Two of the most common diseases in the United States -- cancer and diabetes -- are not often linked together in the public mind. But they may have a stronger link than most people think. Cancer patients who already have diabetes have a greater chance of dying of the disease than cancer patients who do not have the blood-sugar disorder, according to a report in the Journal of the American Medical Association.
Breast, colorectal and endometrial cancer patients with diabetes have a significantly higher risk of dying.
If researchers can tease out the reason why, they might have an opportunity to reduce cancer deaths by focusing on diabetes-related health problems, says lead author Frederick Brancati, M.D., a professor of medicine and epidemiology at Johns Hopkins Bloomberg School of Public Health, in Baltimore, Maryland.
About 24 million people in the United States have diabetes, and about eight to 18 percent of newly diagnosed cancer patients also have the disease. Health.com: Top 10 diabetes stories of 2008
"It's something that is hiding in plain sight," says Brancati. "Diabetes is very common, cancer is very common, but no one had really thought to organize the literature and see it."
In the new study, Brancati and colleagues analyzed all the available research and selected 23 well-designed studies that looked at cancer patients who had diabetes at the time of diagnosis. This type of study, known as a meta-analysis, can highlight problems that no single study typically can.
The researchers found that people with diabetes had about a 1.4 times greater risk of dying of cancer than patients without blood-sugar problems. Mortality seemed to be higher for many types of cancer, but it was only statistically significant for three types: breast, endometrial, and colorectal. Health.com: What's your breast cancer IQ?
"It's a big deal if we can take a cancer and knock its mortality rate down by 5 percent or 10 percent," says Dr. Brancati. "Here's a potential 40 percent. We don't know from this kind of study how much we can really chip away at it, but it does say there's a target out there."
Some research suggests that people with diabetes are at greater risk of developing cancer in the first place, possibly due to their weight. Many people with type 2 diabetes are also obese, and obesity is linked to a greater risk of some types of cancer.
The researchers say there are many reasons why people with diabetes might have a higher cancer-mortality risk.
• Something about diabetes, possibly high blood sugar, may cause tumors to grow faster.
• People with diabetes are at greater risk of other health problems, such as heart and kidney disease. This may weaken their ability to withstand chemotherapy or other aggressive treatments, so doctors may treat them less aggressively.
• Diabetics are more susceptible to infections, which may increase mortality after surgery or other treatments.
• People with diabetes may be diagnosed with cancer at a later stage of disease because doctors are focusing on their other health problems. Health.com A mammogram may have saved my life
• A cancer diagnosis may cause patients and doctors to neglect diabetes treatment.
• There may be no relation between cancer and diabetes. People with diabetes are at greater risk of health problems, and may just have a higher mortality risk in general.
One other possible factor is depression, says Richard Hellman, M.D., a clinical professor of medicine at the University of Missouri -- Kansas City School of Medicine. About 10 to 40 percent of people with diabetes also experience depression, and they can have more severe episodes than those who don't have diabetes.
"What happens is that people [who are depressed] delay diagnosis, they delay treatment for needed problems, and their outcomes for almost anything are poorer," he says. "If you have a subset of patients with cancer who have diabetes, and that group has a subset that is depressed, the ones who are depressed will tend to have particularly poor outcomes." Health.com Top 10 celebrity health stories of 2008
Undoubtedly, more research will focus on the link to see if managing diabetes better, or differently, can reduce the cancer-mortality rate.
In the meantime, Hellman says that the findings may encourage oncologists to pay closer attention to their patients' diabetes care, and communicate more with physicians who are managing the diabetes. Health.com: I beat cancer and finished a half-marathon
"If you have cancer and you have diabetes, you have to be more vigilant," he says. "Make sure that your care is well coordinated so that your diabetes care remains excellent, any of your cardiovascular evaluations are up-to-date, and the doctors are talking to each other."
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