Obese and overweight people are more likely to develop colon polyps, a possible precursor to cancer, than are slimmer individuals, according to a new review of past research.
Polyps, also called adenomas, are clumps of cells that grow from the colon's lining. Although less than 10 percent become cancerous, most colon cancers are thought to develop from polyps.
Previous studies have made the connection between obesity and colon cancer - a link recognized by the National Cancer Institute - but the new study is the first to point to a higher risk of adenomas in heavy people.
By focusing on "precancerous" cell changes, researchers were hoping to shed more light on whether cancer screening recommendations should take a person's weight into account.
"Because there is a known association between obesity and cancer, there is a logical extension to expect a connection between obesity and the step before cancer, which is adenoma," said Dr. Hutan Ashrafian from Imperial College, London, who co-authored the study.
The findings can't say whether obesity causes polyps by itself. But if it does, that could be bad news for a world where obesity is on the rise.
According to the World Health Organization, about 500 million people worldwide are obese; colon cancer killed more than half a million people worldwide in 2008, the most recent figures from the WHO show.
For the new research, Ashrafian and his colleagues analyzed data from 23 studies involving more than 100,000 people across the U.S., Asia and Europe, looking at the relationship between polyps and body mass index, or BMI, a measure of weight relative to height.
All the studies followed World Health Organization guidelines that define people with a BMI over 25 as overweight and above 30 as obese. In most studies, polyps were identified during colonoscopy procedures in which a flexible tube tipped with a camera is guided though the rectum and into the colon. Self-reported questionnaires were used in two large studies.
Overall, researchers found that 22 percent of overweight and obese people had colon polyps, compared to 19 percent in people of normal weight, and the polyp risk grew with increasing BMI.
"The findings suggest that obesity may be having an effect (on cancer development) much earlier than we thought," said Ashrafian.
Most polyps don't cause any symptoms so they are usually not picked up before routine colonoscopy. During the procedure, the doctor cuts out polyps if there are any to make sure they don't turn into cancer.
In their report, published in the American Journal of Gastroenterology, the authors recommend timely colon cancer screening among overweight and obese people.
The U.S. Preventative Services Task Force, a government-funded expert panel, recommends colon cancer screening for people ages 50 to 75 (there are a number of different tests, including a cheap stool test, to choose from). In the UK, screening for colon cancer is offered by the National Health Service from age 60 to 69.
The new study points to a need for screening that specifically targets obese people, said Dr. Joseph Anderson from Dartmouth in Hanover, New Hampshire, who was not involved in the study.
There are different ways to screen people, and if a person is obese, they are at higher risk and should have a full colonoscopy, he added.
A partial colonoscopy, also known as sigmoidoscopy, uses a flexible tube to look at the rectum and lower part of the colon, making it less invasive and uncomfortable than a full colonoscopy procedure.
Obese people tend to have adenomas in the upper part of the colon, and sigmoidoscopy doesn't reach that far, said Anderson, who has written an editorial on this study for a forthcoming issue of the American Journal of Gastroenterology.
But Dr. Michael Leitzman, who has researched obesity and adenomas, urges caution.
BMI didn't seem to be linked to faster development of cancer once colon polyps were present, so changing screening practices doesn't seem to be necessary, said Leitzman, from University Hospital Regensburg in Germany.
Still, both Anderson and Leitzman agreed that the study had combined the best of previous research on obesity and colon polyps, and strengthened evidence of the link between obesity and colon cancer.
Anderson did, however, note that the study didn't account for diabetes as a potentially confounding factor. Because of such uncertainties, the data can't prove that losing weight reduces the risk of developing polyps, or reverses the progression of cancer development.
But erring on the side of caution and shedding excess weight is a good idea no matter what, said Anderson.