Relative and combined effects of socioeconomic status and diabetes on mortality: A nationwide cohort study

Both low socioeconomic status (SES) and diabetes mellitus (DM) are important risk factors for mortality. However, little is known about their combined effects and relative contribution to the mortality risk. From a nationwide cohort provided by the National Health Insurance Service in Korea, 153,075 subjects who were over 30 years of age from 2003 to 2004 were followed-up until 2010. The SESs of the subjects in the DM and non-DM (NDM) groups were categorized into 3 groups (highest 30% as S1, middle 40% as S2, and lowest 30% as S3) based on the subjects' income levels. During the 7.9-year follow-up, 3933 deaths occurred. When the subjects were stratified into 6 groups by their socioeconomic and diabetes status, a linearly increasing pattern of the hazard ratio (HR) of mortality from the higher SES without diabetes group (NDM-S1, as a reference) to the lower SES with diabetes group (DM-S3; HR, 2.04, 95% confidence interval (CI), 1.80-2.36) was observed (P for trend < 0.001). Notably, subjects with DM in the highest SES group (DM-S1) had a significantly higher mortality risk than did non-DM subjects in the lowest SES group (NDM-S3). This pattern was maintained in cause-specific mortality but was more prominent in cardiovascular disease (CVD) and less prominent in cancer mortality. The association was not affected by gender; however, in individuals <60 years of age, the combined effects of SES and DM on mortality were more prominent (DM-S3; HR, 3.68, 95% CI, 2.95-4.60) than in those ≥60 years of age. Low SES and DM were major determinants of mortality and synergistically increased the risks of all-cause, CVD, and cancer mortality.