Diabetes mellitus type 2 - an independent risk factor for cancer?

Epidemiological findings have shown up to two-fold increases in the risks of cancers of the colorectum, breast, endometrium, kidney (renal cell tumours), liver and pancreas among diabetes patients. In the present review, we address the question whether, on the basis of these epidemiological observations, type 2 diabetes should be considered a specific and independent risk factor for these various forms of cancer, due to its particular metabolic characteristics of glucose intolerance and hyperinsulinemia. On the basis of further epidemiological evidence among non-diabetic individuals, as well as recent studies examining the effects of different types of diabetes treatment on cancer risks, we conclude that chronic elevations in fasting and non-fasting blood levels of glucose and/or insulin are plausible independent risk factors for cancer, but that much of the increase in cancer risks associated with these two metabolic factors may occur within the normoglycaemic and insulinemic (non-diabetic) ranges. Furthermore, for some tumour types (e. g. cancer of the endometrium) the associations of risk with type 2 diabetes may to a large extent be due to, and at least partially confounded by, other obesity-related alterations in (e. g. sex steroid) metabolism that in part are independent of glucose and/or insulin metabolism. Specifically for pancreatic cancer, a major question, addressed in detail by other reviews, is whether associations of risk with plasma glucose, insulin or overt type 2 diabetes could be either a cause, or possibly also a consequence of tumour development (or both).