Degree of Food Processing and the Risk of Immune-Mediated Inflammatory Diseases: A Prospective Analysis of the SUN Cohort

Background and Objectives: The role of diet in the risk and clinical course of immune-mediated inflammatory diseases (IMIDs) is an area of ongoing research, in which prospective evidence regarding the degree of food processing is limited. We prospectively assess the association between ultra-processed food (UPF) and minimally or unprocessed food (MUPF) consumption and incident psoriasis, rheumatoid arthritis and vitiligo, as well as a composite exploratory IMID outcome, in a Spanish cohort. Methods: We followed 15,874 IMID-free participants from the SUN Project (median follow-up: 15.1 years). Baseline diet was assessed via a validated FFQ and categorized by NOVA classification. Multivariable Cox models estimated hazard ratios (HRs) and 95% confidence intervals (CIs) across consumption tertiles (% g/day). Results: During follow-up, 298 self-reported diagnosed incident IMID cases occurred (1.31/1000 person-years, mostly psoriasis and rheumatoid arthritis). After adjusting for sociodemographic, lifestyle, and clinical factors, the highest UPF consumption was associated with an increased risk of self-reported diagnosed psoriasis (T3 vs. T1: HR = 1.63, 95% CI: 1.08–2.45) and rheumatoid arthritis (T3 vs. T1: HR = 1.97, 95% CI: 1.19–3.26; p-trend = 0.006), whereas no significant association was observed for vitiligo. Similar trends were observed for the exploratory composite IMID endpoint (T3 vs. T1: HR = 1.80, 95% CI: 1.31–2.45). Conversely, higher MUPF intake was associated with a lower risk of self-reported diagnosed psoriasis (T3 vs. T1: HR = 0.60, 95% CI: 0.40–0.92; p-trend = 0.014) and exploratory composite IMID endpoints (T3 vs. T1: HR = 0.64, 95% CI: 0.47–0.87; p-trend = 0.003). Conclusions: Higher UPF consumption is associated with an increased risk of self-reported diagnosed psoriasis and rheumatoid arthritis, whereas MUPF intake appears to be inversely associated with self-reported diagnosed psoriasis risk. These findings contribute to the ongoing evidence regarding the degree of food processing as a potential factor in the epidemiological profile of specific autoimmune conditions. However, given the observational design of the study, these findings reflect longitudinal associations and do not imply causation.