Personal History of Diabetes as Important as Family History of Colorectal Cancer for Risk of Colorectal Cancer: A Nationwide Cohort Study.

INTRODUCTION: Diabetes mellitus (DM) and colorectal cancer (CRC) share some risk factors, including lifestyle and metabolic disturbances. We aimed to provide in-depth information on the association of CRC risk, especially early-onset CRC, with DM, family history of CRC, and age at DM diagnosis. METHODS:A nationwide cohort study was conducted using Swedish family cancer data sets, inpatient, and outpatient registers (follow-up: 1964-2015), including all individuals born after 1931 and their parents (12,614,256 individuals; 559,375 diabetic patients; 162,226 CRC patients). RESULTS: DM diagnosis before the age of 50 years was associated with a 1.9-fold increased risk of CRC before the age of 50 years (95% CI for standardized incidence ratio: 1.6-2.3) vs 1.3-fold risk of CRC at/after the age of 50 years (1.2-1.4). DM diagnosis before the age of 50 years in those with a family history of CRC was associated with 6.9-fold risk of CRC before the age of 50 years (4.1-12) and 1.9-fold risk of CRC at/after the age of 50 years (1.4-2.5). Diabetic patients had a similar lifetime risk of CRC before the age of 50 years (0.4%, 95% CI: 0.3%-0.4%) to those with only a family history of CRC (0.5%, 0.5%-0.5%), double that of the population (0.2%, 0.2%-0.2%). DISCUSSION: Our large cohort with valid information on DM and family history of cancer showed that DM is associated with increased risk of CRC in a magnitude close to having family history of CRC. Associations of DM and CRC family history with increased CRC risk were most prominent in young adults. These findings warrant further studies on harms, benefits, and cost-effectiveness of CRC screening in patients with diabetes, especially type 2, at earlier ages than in the general population.