In the study, men with diabetes were found to have more liver, kidney, and pancreatic cancers than men without the disease. And women with diabetes had more stomach and liver cancers than women who didn't have diabetes.
The Japanese study presents some of the strongest evidence yet linking diabetes and cancer, but it is not yet clear if diabetes actually causes malignant disease, researchers from the National Cancer Center, Tokyo, wrote.
Diabetes Rates on the Rise
As with other industrialized countries, including the U.S., diabetes rates have skyrocketed in Japan in recent years, and they continue to rise.
Approximately 150 million people worldwide have diabetes. But that number is expected to double within the next two decades as the population ages.
"The remarkable increase in the diagnosis of diabetes in Japan in recent years may affect future trends in the incidence and type of cancer," the Japanese researchers write in the Sept. 25 issue of the Archives of Internal Medicine.
Researcher Manami Inoue, MD, PhD, and colleagues followed nearly 98,000 men and women in Japan for 9 to 13 years. Participants were between the ages of 40 and 69 when they entered the study.
At enrollment, 3,097 of the men (6.7%) and 1,571 of the women (3.1%) had diabetes or had a history of having the disease. By the end of the study's follow-up in December 2003, 3,907 men, including 366 with diabetes, and 2,555 women, including 104 with diabetes, had developed cancer.
Total cancer risk was found to be 27% higher for men with diabetes than for men without the disease. For women with diabetes, they found an increased risk for stomach and liver cancers specifically.
The Role of Obesity
Obesity is one of the biggest risk factors for type 2 diabetes; it is also a well-established risk factor for certain cancers, including those of the colon, endometrium (inner lining of the uterus), breast, kidney, and esophagus.
But evidence from this and earlier studies suggests that being overweight alone may not fully explain the increase in cancers among diabetes patients.
In a study reported last year that included more than 1 million South Koreans, having diabetes was associated with a 30% increase in risk for dying from cancercancer, even though few of the study participants were overweight.
Eugenia Calle, PhD, the American Cancer Society director of analytic epidemiology, tells WebMD that the link between diabetes and certain cancers has been well established over the years.
"I would say that there is little question that diabetes is an independent risk factor for cancers of the liver, pancreas, colon, and kidney and for endometrial cancer," she says.
American Diabetes Association president Larry Deeb, MD, tells WebMD that the new findings add to the evidence that people who take steps to lower their risk of developing type 2 diabetes or control the disease once they have it may also be lowering their cancer risk.
That means eating right and getting plenty of exercise, he adds.
"Preventing diabetes can certainly lower your cardiovascular risk, and it appears that the same is true for certain cancers," he says.