The study, led by Chaoyang Li, MD, from the Centers for Disease Control and Prevention, also showed that cancer prevalence is increased in adults with type 2 diabetes who currently use insulin.
Dr. Li stated that, although associations were seen for all cancers, that among men, having diabetes diagnosed for 15 years or longer was specifically linked with a higher prevalence of colon cancer, melanoma, nonmelanoma skin cancer, and cancer of the urinary tract, compared with those who had diabetes less than 15 years. For women, the association was seen for breast cancer, cancer of the reproductive tract, and skin cancer. The association of skin cancer with duration of diabetes for both sexes was one of the unique findings of the study, he added.
Dr. Li also warned that caution is required in interpreting these results. "This is a population-based epidemiologic study. The findings are useful in hypothesis screening for the possible association between diabetes and cancer but, should not be over generalized to clinical practice or individual patients."
In their published study online, Dr. Li and colleagues explain that little is known about the association between diagnosed diabetes and cancer and that prior studies looking at whether insulin therapy affects cancer risk in people with diabetes have been inconclusive.
To determine whether longer duration of diagnosed diabetes and current insulin use are associated with increased prevalence of cancer among adults with diagnosed diabetes, they analyzed a large population-based sample from the 2009 Behavioral Risk Factor Surveillance System (BRFSS). After adjustment for potential confounders, Dr. Li and colleagues found that the greater the duration of diagnosed diabetes, the higher the prevalence of diagnosed cancers (P < .0001 for linear trend).
Among adults with type 2 diabetes, there was an estimated 1.6-fold increased prevalence for cancers of all sites among men and a 1.8-fold increase among women who reported being diagnosed with diabetes 15 or more years ago compared with those reporting diabetes diagnosis less than 15 years ago.
For patients with type 2 diabetes who currently use insulin, the prevalence estimate for cancers of all sites was about 1.3 times higher than among those who did not use insulin, for both men and women, even after adjustment for duration of diagnosed diabetes.
The study found a higher prevalence of diagnosed cancer among adults who had type 2 diabetes and currently used insulin than those who had type 2 diabetes but did not currently use insulin or those who had type 1 diabetes.
"The difference in the prevalence of diagnosed cancer between type 1 diabetes and type 2 diabetes is possibly attributable to factors related to the underlying etiologies and lifestyle habits," Dr. Li added. "We did not conduct analyses according to type of cancer among persons with type 1 diabetes due to inadequate sample size."
Among men who had been diagnosed with diabetes 15 or more years ago, there was a more than 3-fold higher prevalence for colon cancer and a similar increase for urinary-tract cancers (including both bladder and kidney) than those with diabetes for less than 15 years.
For women, there was an approximately 2-fold higher prevalence of cancers of the breast and reproductive tracts in those with longer duration of diagnosed diabetes compared with shorter. "These results support the notion that exposure to a high circulating concentration of insulin might result in mitogenic effects on breast tissue that stimulate the growth of endometrial stromal cells," Dr. Li suggests.
There was also a strong association between duration of diagnosed diabetes and prevalence of skin cancer among both men (adjusted prevalence ratio, 1.6 for longer vs shorter duration) and women (adjusted prevalence ratio, 2.2), a "unique" finding of the study, say the scientists.
With regard to insulin use, this was specifically associated with a higher prevalence of skin cancer among diabetic men and cancer of the reproductive tract among diabetic women, Dr. Li said.
Future clinical studies are needed to identify the risk factors that could be targeted to prevent or reduce the risk of cancer among diabetes sufferers, he said; these could include hyperglycemia, insulin resistance, antidiabetes medication, lipid and lipoprotein profiles, health and behavioral risk factors, and diabetes-related complications.
Diabetes Care. Published online January 8, 2013. Abstract