Determinants of Adherence and Their Relationship with Clinical Response to a Vegan Mediterranean Diet in Women with Fibromyalgia: A Secondary Analysis of the FIBROVEG Randomized Trial

Plant-based dietary interventions have emerged as promising strategies for fibromyalgia (FM), although their effectiveness may depend on participants’ adherence to the prescribed dietary pattern. This secondary analysis of the FIBROVEG randomized controlled trial investigated dietary adherence, its determinants, and its association with clinical response in women with FM. Twenty-two participants were randomly assigned to follow either a vegan Mediterranean diet or a traditional Mediterranean diet for six weeks (ClinicalTrials.gov: NCT06804460). Adherence was assessed using a predefined food-group-based score, while diet satisfaction and perceived barriers were evaluated using validated questionnaires. Adherence was moderate to high in both groups but was greater in the vegan Mediterranean diet group than in the traditional Mediterranean diet group (86.4 ± 11.3% vs. 72.7 ± 11.9%). In the vegan Mediterranean diet group, lower adherence was associated with a busy lifestyle, time required for meal preparation, and unfamiliar foods. Higher adherence was correlated with greater reductions in low-density lipoprotein cholesterol, neutrophil-to-lymphocyte ratio, and fatigue, whereas no clear associations were observed in the traditional Mediterranean diet group. These findings suggest that a vegan Mediterranean diet is feasible in the short term under structured conditions in women with FM and that greater adherence may contribute to more favorable cardiometabolic and fatigue-related outcomes.