Does lifestyle explain the relationship between socioeconomic position and multimorbidity of cancer and cardiometabolic diseases? A mediation analysis applied to the European Prospective Investigation into Cancer and Nutrition.

BACKGROUND: Multimorbidity is socially patterned, with lower socioeconomic position (SEP) linked to higher risk. We examined whether a Healthy Lifestyle Index (HLI) mediates the SEP-multimorbidity association and whether pathways differ by sex. METHODS: We used data from 244 886 participants in the European Prospective Investigation into Cancer and Nutrition study. HLI was derived from smoking, alcohol consumption, physical activity, body mass index and diet. SEP was categorised into low, medium and high-SEP based on education. Multimorbidity was defined as the coexistence of at least two diseases among cancer, type 2 diabetes and cardiovascular diseases. Logistic regression assessed SEP-HLI association, Cox regression SEP-multimorbidity and HLI-multimorbidity associations. Counterfactual mediation analysis estimated the natural indirect effect (NIE) and pure direct effect (PDE). Analyses were stratified by sex. RESULTS: Participants from lower SEP categories were older with worse health outcomes. Women had a healthier lifestyle than men across all SEP levels. In men, the hazard ratio of developing multimorbidity was 1.40 (95% CI: 1.26 to 1.54) for those with low SEP compared with high SEP, in women 1.74 (95% CI: 1.52 to 2.00). Comparing low versus high SEP, PDE for men was 1.28 (95% CI: 1.15 to 1.41), NIE was 1.09 (95% CI: 1.07 to 1.11) (proportion mediated (PM)=29%). In women, PDE was 1.65 (95% CI: 1.47 to 1.90), NIE 1.05 (95% CI: 1.03 to 1.06) (PM=11%). CONCLUSIONS: Lifestyle behaviours partly mediated the SEP-multimorbidity association, underscoring the need to integrate considerations of socioeconomic disparities into the planning of lifestyle interventions.