Diabetes is estimated to afflict 6% of the world's population, and its prevalence has grown sharply in the United States (across all age groups, genders, and ethnicities for which data are available). In 1995, only three states had an age-adjusted (18 years and older) diabetes prevalence greater than 6%, whereas by 2010 it was over 6% in all 50 states plus Puerto Rico. As of 2010, there were 18.8 million diagnosed and an estimated 7 million undiagnosed cases of diabetes in the U.S., a country of 313 million people.
It's been known for many years that diabetes brings with it a greatly elevated risk for a number of comorbidities, including obesity, hypertension, cardiovascular disease, non-alcoholic fatty liver disease, and eye and kidney damage. To this list, we now have to add elevated risk of cancer, as well.
The medical picture isn't a simple one, because for one thing, the diabetes drug metformin is widely believed to a have anti-cancer properties. Also, obesity (a common diabetes cormorbidity) is, by itself, a cancer risk. And paradoxically, the risk for prostate cancer is actually somewhat lower in diabetic men than in non-diabetic men.
While most studies have focused on Type 2 diabetes, there is ample evidence for increased cancer risk in Type 1 (insulin-dependent) diabetes. Also, although cancer risk is elevated for all diabetic groups, there is strong evidence of greater-than-usual cancer risk in Type 2 patients who use insulin. The magnitude of increased risk is about 20% for Type 1 patients, according to one study, but it's more like 60% for Type 2 men who use insulin and 80% for Type 2 women who use insulin. The risk applies primarily to cancers of liver, pancreas, endometrium, colon/rectum, breast, and bladder.
One of the most comprehensive (and recent) studies on diabetes and cancer (Li et al., "Prevalence of Diagnosed Cancer According to Duration of Diagnosed Diabetes and Current Insulin Use Among U.S. Adults With Diagnosed Diabetes: Findings from the 2009 Behavioral Risk Factor Surveillance System," Diabetes Care, January 2013) involved 25,964 U.S. men and women, of whom 4.7% had Type 1 diabetes, 70.5% were Type 2 diabetic without insulin use, and 24.8% were Type 2 with insulin use. The study authors were aware of, and controlled for, such confounding factors as age, educational status, obesity, tobacco use, alcohol consumption, and other factors. Even when these factors were accounted for, cancer risk was elevated across all groups, and the risk increased with the length of time since diabetes diagnosis.
One of the unique findings of the Li et al. study was elevated risk of skin cancer in diabetics. This effect had been noted before, in a Swedish study that found a five-fold increased risk of skin cancer in Type 1 diabetics. Li et al. have taken an appropriately cautious approach, noting:
Future research is warranted to determine whether there are biological mechanisms linking cutaneous manifestations of diabetes and development of skin cancers or whether the cutaneous manifestations of diabetes increase the probabilities of early clinical detections for skin cancers.
What can diabetic patients do to reduce their cancer risk? The available research seems to imply that if you can take metformin rather than insulin, doing so will cut your cancer risks. It also seems to be the case that obesity (whether associated with diabetes or not) is correlated with elevated cancer risk, so anything you can do to lower your weight will likely also lower your cancer risk. Substantial evidence suggests that anti-inflammatory drugs like aspirin and ibuprofen protect against many cancers. And it's well-established that sunshine and vitamin D are protective against many cancers.
So there's actually a great deal you can do to cut your cancer risk. The most important advice I can give you is: Be proactive in your own health care. Do your own research. (Get familiar with Google Scholar.) Don't rely on doctors to tell you everything you need to know.
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