after lung cancer.

In the new study, Cho and colleagues looked at nurses enrolled in a large study. They found more than 39,000 women who were free of colon cancer and then underwent at least one endoscopic examination between 1984 and 2002. Polyps — benign growths that can lead to colon cancer — were found in more than 2,400 of the women.

Women who ate the most choline in their food were 1.45 times more likely to have polyps, the team reported in the Aug. 7 online issue of the Journal of the National Cancer Institute.

Having more polyps doesn’t necessarily mean more cancer, and future research will explore whether those who ate the most choline actually developed tumors, Cho said. Studies also need to look into the impact of choline on men.

Why might choline contribute to polyps, and possibly colon cancer, in the first place? The nutrient is a major component of the membranes of cells, Cho said, “and the tumor cell may need choline.”

Currently, health officials recommend that people prevent colon cancer by eating a lot of fiber along with fruits and vegetables. Red meat, meanwhile, is thought to increase risk.

That dietary advice isn’t likely to change even if choline turns out to be a possible villain, said Regina Ziegler, a senior investigator with the National Cancer Institute, who co-wrote a commentary accompanying the new study. “What they’re finding is consistent” with the recommendations, she said.

As for now, “people shouldn’t run out and start either taking more choline or less choline,” she said.