The interaction between income and mental health conditions for subsequent physical health conditions: a nationwide Danish cohort study from 2000 to 2021

BACKGROUND: Mental health conditions and low income are each robustly associated with increased risks of subsequent physical health conditions. However, whether and how these risks are shaped by an interaction between mental health conditions and income is unclear. We aimed to assess absolute and relative risks of a broad range of physical health conditions across income strata in people with and without mental health conditions. METHODS: In this nationwide, register-based cohort study, we included all people residing in Denmark on Jan 1, 2000, through the Danish Civil Registration System, and followed them up through to Dec 31, 2021, excluding those with missing or negative income. We used Danish hospital registers to collect data on mental health conditions; the Danish National Patient Register, the Danish National Prescription Register, and the Cause of Death Register for data on physical health conditions; and the Income Statistics Register for data on income. We applied Poisson regression models with interaction terms between 11 categories of mental health conditions and income quintiles to estimate incidence rate ratios and rate differences per 100 000 person-years for the subsequent diagnosis of 31 specific physical health conditions across nine broadly defined categories, examining overall and sex-stratified associations. FINDINGS: Of the 5 323 158 people eligible, we followed up 5 279 634 people for 98·7 million person-years. Of these 5 279 634 people, 2 671 263 (50·6%) were female and 2 608 371 (49·4%) were male. Those with previous mental health conditions had elevated rates of most physical health conditions across all income quintiles, with parallel income gradients across mental health condition status. The absolute excess burden was substantially elevated in people with mental health conditions with lower incomes versus higher incomes; for example, we detected 716 (95% CI 695-737) additional cases of chronic obstructive pulmonary disease per 100 000 person-years in the lowest income quintile versus 337 (95% CI 314-359) in the highest income quintile. Patterns were consistent across most mental health and physical health condition pairs, but notable exceptions included developmental health conditions (with higher physical condition rates at higher incomes), substance use and schizophrenia spectrum disorders (attenuated gradients), and eating disorders (disorder-specific patterns). Sex-stratified analyses showed similar patterns in both sexes, with slightly higher rates in males. INTERPRETATION: Mental health conditions and low income seem to operate as independent, additive risk factors rather than synergistic ones, resulting in concentrated disease burden in economically disadvantaged populations with mental health conditions. Integrated strategies that address both mental health and socioeconomic determinants are needed to reduce medical morbidity. FUNDING: Independent Research Fund Denmark and the Lundbeck Foundation.