Most people wouldn't think diabetes and cancer have anything to do with each other. But a group of experts from the American Cancer Society and the American Diabetes Association thinks they do.
A consensus statement from that panel says there's accumulating evidence that people with diabetes are, in fact, more prone to certain cancers. The analysis is published in the latest CA: A Cancer Journal for Clinicians.
It turns out that diabetics are twice as likely to get cancer of the liver, pancreas and uterine lining. Their risk of colon, breast and bladder cancer is 20 to 50 percent higher than non-diabetics'. There doesn't seem to be any higher risk for others, such as lung cancer. And the risk of prostate cancer is actually lower among diabetics.
Doctors have noticed since the 1950s that their diabetic patients seem to get cancer more often than usual. But it wasn't until last year that researchers pulled together data from various studies and found an association.
Also last year, several epidemiology studies got a lot of attention when they suggested that a synthetic long-acting insulin called glargine seems to increase the risk of cancer. But that's still highly debatable.
Other studies indicate that metformin, the most common drug for Type 2 diabetes, may actually reduce the risk of getting cancer or dying from it.
And just how might diabetes increase the risk of cancer? On that point there's only speculation. Here are some possibilities:
Shared risk factors: Both cancer and diabetes become more common as people age; gain weight; eat diets poor in fruits, vegetables and whole-grain cereals; or smoke. Men have higher risk of both diabetes and cancer.
Off-kilter metabolism: Many cells in the body have surface receptors for insulin and insulin-like growth factors that have been shown in the test tube to stimulate the growth and metastasis of cancer cells. About half of Type 2 diabetes and all Type 1's take insulin daily, and their blood-insulin levels spike higher than normal. Diabetic patients also have episodes of higher-than-normal blood sugar, which might promote cell cancer growth.
The fat-tissue hypothesis: Being overweight and obese is the most prominent risk factor for diabetes, and scientists now know that fat tissue is like an endocrine gland, putting out a number of hormone-like substances. These might play a role in turning normal cells into cancerous ones, or promote cancer cell growth. Cancer in overfed laboratory animals behaves more aggressively.
All these questions need a lot more research. But the consensus panel says it will be hard to do good human studies because the issues are so complex and lots of people would have to be followed for many years. They call for more good "observational" studies comparing cancer incidence in diabetics and nondiabetics and correlating cancer incidence with blood chemistry and different treatments.
Meanwhile, the evidence so far justifies even more attention to healthy diets, weight control and physical activity among diabetics, the consensus panel says. And doctors should look carefully for any signs of early cancer in patients with diabetes.