Chronic medical conditions and metabolic syndrome as risk factors for incidence of major depressive disorder: A longitudinal study based on 4.7 million adults in South Korea

The assessment of comorbid physical illness and metabolic or cardiovascular risk factors as potential risk factors for onset of major depressive disorder (MDD) is crucial. We aimed to investigate potential risk factors for the development of MDD among individuals with chronic medical conditions and metabolic and behavioral risk factors using a large population-based retrospective cohort from the data of the National Health Insurance Service (NHIS) in South Korea.The population-based retrospective cohort included data from 2,370,815 adults (age ≥20 years) diagnosed with MDD between January 1, 2010, and December 31, 2016 and age- and gender-matched 2,370,815 healthy controls obtained from the claims data of the NHIS. The data of the regular health checkup provided by the NHIS were also included (age ≥40 years). Logistic regression analyses were performed to investigate the potential risk factors for the incidence of MDD.Chronic medical conditions such as Parkinson's disease (odds ratio [OR] = 7.808, 95% confidence interval [CI] = 7.517-8.11), epilepsy (OR = 6.119, 95% CI = 6.019-6.22), multiple sclerosis (OR = 5.532, 95% CI = 4.976-6.151), Huntington's disease (OR = 5.387, 95% CI = 3.258-8.909), migraine (OR = 4.374, 95% CI = 4.341-4.408), stroke (OR = 4.074, 95% CI = 4.032-4.117), and cancer; metabolic syndrome (OR = 1.049, 95% CI = 1.041-1.057) and several of its components including central obesity, elevated fasting blood glucose and triglyceride levels, and reduced high-density lipoprotein level; and cigarette smoking, frequent alcohol consumption, and low physical activity are potential risk factors for the development of MDD.Our results may support previous evidence on the association between physical conditions and the incidence of MDD as reported by individual population-based studies with modest sample sizes.