Background: The role of dietary patterns in the primary prevention of specific autoimmune conditions remains poorly understood. Objectives: This study aimed to evaluate the association between Mediterranean diet (MedDiet) adherence and risk of incident rheumatoid arthritis and psoriasis. Methods: We conducted a prospective analysis of 15,874 immune-mediated inflammatory disease-free participants from the Seguimiento Universidad de Navarra Project cohort (median follow-up: 15.1 y). The outcomes were the incidence of rheumatoid arthritis and psoriasis. Adherence to the MedDiet was repeatedly assessed using validated food frequency questionnaires with the MedDiet score and classified into energy-adjusted tertiles. Incident cases were identified through biennial questionnaires. Primary analyses used Cox proportional hazards models with cumulative mean dietary exposure. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated from multivariable models, stratified by age and follow-up period and adjusted for sociodemographic, lifestyle, and clinical confounders. Results: During follow-up, we identified 165 incident cases of psoriasis (0.73 per 1000 person-years) and 119 incident cases of rheumatoid arthritis (0.53 per 1000 person-years). High MedDiet adherence (T3 compared with T1) was inversely associated with rheumatoid arthritis risk (HR = 0.53; 95% CI: 0.32, 0.89; P-trend = 0.020). For psoriasis, higher adherence showed a nonsignificant inverse association (HR = 0.86; 95% CI: 0.57, 1.32; P-trend = 0.491). In exploratory item-by-item analyses for rheumatoid arthritis, lower meat intake and higher fruit/nut intake were associated with lower disease risk (HR = 0.41; 95% CI: 0.27, 0.61; HR = 0.64; 95% CI: 0.43, 0.95, respectively). Conclusions: High MedDiet adherence was inversely associated with risk of incident rheumatoid arthritis among Spanish university graduates, whereas the association for psoriasis remained inconclusive. This trial was registered at clinicaltrials.gov as NCT02669602 (https://clinicaltrials.gov/study/NCT02669602).
