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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.

Background Colorectal cancer (CRC) incidence is increasing among young adults below screening age, despite the effectiveness of screening in older populations. Individuals with diabetes mellitus are at increased risk of early-onset CRC. We aimed to determine how many years earlier than the general population patients with diabetes with/without family history of CRC reach the threshold risk at which CRC screening is recommended to the general population. Methods and findings A nationwide cohort study (follow-up:1964–2015) involving all Swedish residents born after 1931 and their parents was carried out using record linkage of Swedish Population Register, Cancer Registry, National Patient Register, and Multi-Generation Register. Of 12,614,256 individuals who were followed between 1964 and 2015 (51% men; age range at baseline 0–107 years), 162,226 developed CRC, and 559,375 developed diabetes. Age-specific 10-year cumulative risk curves were used to draw conclusions about how many years earlier patients with diabetes reach the 10-year cumulative risks of CRC in 50-year-old men and women (most common age of first screening), which were 0.44% and 0.41%, respectively. Diabetic patients attained the screening level of CRC risk earlier than the general Swedish population. Men with diabetes reached 0.44% risk at age 45 (5 years earlier than the recommended age of screening). In women with diabetes, the risk advancement was 4 years. Risk was more pronounced for those with additional family history of CRC (12–21 years earlier depending on sex and benchmark starting age of screening). The study limitations include lack of detailed information on diabetes type, lifestyle factors, and colonoscopy data. Conclusions Using high-quality registers, this study is, to our knowledge, the first one that provides novel evidence-based information for risk-adapted starting ages of CRC screening for patients with diabetes, who are at higher risk of early-onset CRC than the general population., Uzair Ali Khan and colleagues describe the risk of colorectal cancer in patients with diabetes., Author summary Why was this study done? Diabetes is associated with increased risk of colorectal cancer (CRC), especially in young adults before age 50. CRC incidence is increasing among young adults who are not targeted for screening. Diabetes has not been considered as a risk factor in any CRC screening guideline. What did the researchers do and find? For each single age, we calculated the risk of developing CRC in the next 10 years; for example, at age 50, which is the most common age for starting CRC screening, the risk of developing CRC during next 10 years (age 50 to 59) in the Swedish population was 0.44% in men and 0.41% in women. Men and women with diabetes reached the risk levels for 50-year-old individuals (0.44% and 0.41%, respectively) at about age 45 instead of age 50, i.e., nearly 5 years earlier than the general population, whereas patients with an additional family history of CRC reach these screening risk thresholds, 12 to 21 years earlier than the general population. What do these findings mean? These findings for the first time provide evidence-based information about the best starting age of screening for CRC in patients with diabetes. A major strength of this study would relate to the extremely large and comprehensive national (Swedish) datasets available and the duration involved (all Swedish residents born after 1931 and their parents, followed up to 2015). Clinicians could inform patients with diabetes (with or without family history of CRC) about this possibility and encourage individualized counseling for CRC screening.