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Long-Term Smoking Increases Colorectal Cancer Risk, Study Shows

By: Rebecca Viksnins Snowden

Long-term cigarette smoking is associated with a higher risk of colorectal cancer, even after accounting for known risk factors such as race, body mass index, and a family history of the disease, according to a new study by American Cancer Society (ACS) researchers published today in Cancer Epidemiology, Biomarkers, and Prevention.

"This provides one more reason not to smoke, or to quit as soon as possible," said author Michael J. Thun, MD, MS, American Cancer Society vice president emeritus, epidemiology and surveillance research. "Colorectal cancer should be added to the list of cancers caused by smoking."

From 1992 to 2005, ACS researchers tracked the habits of nearly 185,000 men and women aged 50 to 74 years old, participants in CPS II Nutrition Cohort, a subgroup of the Cancer Prevention Study II (CPS-II), started by the American Cancer Society in 1982. Researchers examined the relationship between cigarette smoking and colorectal cancer, controlling for 13 potential risk factors, including body mass index, education, family history of the disease, race, diet, multivitamin use, aspirin use, alcohol consumption, red and processed meat intake, sex, vegetable consumption, as well as colorectal cancer screening history.

Current and former smokers were more likely to develop colorectal cancer than lifelong nonsmokers (defined as smoking fewer than 100 cigarettes in their lives). The longer a person smoked, the greater the risk. Those who smoked for 40 years or longer or who did not quit before age 40 had a 30% to 50% increased risk of developing colon or rectal cancer. Among former smokers, risk decreased as the amount of time since quitting increased.

While other studies have found a higher risk of colorectal cancer among smokers, it wasn't clear that smoking was an independent risk factor because many smokers have other characteristics that also raise colorectal cancer risk. This study shows that the risk from smoking persists even when controlling for known colorectal cancer risk factors.

"These findings contributed to the evidence recently reviewed by the International Agency for Research on Cancer (IARC) in October of this year," Thun said. "IARC upgraded the evidence that smoking causes colorectal cancer from ‘limited’ to ‘sufficient’."

Seventeen types of cancer are now classified by the IARC as related to cigarette smoking.

The researchers didn't find a link among smokers who quit before the age of 40 or those who hadn't smoked for 31 years or more -- good news for those who want to quit.