Japanese adults with diabetes may have a higher risk of cancer overall and in several specific organs, including the liver, pancreas and kidney, according to results of a large study published in the September 25 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.

Researchers have long suspected that there might be an association between diabetes and cancer, but no conclusive evidence has been obtained, according to background information in the article. Diabetes is rapidly becoming more common in Japan, as it is in many other countries. More than 7.4 million Japanese individuals were estimated to have diabetes in 2002, and by 2025, 8.7 percent of the population is expected to develop the disease. “Clarification of the association between diabetes mellitus and cancer in populations with an increasing prevalence, such as Japanese persons, is a crucial task, not only from the causative point of view but also with regard to the formulation of clinical strategies and public health policies for the target population,” the authors write.

Manami Inoue, M.D., Ph.D., National Cancer Center, Tokyo, and colleagues studied the association in 97,771 Japanese individuals (46,548 men and 51,223 women) age 40 to 69 who were enrolled in the study between 1990 and 1994. The participants, who had an average age of 51 at the beginning of the study, completed a lifestyle questionnaire at that time that included information about smoking, alcohol drinking, medical history, physical activity and food and beverage intake. They were also asked if they had ever been diagnosed with diabetes or taken diabetes medications. Researchers consulted the national registry of Japanese residents, major hospitals, cancer registries and death certificates to track deaths and cancer cases.

At the beginning of the study, 3,097 men (6.7 percent) and 1,571 women (3.1 percent) had a history of diabetes. By the end of the study’s follow-up in 2003, 6,462 participants had developed cancer, including 3,907 men (366 of whom had diabetes) and 2,555 women (104 with diabetes). Men with diabetes had a 27 percent higher risk of developing cancer than men without diabetes; the risk was especially high for liver, kidney and pancreatic cancer. Among women, those with diabetes had a 21 percent higher risk of cancer than those without (although this increased risk was not statistically significant). However, there was a significantly higher risk for stomach and liver cancer and a borderline higher risk for ovarian cancer.

It is unclear exactly how diabetes might contribute to cancer; many discussions of the issue have focused on the particular type of cancer, such as liver or pancreatic cancer, the authors write. Researchers suspect that excess insulin in diabetic patients may promote growth in the cells of these organs, increasing cancer risk. In addition, changes in sex hormone levels associated with diabetes could contribute to ovarian cancer in women and prostate cancer in men. “Despite the biological plausibility of the association, several issues should be considered when discussing the role of diabetes mellitus as a cause of cancer,” the authors warn. For example, common health conditions and risk factors such as obesity might contribute to both diabetes and cancer, and some types of cancer may actually cause diabetes. In addition, those being treated for diabetes often visit a physician more frequently than those without a chronic health condition, and this increased vigilance could lead to more cancer diagnoses. “These issues should likely be considered as alternative factors affecting the association between diabetes mellitus and cancer, directly or otherwise.”

Regardless of whether diabetes causes cancer, cancer causes diabetes or a common third cause links them both, it is likely that the rapidly increasing incidence of diabetes among Japanese residents in recent years heralds a future increase in cases of cancer, especially those kinds most closely linked with diabetes, they conclude.

(Arch Intern Med. 2006;166:1871-1877.)