Most fibromyalgia (FM) patients are women, with a female-to-male ratio of 3:1. While sex differences in pain perception and description have been reported, the impact of sex and gender on FM severity remains controversial. Additionally, sex-related differential item functioning (DIF) in FM assessment tools has not yet been explored. The primary aim of this study was to analyze sex- and gender-related differences in FM severity using data from a web-based FM registry. The secondary aim was to assess sex-related DIF in three commonly used questionnaires: the Polysymptomatic Distress Scale (PDS), the modified Fibromyalgia Assessment Status (ModFAS), and the revised Fibromyalgia Impact Questionnaire (FIQR). Data from 331 male and 331 female patients, matched for age and body mass index (BMI) and fulfilling ACR 2010/2011 criteria, were retrospectively collected from the Italian Fibromyalgia Registry. Multivariate analyses were conducted on the overall and sex-stratified populations. Sex-related DIF was assessed using a hybrid Ordinal Logistic Regression/Item Response Theory method. Female sex was significantly associated with greater physical impairment, despite no differences in overall disease severity. In stratified analyses, married status influenced disease impact and burden in women, whereas BMI was associated with higher disability in men. Significant sex-related DIF was detected in one item of the Symptom Severity Scale of the PDS. Women with FM experience greater physical impairment than men, despite similar disease severity. Sociodemographic factors influence FM differently across sexes. Despite minor DIF, the three FM-specific questionnaires appear valid for use in both male and female patients.

Background: The Mediterranean diet (MedDiet) has been associated with anti-inflammatory effects and potential benefits in several chronic conditions. However, adherence to the MedDiet and its relationship with lifestyle factors and disease severity across different rheumatological diseases remain poorly characterized. Objectives: This study aimed to evaluate differences in MedDiet adherence among patients with rheumatoid arthritis (RA), psoriatic arthritis (PsA), and fibromyalgia (FM), and to explore its association with cardiovascular comorbidities, bowel habits, and disease-related outcomes. Methods: In this monocentric cross-sectional study, adherence to the MedDiet was assessed using the 14-item PREDIMED questionnaire. Self-reported data on sociodemographic characteristics, cardiovascular comorbidities, bowel habits, and dietary behaviors were collected through questionnaires. Disease activity and severity were assessed using validated disease-specific measures. Differences in MedDiet adherence across diagnostic groups were evaluated using non-parametric tests. Multivariable models were performed to examine associations between MedDiet adherence and cardiovascular comorbidities or disease outcomes, adjusting for potential confounders including age, sex, BMI, smoking status, and educational level. Results: A total of 422 participants were included (RA n = 165, PsA n = 85, FM n = 172). Significant differences in MedDiet adherence were observed across diagnostic groups (p < 0.001), with the highest adherence in RA, intermediate values in PsA, and the lowest in FM. Compared with the other groups, a higher proportion of FM participants reported food intolerances (46.5%) and restrictive diets, including lactose-free (34.9%) and gluten-free (15.1%) diets. In the FM group, high adherence to the MedDiet was significantly associated with lower FIQR scores (β = −16.9; 95% CI −32.1 to −1.7; p = 0.01) and lower PDS scores (β = −4.34; 95% CI −7.81 to −0.86; p = 0.01). Sensitivity analyses using the continuous PREDIMED score confirmed these associations. Conclusions: Adherence to the MedDiet differs across rheumatological diseases, with the lowest adherence observed in FM. Higher adherence was associated with lower disease severity and impact in FM. These findings highlight the potential relevance of nutritional counselling in rheumatological diseases and support the need for longitudinal and interventional studies evaluating the role of the MedDiet within multidisciplinary disease management.