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Diabetes mellitus and cancer are diseases of epidemic proportions across the globe. These diseases are influenced by many factors, both genetic and environmental. A possible association between diabetes and cancer risk has long been speculated. Increased incidence of several cancers has been observed in diabetes patients, notably pancreatic, hepatic, colorectal, breast, urinary tract, and endometrial cancers. In contrast, a decreased incidence of prostate cancer is observed in diabetes patients, implying a protective effect. Precise knowledge of the complex associations and interactions between these two conditions is of great importance for their prevention and treatment. Multiple potential mechanisms have been proposed, but they have tended to be site-specific. Possible common mechanisms for a biological link between diabetes and cancer include hyperinsulinemia, hyperglycemia, and inflammation. Today, 366 million people live with diabetes globally, and this figure is expected to increase. Thus, if diabetes is associated with even a small increase in cancer risk, this may have important consequences at the population level. The aim of this review is to summarize recent epidemiological evidence of an association between diabetes and total cancer and specific sites of cancer, and to consider causal associations between these diseases. (Cancer Sci 2013; 104: 9–14)