Introduction:
Multiple Sclerosis (MS) is an inflammatory immune-mediated disease of the Central Nervous System. Recent studies have linked alterations in the overall profile of the gut microbiome with MS, as well as with the activity of the disease. Here, we aim to evaluate the potential effect of Mediterranean Diet (MedDiet) on the gut microbiome in a cohort of patients with a first demyelinating episode in the context of Clinically Isolated Syndrome (CIS) and/or Relapsing-Remitting MS (RRMS).

Material and Methods:
This ongoing study includes 31 patients with a first demyelinating episode and 27 age, sex and BMI matched Greek individuals. Fecal samples were profiled using 16S rRNA sequencing (V3/V4 regions). Microbial sequences were clustered into operational taxonomic units and aligned to SILVA132 database. Dietary intake was assessed using a validated semi-quantitative FFQ. Adherence to MedDiet was assessed using a nine-item composite index, the Mediterranean Diet Score (MDS), and subjects were classified into two dietary groups according to the median MDS. Data analyses were conducted using in-house R scripts.

Results:
MDS did not statistically differ between healthy and patients, however, patients consume less servings/day of vegetables and red meat compared to the healthy individuals. Alpha diversity of the gut microbiome was similar across dietary groups in both healthy and patients with a first demyelinating episode. Principal coordinate analysis based on Bray-Curtis dissimilarity revealed that MDS and disease status are significantly covaried with the gut microbiome diversity, although they explained a modest proportion of variance. Among patients, differential abundance testing across dietary groups revealed association of greater adherence to MedDiet with abundance of specific previously reported beneficial microbial genera, including Prevotella and Faecalibacterium.

Conclusions:
Our preliminary data provide evidence of a potential role of MedDiet in modulating gut microbiota composition in people with a first demyelinating episode, thus elucidating possible promising intervention approaches.