Introduction:
Recently, diet has received considerable attention as an important factor in inflammatory arthritis. The objective of this study was to evaluate disease activity and fatigue in patients with rheumatoid arthritis and spondyloarthritis (including psoriatic arthritis) depending on their dietary preferences.

Material and methods:
We retrospectively evaluated data of 774 patients collected by the Mida Rheuma App from January 2022 to December 2022. We assessed demographic parameters, major dietary patterns, and food groups (dairy products, fruits, vegetables, processed meat, meat, oily fish, white fish, and sugar) by each patient’s consumption intensity. Disease activity was measured by Routine Assessment of Patient Index Data 3 (RAPID3), Patient’s Global Assessment of Disease Activity (PtGADA), and Patient’s Global Assessment of Pain Intensity (PPAIN). Fatigue was assessed by the Brief Fatigue Inventory (BFI). Depending on the food group, patients were divided into a high-consumption group (HCG; consuming the product 0–2 times per day/per week) and a lowconsumption group (LCG; consuming the product 3 and more times per day/per week). Patients on a specific diet which they followed before inclusion in the analysis were compared with the “eat all group” (EAG).

Results:
The Mediterranean diet group had significantly lower (p < 0.05) RAPID3, PtGADA, and PPAIN (by 9.22%, 8.98%, and 14.3%, respectively), compared with the EAG. In the processed meat HCG, RAPID3, PtGADA, PPAIN, and BFI were significantly higher (14.5%, 18.3%, 16.1%, and 11.8%, p < 0.05) vs. the LCG. The PtGADA, PPAIN, and RAPID3 were significantly lower (p < 0.05) in the oily fish HCG vs. the LCG (18.9%, 18.1%, and 10.6%).

Conclusions:
Our study results suggest that Mediterranean diet and oily fish have a potentially positive impact on patient-reported disease activity and fatigue in patients with inflammatory arthritis. In contrast, processed meat consumption has a negative effect.