Adherence to a Mediterranean type of diet in the world: a geographical analysis based on a systematic review of 57 studies with 1,125,560 participants

Abstract This systematic review aimed to assess the level and time-trends of adherence to Mediterranean-type diets (MTD) among the general population, globally. According to Preferred Reporting Items for Systematic Reviews and Meta-Analyses, a comprehensive literature search of the MEDLINE and Scopus databases was carried out, until 04/09/2023, based on specific criteria. Fifty-seven studies with 1,125,560 apparently healthy adults from Europe (n = 37), US (n = 8), Asia (n = 8), Australia (n = 4) and Africa (n = 1) were included. Adherence to an MTD was moderate with a significant decline observed in the last decade. European countries, mainly driven by Mediterranean countries, showed higher adherence than other regions. Geographical analysis revealed that adherence to an MTD is related to both geographic location and socioeconomic status throughout the world.


Introduction
It is difficult to understand how a regional dietary pattern, like the Mediterranean diet, can be adopted by other populations around the world.Considering that the Mediterranean diet is one of the most extensively studied dietary patterns in nutrition epidemiology, which has shown a series of health benefits, and is well appreciated by several scientific organisations, its wide adoption by various, mainly western, populations, should not be a surprise.Mediterranean diet was introduced in 1960s by Ancel Keys, a physiologist from US and principal investigator of the Seven Countries Study, as a dietary pattern low in saturated fat and high in vegetable oils, which has shown considerable cardiovascular health benefits (Keys et al. 1986).However, it should be acknowledged that there are several variations regarding what the Mediterranean diet entails; thus, the most probationary term is Mediterranean type of diet (MTD), instead of Mediterranean diet.In an attempt to achieve a consensus of what the Mediterranean diet is, it could be stated that it is mainly characterised by a high intake of vegetables (including leafy green vegetables), fruits, grains, nuts and legumes, and the use of olive oil, a moderate intake of fish, poultry, dairy products and a rare intake of eggs, sweets, red meat and processed products (Noah and Truswell 2001;Karamanos et al. 2002;Davis et al. 2015).
The pleiotropic beneficial effects of MTD on health, i.e. cardiovascular diseases, cancer, cognitive function, diabetes mellitus, have been well established and supported by scientific health organisations, including the World Health Organisation, and scientists, making this dietary pattern seem like a "panacea" (Sofi et al. 2008(Sofi et al. , 2014;;Koloverou et al. 2014;Dominguez et al. 2021, Gantenbein andKanaka-Gantenbein 2021;Yannakoulia et al. 2015).For example, concerning the primary prevention of cardiovascular disease, the recent European Society of Cardiology (ESC) guidelines encourage adherence to the Mediterranean or a similar diet (Visseren et al. 2021), the American Heart Association/American College of Cardiology (AHA/ACC) 2019 guidelines suggest following plant-based and Mediterranean diets or diets that include an increased intake of vegetables, fruits, legumes, nuts, as well as whole grains and lean animal protein (emphasizing fish over meat) (Arnett et al. 2019).In addition, the World Cancer Research Fund (WCRF) and the American Institute of Cancer Research (AICR), in a 2018 report, recommended following a healthy dietary pattern, the components of which resemble those of an MTD (WCRF/AICR 2018), and, similarly, the International Agency for Research on Cancer of the World Health Organisation (IARC/WHO) recommends following a healthy diet, which incorporates the right amount and variety of different foods (i.e.plenty of vegetables, fruits, whole grains, pulses, limited amounts of high-calorie foods, red meat, foods high in salt and avoiding sugary drinks and processed meat) (IARC/WHO 2023).
As an appreciation of Mediterranean's dietary pattern contribution to human health, the United Nations Educational, Scientific and Cultural Organisation (UNESCO), in 2010, inscribed the Mediterranean Diet to the Intangible Cultural Heritage of Humanity list.According to UNESCO, the Mediterranean lifestyle, in general, considers consuming foods as a social occasion that includes the conviviality of eating, as well as foods that are local, seasonal and biodiverse (UNESCO 2010).For these reasons, MTD is considered a sustainable diet for the planet at least for the Mediterranean region (Donini et al. 2016;Dixon et al. 2023).However, despite the undoubtable beneficial health effects of MTD, and the attention that this dietary pattern has received worldwide during the past decades, a decline in MTD adherence has been observed in most Mediterranean populations, and not only (Panagiotakos et al. 2006;da Silva et al. 2009;Gray et al. 2018;Vilarnau et al. 2019;Georgoulis et al. 2022;Obeid et al. 2022).Urbanisation and socio-economic reasons may have negatively affected not only dietary habits but the living environment, as well (Popkin and Ng 2022;Dixon et al. 2023).The aim of this systematic review was to assess the level and time trends of adherence to MTD among the general adult population, based on observational studies carried out around the world, since 1980s.

Eligibility criteria
Only observational studies were included.For the definition of MTD we followed the description by UNESCO (2010).Specific inclusion and exclusion criteria were formed according to the CoCoPop (Condition, Context, and Population) method (Munn et al. 2015).(see Supplementary Table 1).The selected observational studies were grouped by region and year that they were conducted, sex of the participants and diet score used.

Information sources
According to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, a comprehensive literature search of the MEDLINE (via PubMed, Web of Science, EBSCOhost) and Scopus databases was carried out, until 04/09/2023 (Munn et al. 2015, Page et al. 2021).Moreover, with the same time-limit, references of systematic reviews and meta-analyses on the adherence to MTD (mainly against clinical outcomes) were also manually searched and included.Duplicates were then removed.

Search strategy and selection process
A systematic literature search was performed independently by two of the authors (ED, MK).Disagreements between the two authors were resolved by a third author (DP).Boolean operators (i.e.AND, OR, NOT) were used to make the search specific.The search string included the following keywords, according to MeSH (Medical Subjects Headings): "Health", "Nutrition", "Diet", "Mediterranean diet", "Mediterranean", "Mediterranean dietary pattern", "Diet Score", "Dietary Pattern", "MedDietScore", "Mediterranean diet score", "Mediterranean Diet Scale", "Alternate Mediterranean Diet Score", "MEDAS", "adherence", "compliance", "prevalence", "observation".The sensitivity of the systematic search was also verified by back referencing the collected systematic reviews, meta-analyses, and independent studies.A hierarchical approach, i.e. screening the title, abstract followed by the full-text manuscript, was used to search for studies (Figure 1) (adapted from Page et al. 2021).

Data collection process and data items
From the 21,567 studies that were initially identified, 1,284 remained after rigorous screening based on their title and/or abstract and the inclusion and exclusion criteria (Supplementary table 1), and then 73 were assessed for eligibility.The information extracted included: "First author name, year" (i.e. for citation purposes), "Year of dietary assessment" (if the year was not specified, the year that the study was published was used), "Region", "Country", "Sample size", "Sex, % men", "Score to assess the adherence to a Mediterranean type diet", "Score range", "Score mean" (obtained or computed), "% of adherence to a Mediterranean type diet" (computed).Of the 73 studies found, 57 had available/obtainable data via specific diet scores, concerning the adherence to MTD for the purposes of this systematic review (Supplementary table 2).

Study risk of bias assessment
Concerning study risk of bias, in this systematic review the aim was not to assess an effect size, therefore, reporting bias such as publication bias and certainty assessment were not applicable.Nonetheless, a critical appraisal of the selected 57 studies, which included prevalence data on adherence to MTD, was performed, by two independent authors, based on the checklist of the Joanna Briggs Institute (Munn et al. 2015).This checklist is a novel tool (the first of its kind), to assess the quality of studies reporting prevalence or incidence data (e.g.cross-sectional or cohort studies); it is easy to use as it only includes 9 questions, and by extent has been used by many recent systematic reviews of observational studies (Munn et al. 2015;Akhtar et al. 2023;Spoelma et al. 2023).

Data extraction and analysis
The mean adherence to MTD was either obtained (if the study reported mean adherence via a specific Mediterranean Diet score (MDS) or index, and variance of the mean value) or computed using the weighted midpoint of each percentile class and averaged to the number of percentile classes (if the study presented MTD adherence as a categorical variable of a specific diet score, e.g.high/low, low/moderate/high, centiles etc.).To estimate the level of MTD adherence, the mean scores derived (directly or indirectly as mentioned above) from the retrieved studies were centralised and then transformed to a 0-100 range scale; with 100 score indicating complete adherence to each study-specific defined MTD.Time-trend analysis was performed using linear and non-linear generalised regression models to evaluate shifts in MTD adherence.Regression splines were also estimated and plotted against the year of dietary assessment, to smooth noise in data series.Furthermore, subgroup analyses were performed, to pinpoint potential differences between men and women or different regions, or diet scores used to assess MTD adherence.Data analyses also included descriptive statistics (i.e.mean(standard deviation), frequencies, and 95% confidence intervals-95%CI), and were performed using STATA version 17 (STATA Corp, College Station, Texas, USA).

Geographical analysis and mapping MTD adherence
To explore the influence of geographic location on MTD adherence, countries were divided into groups by geographic area and by Mediterranean-type ecosystems (MTEs) (Archibold 1995).The MTEs are characterised by, among other features, common climatic conditions with mild, wet winters and warm, dry summers that favour the cultivation and availability of foods with high consumption value in the Mediterranean diet.Five MTEs cover different regions of the world on the southwestern edges of large landmasses located at approximately 30-40° north or south latitude: the Mediterranean basin, California, central Chile, southern Africa, and southwestern and southern Australia (Archibold 1995).Countries were grouped together with common geographic characteristics based on their geographic location, their coverage by MTEs, and the proportion of each country's population living in these ecosystems.
A geodatabase was created in a Geographic Information System (GIS) environment including the collected data and the spatial layer of the world countries.A thematic map was created to illustrate the latest available information on the level of MTD adherence, by country.For countries where the latest available year of dietary assessment was the same in more than one study, the level of MTD adherence from the study with the largest sample was mapped.
ArcGIS version 10.2 software (ESRI Inc., Redlands, California, USA) was used for the geographic analysis procedures and mapping.

Human Development Index and socioeconomic factors
To further evaluate the level of MTD adherence with country-specific socio-economic characteristics, the Human Development Index (HDI) was used (UNDP 2022).The HDI has been continuously released from 1990 till the present and is included in the annual Human Development Report of the United Nations Development Programme (UNDP).The HDI captures three key dimensions of human development: a long and healthy life, knowledge, and a decent standard of living (UNDP 2022).For countries with a study dietary assessment year prior to 1990, which is the first year HDI was released, HDI and dimensional indexes of 1990 were considered.For countries with more than one study from the same or adjacent year of the dietary assessment, the weighted mean adherence score was calculated based on the sample size of the studies.The dimension indexes were calculated based on UNDP Human Development Reports methodology (UNDP 2022).

MTD adherence
The mean MTD adherence score, as calculated from the entire database, was 49.3 out of 100 (SD 9.3) (with 100 indicating complete adherence to the study-specific defined MTD); the range of the level of adherence varied between 41.0 and 64.0 out of 100.Highest level of adherence (i.e.64.0), was observed in a study from Africa (Morocco), followed by 37 studies conducted in European region (51.3),Australia (i.e.48.8, n = 4), Asia (44.3, n = 7), North America (43.4,n = 8), and the one study from South America (Chile) (41.0).Sample-weighted regression analysis showed that a slight, but not significant decrease in the level of MTD adherence was observed during 1980 and 2020 (b-coefficient: −0.11, 95%CI: −0.40, 0.18, p = 0.43) (Figure 2).However, when the analysis was focused on the more recent years (i.e. after 2010), this decrease was highly significant (b-coefficient: −1.40, 95%CI: −2.55, −0.55, p = 0.019).Furthermore, MTD adherence score was not associated with the studies' sample size (p = 0.97), the sex-ratio of the studies (p = 0.75) and the type of diet score used to assess adherence (p = 0.30).

Geographical analysis of MTD adherence
The geographic analysis revealed the spatial heterogeneity of MTD adherence across three groups of countries: (a) the countries that either fully or partially cover the MTEs and where a large proportion of the population lives, (b) the other European countries, and (c) the countries of the other regions (Figure 3).Most MTE countries (group A), including Mediterranean countries and Australia, had higher levels of MTD adherence, except for Cyprus and Israel, which were among the lowest.Lower adherence was found in most other European countries (group B) and countries in other regions (group C) (Figure 4).

Socioeconomic status and MTD adherence
Most of the countries included in the present analysis have a high HDI (Supplementary table 3).Based on the HDI time-series data, it was found that the increase in HDI and especially in income index was accompanied by an increase in adherence (Supplementary table 3).The decline in the income index in Greece and Spain during the initial phase of the socioeconomic crisis in the 2010s was accompanied by a decline in adherence levels.In addition, increases in the education index seemed to be associated with higher adherence (i.e. in Italy, Iran, and the UK), while decreases in adherence seemed to be associated with decreases in the education index (i.e.Lithuania).

Main findings
In this systematic review of 57 observational studies with 1,125,560 adult participants around the world, it was revealed that MTD adherence is moderate (i.e. on average the level of adherence did not exceed 50% of the full compliance to this dietary pattern) with a significant decline observed in the last decade.European countries, mainly driven by countries in the Mediterranean area, showed higher adherence than other regions of the world, nevertheless, the level of adherence was still moderate.Despite the inherent methodological limitations of this analysis, in which a variety of divergent observational studies were combined and analysed together, cultural and environmental particularities, trends in agriculture economies and food policies, as well as food availability, were revealed as reasons that could explain the poor adherence to this, widely acknowledged and well appreciated, healthy dietary pattern, the Mediterranean dietary pattern (Tourlouki et al. 2011;Lăcătușu et al. 2019;Castaldi et al. 2022;Baudry et al. 2023).

Factors influencing MTD adherence
The decline in the MTD adherence even in the Mediterranean region that we observed in this systematic review, could be attributed to a wide "nutrition transition phenomenon" that has impacted the whole world (Popkin and Ng 2022).Due to urbanisation, as well as the abundant food supply of high-calorie, less nutritious, ultra-processed food products (Cooksey-Stowers et al. 2017), along with various socio-economic reasons, such as food insecurity (Morales and Berkowitz 2016) and immigration (Lee et al. 2022), dietary patterns have become more processed (Harriden et al. 2022) and it has been observed that populations tend to move away from traditional healthy diets such as the Mediterranean, the Japanese or other local diets (Vilarnau et al. 2019;Yoneda and Kobuke 2020).

Mediterranean diet as a part of a healthy lifestyle pattern globally
The Mediterranean diet is actually closer to a lifestyle pattern, rather than just a diet that only includes what people eat (Yannakoulia et al. 2015).The use of the word "diet" is more appropriate in its original sense in Ancient Greek, where "to diet" means to follow a particular way of eating and living.In the most recent Mediterranean Diet pyramids, lifestyle habits such as physical activity and eating conditions, as well as environmental characteristics of foods of this dietary pattern are also included (Bach-Faig et al. 2011;Serra-Majem et al. 2020).The Mediterranean lifestyle is a physically active régime, observed mainly in rural areas of the region, that is accompanied by low-stress levels and activities that are known to reduce psychological stress (UNESCO 2010, Georgousopoulou et al. 2017, Trichopoulou 2021, Sotos-Prieto et al. 2022;Yannakoulia et al. 2015) This lifestyle includes habits such as adequate sleep and having daytime naps, often after the midday meal (called "siesta" in Spanish), engaging in social or community events (e.g.going out with friends) which may usually include eating and/or celebrating festivities with family and friends around the table.Moreover, contrary to the current abundance and increased consumption of highly-processed pre-prepared ready-to-eat meals (Popkin and Ng 2022), dietary habits in a Mediterranean lifestyle are characterised by moderation, frugality and might be time-consuming; preparing and cooking food encompasses participating in the handling of foods, sometimes even in all steps from production to table (e.g.making cheese as a way to preserve milk), while eating occurs under specific conditions, such as sharing lengthy meals at the table, together with companions, with pleasure and without stress or distractions (Moro 2016).Finally, it should be mentioned that in the Mediterranean way of living, the role of women is of importance, not only in terms of cooking and feeding family members, but also regarding the safe-keeping and transmission of traditional cooking techniques, rituals, and festivities (UNESCO 2010).
However, can an MTD be a global example of a healthy and sustainable diet?In 2019, the EAT-Lancet Commission nutritionally defined a sustainable diet, while acknowledging the Mediterranean diet as an ecologically sustainable and environmentally friendly diet (Willett et al. 2019).In addition to this, the Mediterranean diet has been used as a case study in a consensus proposal for pinpointing nutritional indicators to assess the sustainability of a healthy diet (Donini et al. 2016), and in another study four components of the Mediterranean diet were researched i.e. health benefits, low environmental impact, biodiversity and socio-cultural food values and positive local economic returns (the Med-Diet 4.0 framework), highlighting that the Mediterranean diet can be used as a sustainable diet with appropriate country-specific and culturally appropriate variations (Dernini et al. 2017).Recently, Serra-Majem et al. (2020), updated the Mediterranean Pyramid to also illustrate the environmental aspects of food products, along with the components of "biodiversity and seasonality", "traditional, local and eco-friendly products" and "culinary activities" in the Mediterranean Basin, but this version of the Mediterranean Pyramid could also serve as a guide for other regions.
Outside the Mediterranean region, the adoption of the Mediterranean diet is recommended for the American population in the most recent (2020-2025) Dietary Guidelines for Americans (2022), while a study showed that in the US, the Mediterranean along with vegan and climatarian diets had the lowest environmental impacts (i.e.low carbon footprint) (Dixon et al. 2023).Given that the Mediterranean diet precedes the definition of a sustainable diet, the question may go both ways: is the Mediterranean diet a sustainable diet or rather, are sustainable diets, by definition, Mediterranean type of diets?

Geographical and socioeconomic features regarding MTD adherence
MTD adherence has been found to be related to socioeconomic status (Bonaccio et al. 2016).Exploratory socioeconomic analysis revealed that increasing adherence to MTD coincided with increasing HDI in several countries.The observation that changes in MTD adherence appeared to be related to changes in education and income indices supports previous research indicating that higher levels of education and income are associated with greater MTD adherence (Bonaccio et al. 2012, Cavaliere et al. 2018).The association between education and MTD may be explained by the relationship between education level and individual awareness of healthy dietary patterns (Schröder et al. 2016), while high consumption of low-energy and nutrient-dense foods is more expensive than high consumption of low-energy and energy-dense foods (Bonaccio et al. 2013).In contrast, MTE countries with lower socioeconomic status showed higher MTD adherence, whereas other European countries and countries in other regions with the highest HDI values seemed to have lower adherence.Thus, climatic conditions in MTEs that favour the cultivation and availability of foods with high consumption value in MTD seem to lead to higher adherence.Therefore, it appears that MTD adherence is related to both geographic location and socioeconomic status of the country, and this interaction needs to be studied more thoroughly.Nevertheless, an MTD seems to be widely adopted beyond the Mediterranean area, both in areas with similar climatic conditions (e.g.Australia) and in other geographical regions (e.g.UK).

Challenges in assessing MTD adherence globally
One of the main methodological problems faced in studying the association of MTD with human health is defining the Mediterranean diet.With the social, cultural, and religious variety of populations living in the Mediterranean region, different MTD have been presented throughout the years, making the definition of Mediterranean diet a very difficult task (Noah and Truswell 2001).However, there is consensus that the Mediterranean Diet entails a high intake of olive oil (i.e.favourably extra virgin), vegetables (including leafy green vegetables), fruits, grains, nuts and legumes, a moderate intake of fish, poultry, dairy products and a low intake of eggs, sweets, red and processed meat, as well as moderate consumption of alcoholic drinks, mainly in the form of wine (Willett et al. 1995;Bach-Faig et al. 2011;Davis et al. 2015;Serra-Majem et al. 2020).
Based on UNESCO, the Mediterranean diet includes foods and food systems that respect and conserve the specific territory of a region and its biodiversity while staying in line with traditional cultural activities of food production such as farming or fishing.Such food systems, although scarce, still exist to this day, in some Mediterranean communities such as Koroni (Greece), Soria (Spain), Cilento (Italy) and Chefchaouen (Morocco), and are prime examples of sustainable food systems.In fact, MTD and sustainable diets share similar characteristics and components, such as beneficial health effects, bio-diversity, environmental and cultural traits, eco-friendly, local and seasonal foods, cultural heritage, food security and accessibility (Johnston et al. 2014).Some of these characteristics of a traditional Mediterranean diet (e.g.locality, traditionality) may only apply to the Mediterranean region, but with some modifications (e.g.MTD), could also be adopted by other regions.
Moreover, adherence to a dietary pattern is usually measured through diet scores, such as the various MDS; these scores are a-priori-defined tools to evaluate adherence to a specific dietary pattern and/or guidelines, in this case, MTD (Hu 2002).There is a plethora of MDS.Most scores use approximately 9-15 questions (i.e.items) and the potential answers may be binary (i.e.yes/no) or multinomial (e.g.servings of times per day, week, month etc.) (Bountziouka et al. 2012b).Albeit their practicality, all diet scores are subject to methodological issues.The development of dietary pattern scores is based on mathematical equations; robust as these statistical methodologies may be, dietary scores are dependent on the variation of the retrieved dietary information (e.g.within-subject variation) (Bountziouka et al. 2011b).Additionally, the most prominent error in nutritional epidemiology is the reporting error, which may lead to over-or under-estimation of the true dietary intake depending on individual characteristics (e.g.socio-demographic, physiological, psychological, lifestyle characteristics) (Rumpler et al. 2008, Bountziouka et al. 2012a).Moreover, the length and depth of the tool might affect the results; it has been observed that the number of items of a diet score affects the accuracy of the score, although, in another study, the number of food items or consumption responses of nutrition assessment tools (i.e.diet score or food frequency questionnaire-FFQ) did not affect the repeatability of the tool (Kourlaba and Panagiotakos 2009, 2010, Bountziouka, et al. 2012b).Furthermore, in the study of Kourlaba and Panagiotakos (2010), it was concluded that the indirect calculation of a score (MedDietScore via FFQ) might lead to the overestimation of diet quality (i.e.MTD adherence).As noted, many similar, but also different diet scores, with different food components and score ranges, have been proposed and used to assess adherence to MTD; this fact might have affected the evaluation of the level of adherence to this dietary pattern.Scientists have suggested that shorter score ranges lead to less accurate diet scores in terms of health risk assessment (Krebs-Smith et al. 1995, Food and Agriculture Organization of the United Nations 2008, Kourlaba and Panagiotakos 2009); this characteristic may also lead to an overestimation or underestimation of the true level of adherence.In the present analysis, no association was observed between the diet scores' range and the level of adherence observed, answering the previous research question.Taken together, there is an urgent need to establish a global, widely accepted MDS, based on a unanimous definition of what the Mediterranean diet entails, mainly in terms of foods/food groups included.It would also be interesting for a new comprehensive Mediterranean lifestyle score to be established, which could include not only foods or food groups, but lifestyle habits associated with the Mediterranean lifestyle (e.g.physical activity), as well as the components that make the Mediterranean diet a prime example of a sustainable dietary pattern (e.g.seasonality).

Strengths and limitations
To the best of our knowledge, this is the first study to systematically review the level of MTD adherence, worldwide.Following the PRISMA guidelines, as well as the references of included individual studies, systematic reviews, and meta-analyses, we tried to minimise the possibility of losing studies.Moreover, to strengthen our analysis and reduce bias, we excluded studies with older adults, as it has been observed that during senescence, changes in dietary habits occur due to multiple reasons (e.g.dysphagia, swallowing problems, economic reasons), including fewer diverse dietary choices which negatively affect overall diet quality (Steenhuis et al. 2011, Nawaz andTulunay-Ugur 2018;Tsou et al. 2022).
However, there are several limitations that should be acknowledged.We searched the two major databases for biomedical research (i.e.MEDLINE, and Scopus, as search engines that include studies from a wider spectrum of sciences), but some papers might have been missed if they had been published in local or regional journals that were not archived in these databases.No common diet adherence score was used in individual studies retrieved, making the synthesis of the results challenging.Some studies did not report actual mean values of the diet scores, but percentiles, and, thus, the calculation of score mean values was made under some assumptions.In most studies, the diet scores were indirectly derived through FFQ (as it is common in large prospective epidemiological studies), whereas in other studies, MTD adherence was evaluated directly through a special score.Although the sample consists exclusively of studies with >1000 participants and the analyses were weighted according to each study's sample size, it is important to acknowledge that these studies are not representative of the total population in each country but might be representative of the differences between geographical regions.

Conclusions
Based on the US News & World Report (2023) the Mediterranean diet was ranked as the best diet overall out of 24 diets and has kept this title for the past six consecutive years.Despite this popularity, the numerous beneficial health effects that accompany the Mediterranean diet, and its sustainable nature, the findings of this systematic review confirmed previous individual studies' reports, showing that the current level of adherence to MTD is moderate and has globally declined the last decade, even in the place of its origin (i.e. the Mediterranean region).It has become evident that the dissemination of knowledge concerning the health benefits of MTD is a mission that has been successfully accomplished, while the current duty of health professionals should be to aid individuals to actually follow this beneficial dietary pattern.However, achieving this goal is a rather challenging task.Changes must be made on all levels, through organised public health actions, in the individual, interpersonal, community, social and political environments.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 1 .
Figure 1. the flow diagram of study identification, screening, and inclusion process (adapted from Page et al. 2021).

Figure 2 .
Figure 2. time-trend of the mean adherence score to a Mediterranean type of diet according to the 57 observational studies with 1,125,560 included participants in the present systematic review.dotted line presents smoothed splines of the actual adherence score to specific time points.

Figure 3 .
Figure 3. level of adherence to a Mediterranean type of diet, by country and group of countries, based on the latest available year of dietary assessment.

Figure 4 .
Figure 4. level of adherence to an Mtd by year of dietary assessment (aggregated to time periods), country and group of countries.for countries with data from more than one year of dietary assessment in the same time period, the weighted mean adherence to an Mtd was calculated based on the sample size of the studies.abbreviations: cl, chile, cn, china, el, Greece, eS, Spain, fr, france, Hr, croatia, Ir, Iran, It, Italy, Kr, Korea, Ma, Morocco, Mtd, Mediterranean type diet, Mte, Mediterranean-type ecosystems, nl, netherlands, Se, Sweden, uSa, united States of america.