Type 2 diabetes is associated with increased prevalence of colorectal cancer, according to two population-based studies reported in the American Journal of Gastroenterology. Many studies that have examined the connection between diabetes and colorectal cancer have yielded contradictory results, Dr. Paul J. Limburg and his research team note. They attribute the variability to deficiency in methods, such as incomplete characterization of diabetes status, variably defined control populations, and failure to assess potential confounders or to differentiate the location of tumors.
To avoid these pitfalls, Dr. Limburg, from Mayo Clinic College of Medicine in Rochester, Minnesota, and his group evaluated data from the Rochester Epidemiology Project, which has medical information for close to 86,000 residents. They analyzed the relationship between type 2 diabetes from 1970 to 1994 and incidence of colorectal cancer through 1999.
There were 1975 residents with diabetes and 51 incident cases of colorectal cancer. After accounting for age and calendar period, the standardized incidence ratio (SIR) was 1.39, but the association was statistically significant only among men (SIR 1.67).
Because the association was significant for diabetics who were current or former smokers, the authors suggest that men’s higher propensity to smoke may have influenced the difference in risk.
"Colorectal cancer screening (preferably by structural evaluation of the entire colorectum) and tobacco cessation should be strongly emphasized for type 2 diabetes patients," Dr. Limburg and his associates advise.
In the second report, Dr. Dayna S. Early and colleagues at the Washington University School of Medicine, St. Louis, point out that despite the documented relationship between type 2 diabetes and colorectal cancer, there has been no evidence of an association between diabetes and colorectal adenomas.
To investigate, Dr. Early’s team compared the colorectal adenoma risk between 100 postmenopausal women with type 2 diabetes and 500 healthy controls.
Diabetics had significantly more adenomas (37% versus 24%) and advanced adenomas (14% versus 6%) than the controls. The risk was roughly tripled when comparing obese diabetics with nonobese controls.
After adjustment for age, race, diabetes, hypertension, hypercholesterolemia, BMI and NSAID use, the risks of colorectal adenomas and advanced adenomas were approximately doubled among diabetics.
Both research teams report that proximal neoplasms were more frequent than distal neoplasms among diabetics.
Dr. Early’s group also recommends "the incorporation of diabetes mellitus as a risk factor in future colorectal cancer screening guidelines."
Am J Gastroenterol 2006;101:1866-1879.
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