Obesity is the strongest risk factor for developing colorectal cancer in women, overtaking even smoking, new research suggests.

The study defined obesity as a having a body-mass index, or BMI, of at least 30. BMI can indicate the amount of fat a person carries and is calculated using the person's height and weight.

Of the women who had precancerous polyps, 20 per cent were obese and 14 per cent were smokers.
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The findings were presented Monday at a meeting of the American College of Gastroenterology in Philadelphia.

Researchers examined data from 1,252 women who had colonoscopies. They classified them based on their age, smoking history, family history of colorectal cancer and BMI.

Of the women who had precancerous colon polyps, 20 per cent were obese and 14 per cent were smokers who had smoked a pack a day for ten years or more, the study found.

The connection between colon cancer and a high BMI comes down to insulin, Dr. Joseph Anderson, an author of the study, told CBCNews.ca Tuesday.

He said a higher BMI generally means a person carries extra fat around their abdomen, known as visceral fat. That fat makes a person insulin-resistant, meaning biochemical changes occur in their body that cause further excess weight and obesity.

Insulin resistance, in turn, leads insulin growth factors, a series of enzymes and proteins that are responsible for cell development, to fuel the growth of colon tumour cells, Anderson theorized.

BMI needs to be discussed with patients: author

The way doctors and scientists look at obese women needs to change, Anderson said. "Given the increasing number of obese patients in the U.S., identifying them as high risk may have important screening implications."

Anderson said specialists should consider BMI when treating patients, instead of just focusing on age and family history. "BMI for women comes out right on top. Just giving the age and family history may not be enough."

He said physicians should also discuss lifestyle modifications with their patients. "When we screen people as gastroenterologists, we should be modifying their risk. We should be saying: 'Hey look, you're overweight, I'm going to screen you. Here's how you can lose weight.' "

Anderson said he believes lower body fat — and a lower BMI — could reduce the risk of developing colon cancer in women. "While obesity is positively associated with an increased risk of colorectal cancer, patients who lower their BMI could potentially reduce their risk of developing the disease in the future," he said.