**Review Title** Impact of the level of adherence to Mediterranean Diet on the parameters of Metabolic Syndrome: A systematic review and meta-analysis of observational studies **Review question** Does the impact of Mediterranean Diet on parameters of Metabolic Syndrome differ between low and high adherence? **Searches** PubMed®, Scopus®, Embase®, Web of Science®, Cochrane Central Register of Controlled Trials (CENTRAL) and Google Scholar databases will be used for all searches. A basic search strategy will be developed for PubMed according to the follow search string: (Mediterranean diet) AND (Adherence)” and modified accordingly for other research engines. **Types of study to be included** We will include observational studies (Prospective and retrospective cohort studies and Cross-sectional studies) **Condition or domain being studied** Metabolic syndrome, also known as the syndrome X has been closely related to lifestyle changes deriving from the impact of westernization on diet and health behavior. Thereby, this transition has led to an increase in morbidity and mortality rates. Metabolic syndrome has been characterized by health professionals and scientists as a cluster of predefined metabolic conditions namely, hyperglycemia, dyslipidemia, hypertension and central obesity. Lifestyle modifications, focusing on dietary patterns and physical activity, may significantly improve markers of metabolic syndrome and further reduce the risk development of non-communicable diseases. Among various types of dietary treatments, there is a great deal of evidence around the protective effect of the Mediterranean diet. Therefore, the objective of this study is to examine the impact of low and high adherence to Mediterranean diet on the parameters of metabolic syndrome. **Participants/population** Inclusion criteria: Adult population (>18 years old) reporting low or high adherence to Mediterranean diet according to validated score tools. At least three (up to five) parameters of metabolic syndrome should be described for each study population (waist circumference, HDL level, level of triglycerides, systolic blood pressure and fasting glucose), according to the NCEP ATP III 2005 revision criteria for diagnosis of Metabolic Syndrome. **Exclusion criteria:** Age <18 years old Self –reported low or high adherence to Mediterranean diet Studies reporting two or less parameters of metabolic syndrome for the population examined **Intervention(s), exposure(s)** High adherence to Mediterranean diet **Comparator(s)/control** Low adherence to Mediterranean diet **Context Main outcome(s)** Differences in parameters of metabolic syndrome between low and high adherence **Timing and effect measures** Standardize mean difference **Additional outcome(s)** Not applicable. **Timing and effect measures** Not applicable. **Data extraction (selection and coding)** Our systematic review will be performed in accordance with the meta-analyses Of Observational Studies in Epidemiology (MOOSE) statement (Meta-analysis of Observational Studies in Epidemiology. A Proposal for Reporting. JAMA. 2000;283(15):2008-2012. doi: 10.1001/jama.283.15.2008.). Two reviewers will independently extract data regarding the parameters of metabolic syndrome in low and high adherence to Mediterranean diet. Any disagreement will be resolved by a third reviewer. Authors will be contacted in order to obtain any missing data relevant to the analysis. **Risk of bias (quality) assessment** The quality of the eligible studies will be evaluated using a version of the Newcastle-Ottawa quality assessment scale for case-control and cohort studies and the modified Newcastle Ottawa scale for cross-sectional studies (Wells, G. A, Shea, B., O'Connel, D. et al. The Newcastle-Ottawa scale (NOS) for assessing the quality of non-randomized studies in meta-analyses 2013 http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp). A sensitivity analysis will be performed excluding studies with low quality. **Strategy for data synthesis** Random effects will be used and the inverse variance method. We will report effect sizes as standardize mean differences (SMD) and their 95% confidence intervals (CI). Estimation of heterogeneity will be performed with Cochrane Q test (p<0.1: existence of heterogeneity) and I2 statistic Publication bias will be shown with funnel plots in case of adequate number of studies Text and tables will be used to provide an explanation of study characteristics and findings **Analysis of subgroups or subsets** Different group of people and parameters of metabolic syndrome will be analyzed in case of adequate data. **Contact details for further information** Assoc. Prof. Michail Chourdakis e-mail: mhourd@gapps.auth.gr **Organizational affiliation of the review** Aristotle University of Thessaloniki, Thessaloniki, Greece **Review team members and their organizational** **affiliations** Lydia Chrysoula, Laboratory of Hygiene, Social & Preventive Medicine and Medical Statistics, School of Medicine, Faculty of Health Sciences, Aristotle University of Thessaloniki, Greece. Dimitra Rafailia Bakaloudi, Laboratory of Hygiene, Social & Preventive Medicine and Medical Statistics, School of Medicine, Faculty of Health Sciences, Aristotle University of Thessaloniki, Greece. Evangelia Kotzakioulafi, Laboratory of Hygiene, Social & Preventive Medicine and Medical Statistics, School of Medicine, Faculty of Health Sciences, Aristotle University of Thessaloniki, Greece. Xenophon Theodoridis, Laboratory of Hygiene, Social & Preventive Medicine and Medical Statistics, School of Medicine, Faculty of Health Sciences, Aristotle University of Thessaloniki, Greece Assoc. Professor Michail Chourdakis, Laboratory of Hygiene, Social & Preventive Medicine and Medical Statistics, School of Medicine, Faculty of Health Sciences, Aristotle University of Thessaloniki, Greece. **Type and method of review** Systematic review and Meta-analysis **Anticipated or actual start date** 01 January 2021 **Anticipated completion date** 30 March 2021 **Funding sources/sponsors** None **Conflicts of interest** None **Language** English **Country** Greece **Stage of review** Review Ongoing