Does the duration of diabetes increase the risk of cancer? A nationwide population-based cohort of patients with new-onset diabetes and a matched reference cohort.

Diabetes mellitus and cancer are both common health issues, but the correlation between these two diseases remains unclear. We investigated the association of cumulative exposure of diabetes mellitus as an indication of hyperglycemia in terms of disease duration on multiple cancer types. We hypothesized that the risk of cancer would increase over time after the onset of diabetes. The study population consisted of a population-based cohort of 398,708 people and it was constructed from the Finnish CARING project. The Diabetes group consisted of 185,258 individuals, and the non-diabetic reference group comprised 187,921 individuals. Over 4.1 million person-years were accumulated, and the median follow-up time was 10.55 years. In the diabetes group, 25,899 cancer cases were observed compared with 23,900 cancers in the non-diabetic group. We did not find a clear relationship between the duration of diabetes mellitus and most cancer types examined. However, for cancers of the pancreas, prostate gland, bronchus, and lungs, a temporal relationship was found. Furthermore, even within the cancer types where the relationship was detected, it did not change over time. These findings indicate that diabetes does not independently increase the risk of cancer. Instead, the development of diabetes may be attributed to shared risk factors with cancer, such as obesity and/or insulin resistance accompanied by hyperinsulinemia. Thus, it is likely that the clock for increased cancer risk starts ticking already before onset of diabetes and hyperglycemia.