This study aims to evaluate the effect of the Mediterranean diet on disease activity, quality of life, inflammation, pain, and treatment response in patients with axial spondyloarthritis (ankylosing spondylitis) who are receiving stable biologic therapy. Axial spondyloarthritis is a chronic inflammatory disease that affects the spine and sacroiliac joints, often leading to pain, stiffness, and reduced quality of life. While biologic disease-modifying antirheumatic drugs (bDMARDs) are effective in reducing inflammation, lifestyle factors such as diet may also play an important role in disease outcomes.

In this randomized controlled trial, a total of 110 patients diagnosed with ankylosing spondylitis will be enrolled. Participants will be randomly assigned to either the Mediterranean diet group or the control group. The Mediterranean diet group will receive a structured diet program supervised by a dietitian, while the control group will continue their standard care with general healthy eating advice. Patients will be monitored for dietary adherence, and assessments will be conducted at baseline and after 3 months.

The primary outcome measure is the change in Ankylosing Spondylitis Disease Activity Score with C-reactive protein (ASDAS-CRP). Secondary outcomes include BASDAI, BASFI, BASMI, laboratory markers of inflammation, quality of life (SF-36), fatigue (FACIT-Fatigue), sleep quality (PSQI), anxiety and depression (HADS), medication adherence (MMAS-8), and Mediterranean diet adherence (PREDIMED). The study will provide valuable data on whether a Mediterranean diet can improve clinical outcomes and support the management of axial spondyloarthritis in patients receiving biologic therapy, potentially contributing to future lifestyle-based treatment strategies.

Aim This study is designed to investigate the effects of the Mediterranean diet on disease activity, quality of life, inflammation levels, pain scores, and the frequency of biologic DMARD use in patients with ankylosing spondylitis (AS) receiving biologic disease-modifying antirheumatic drugs (bDMARDs).

Background Ankylosing spondylitis (AS) is a chronic inflammatory disease that causes early inflammation in the sacroiliac joints and, in later stages, also affects the axial skeleton. Structural and functional impairment, together with inflammatory back pain, lead to a marked reduction in quality of life. Genetic factors, intestinal microbiota, mechanical stress on the spine and peripheral joints, innate immunity, oxidative stress, lifestyle, and environmental influences are thought to play important roles in the pathogenesis of AS.

Both pharmacological and non-pharmacological strategies are used in the treatment of AS. The primary goal of treatment is to control symptoms and inflammation, prevent structural damage, and maximize long-term quality of life. However, there is limited evidence supporting the effectiveness of lifestyle modifications. Among environmental factors, diet is a modifiable component with the potential to reduce inflammation. Certain foods may trigger inflammatory processes, whereas nutrients such as antioxidants and omega-3 fatty acids may exert anti-inflammatory effects.

The Mediterranean diet (MD), with its anti-inflammatory and antioxidant potential, is considered a promising dietary model in chronic inflammatory conditions. This dietary pattern emphasizes high consumption of olive oil, unrefined carbohydrates, fresh and dried fruits, vegetables, and fish; low consumption of dairy products and red meat; and moderate consumption of red wine. In AS, such nutrients may reduce inflammation, alleviate pain, decrease structural damage, and potentially improve quality of life. No study has specifically compared the Mediterranean diet in AS patients receiving stable bDMARD therapy with a control group. This trial is intended to address this gap.

Hypothesis The investigators hypothesize that the Mediterranean diet will reduce disease activity, improve quality of life, and lower inflammation in patients with AS.

Study Design and Participants This study is designed as a prospective, single-blind, randomized controlled trial. The required sample size was calculated as 51 patients per group. Considering a 10% dropout rate, 55 patients will be included in each group, resulting in a total of 110 participants. Patients older than 18 years who meet the Assessment of SpondyloArthritis International Society (ASAS) classification criteria for AS will be eligible after providing written informed consent.

Inclusion Criteria

Diagnosis of AS according to ASAS criteria

Age ≥18 years

No dietary changes in the last 3 months

Stable treatment (NSAIDs or DMARDs) for the previous 4 months

Willingness to participate

Exclusion Criteria

Pregnancy or breastfeeding

Nutritional restrictions preventing adherence to the Mediterranean diet

Participation in another diet- or exercise-based intervention program within the last 6 months

History of other rheumatologic or neurological diseases

Patients meeting ASAS criteria for radiographic axial spondyloarthritis and aged over 18 years will be included after informed consent. Only those receiving biologic therapy for at least the previous 4 months with stable treatment will be enrolled.

Randomization and Blinding Participants will be randomized into intervention and control groups using stratified randomization according to age, body mass index, and disease activity (ASDAS-CRP). Randomization will be performed by an independent researcher using an online randomization system, with codes safeguarded to ensure allocation concealment. The outcome assessor will be blinded to group allocation.

Interventions

Control Group (Standard Care): Patients will continue routine medical treatment and will be advised to follow general healthy eating guidelines.

Mediterranean Diet Group: Patients will follow a structured Mediterranean diet program designed with a dietitian. Dietary adherence will be reinforced through telephone follow-ups every 10 days.

Assessments will be performed at baseline and at the end of the 3-month intervention.

Primary Outcome

Change in Ankylosing Spondylitis Disease Activity Score with C-reactive protein (ASDAS-CRP).

Secondary Outcomes

Bath Ankylosing Spondylitis Disease Activity Index (BASDAI)

ASDAS using erythrocyte sedimentation rate (ASDAS-ESR)

Bath Ankylosing Spondylitis Functional Index (BASFI)

Bath Ankylosing Spondylitis Metrology Index (BASMI)

Hospital Anxiety and Depression Scale (HADS)

Quality of life (SF-36)

International Physical Activity Questionnaire (IPAQ)

Pittsburgh Sleep Quality Index (PSQI)

Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-Fatigue)

Health Assessment Questionnaire (HAQ)

Medication adherence (MMAS-8)

Mediterranean diet adherence (PREDIMED score)

Hand grip strength

Body composition analysis (InBody270 bioelectrical impedance analysis)

Power Analysis The required sample size was calculated using G*Power software with 80% power and a type I error of 0.05. A total of 55 participants per group was determined, accounting for a 10% dropout rate.

Statistical Analysis All analyses will be performed using SPSS software. The Shapiro-Wilk test will assess normality of data. Baseline characteristics will be compared using independent t-tests or Mann-Whitney U tests as appropriate. Repeated-measures ANOVA will be used for within- and between-group comparisons of primary and secondary outcomes at baseline and 3 months. Bonferroni correction will be applied for multiple comparisons. Categorical variables will be analyzed using Chi-square or Fisher's exact test. A p-value <0.05 will be considered statistically significant.

Inclusion Criteria:

• Age ≥18 years.

Diagnosis of ankylosing spondylitis according to the Assessment of SpondyloArthritis International Society (ASAS) classification criteria.

No dietary changes within the past 3 months.

Stable treatment with NSAIDs or DMARDs for at least the previous 4 months.

Receiving biologic therapy (bDMARDs) with no changes in medication for the last 4 months.

Willingness and ability to provide written informed consent

Exclusion Criteria:

• Pregnancy or breastfeeding.

Nutritional restrictions preventing adherence to the Mediterranean diet (e.g., severe food allergies).

Participation in another diet- or exercise-based intervention program within the last 6 months.

Presence of another rheumatologic disease or history of neurological disorders.

Individuals younger than 18 years.

Contraindications to body composition analysis (e.g., cardiac pacemaker, pregnancy).

No individual participant data (IPD) will be shared due to patient privacy concerns and local data protection regulations. Only aggregate study results will be published in scientific journals and presented at conferences.

This is a single-blind study. The outcomes assessor will be blinded to group allocation. Participants and care providers will be aware of the assigned intervention due to the nature of dietary modification.