Introduction
Since the first results of the landmark Seven Countries study, it became obvious that the Mediterranean diet (MDi) has a powerful preventative effect on cardiovascular disease(Reference Keys1,Reference Keys, Mienotti, Karvonen, Aravanis, Blackburn and Buzina2) . In the following decades, several cross-sectional, longitudinal, interventional, case–control studies and systematic reviews have demonstrated the positive effect of MDi on the prevention of various diseases(Reference Zupo, Castellana, Piscitelli, Crupi, Desantis and Greco3). The most prominent benefits were reduction in mortality(Reference Trichopoulou, Bamia and Trichopoulos4,Reference Trichopoulou, Costacou, Bamia and Trichopoulos5) and prevention of cardiovascular disease(Reference Yang, Farioli, Korre and Kales6,Reference Estruch, Ros, Salas-Salvadó, Covas, Corella and Arós7) , metabolic syndrome(Reference Estruch, Ros, Salas-Salvadó, Covas, Corella and Arós7,Reference Kesse-Guyot, Ahluwalia, Lassale, Hercberg, Fezeu and Lairon8) , obesity(Reference Shai, Schwarzfuchs, Henkin, Shahar, Witkow and Greenberg9–Reference Romaguera, Norat, Mouw, May, Bamia and Slimani12) type 2 diabetes(Reference Estruch, Ros, Salas-Salvadó, Covas, Corella and Arós7,Reference Martínez-González, de la Fuente-Arrillaga, Nunez-Cordoba, Basterra-Gortari, Beunza and Vazquez13,Reference Koloverou, Esposito, Giugliano and Panagiotakos14) , and breast and upper aero-digestive tract (UADT) cancer(Reference La Vecchia15–Reference Buckland, Travier, Cottet, González, Luján-Barroso and Agudo18). Also, a positive association was found between better adherence to MDi and lower incidence of cognitive problems(Reference Coelho-Júnior, Trichopoulou and Panza19–Reference Psaltopoulou, Sergentanis, Panagiotakos, Sergentanis, Kosti and Scarmeas22), sleep apnoea(Reference Georgoulis, Yiannakouris, Tenta, Fragopoulou, Kechribari and Lamprou23), renal diseases(Reference Bowden, Gray, Swanepoel and Wright24) and some hormone-related cancers such as endometrial cancer(Reference Giacosa, Barale, Bavaresco, Gatenby, Gerbi and Janssens25).
Although MDi is not the only ‘healthy diet’, other dietary patterns failed to demonstrate such a strong beneficial association with disease prevention. An umbrella review comprising eighty different meta-analyses examined a wide range of popular diets, including low carbohydrate, high protein, palaeolithic, low glycaemic index, intermittent energy restriction, Nordic, vegetarian and Dietary Approaches to Stop Hypertension (DASH), and concluded that MDi has shown the most consistent association with cardiometabolic risk factors, without evidence of adverse health effects(Reference Dinu, Pagliai, Angelino, Rosi, Dall’Asta and Bresciani26). Also, MDi has been recognised as a healthy dietary pattern in the Dietary Guidelines for Americans 2015–2020 from the US Department of Agriculture(27). Due to all the aforementioned positive effects on general health, key organisations have recognised MDi as a prominent value for human health and supported its international promotion. In 2013, the United Nations Educational, Scientific and Cultural Organization (UNESCO) proclaimed the MDi as an Intangible Cultural Heritage of Humanity(28).
The role of MDi in the prevention of non-communicable diseases (NCD) and cognitive impairment gains more significance considering the growing population of older adults worldwide and the relevant increased morbidity(Reference Rudnicka, Napierała, Podfigurna, Męczekalski, Smolarczyk and Grymowicz29,Reference Mazza, Ferro, Pujia, Mare, Maurotti, Montalcini and Pujia30) . Although there are established genetic and environmental contributors to NCD risk, modifiable lifestyle-related factors, such as nutrition and physical activity, play an immense role in individual NCD development and prevention(Reference Yu, Rimm, Qi, Rexrode, Albert and Sun31,Reference Koene, Prizment, Blaes and Konety32) .
Since its introduction into the scientific world, MDi has changed in some of its characteristics, prevalence, economic and ecological importance. The base of the MDi still consists of olive oil and plants (vegetables, fruits, cereals, nuts) that provide key nutrients and fibres(Reference Trichopoulou, Bamia and Trichopoulos4,Reference Slavin33–Reference Khoo, Prasad, Kong, Jiang and Ismail35) and should be consumed in high frequencies. Foods eaten in moderate amounts are dairy products, eggs, legumes, white meat and fish (seafood) that are a good source of proteins. Consumption of red meat and processed meats should be in small quantities and limited frequency. Olive oil, a monounsaturated fat, is the principal source of dietary lipids because of its high nutritional quality. A moderate consumption of wine is recommended during meals. Sugar, candies, pastries and sweetened soft drinks are avoided and consumed only on special occasions(Reference Bach-Faig, Berry, Lairon, Reguant, Trichopoulou and Dernini36). Modifications that are introduced in the MDi when applied in non-Mediterranean countries are related mainly to the source of fat and the varieties of local fruit and vegetables(Reference Martínez-González, Hershey, Zazpe and Trichopoulou37). Seed oil (sunflower, rapeseed, soya or other seeds), a polyunsaturated fat, is used in many non-Mediterranean countries. However, these oils do not have the antioxidant capacity of olive oil even though they are better than lard or butter(Reference Martínez-González, Hershey, Zazpe and Trichopoulou37).
In accordance with the strong positive correlation between MDi and general health, it is expected that a positive impact on oral health also exists. However, the status of oral tissues and functions may also affect adherence to MDi. A cross sectional study in older Greeks has shown that increased masticatory performance was independently associated with better adherence to the MDi(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38). The stomatognathic system is the initial part of the digestive tract preparing the bolus for swallowing, and its health status may affect food choices(Reference Pedersen, Bardow, Jensen and Nauntofte39–Reference Kossioni, Hajto-Bryk, Maggi, McKenna, Petrovic and Roller-Wirnsberger41), including the components of MDi. Functional limitations such as tooth loss, pain due to untreated caries, or tooth mobility due to severe periodontal disease may affect masticatory performance(Reference Bousiou, Konstantopoulou, Polychronopoulou, Halazonetis, Schimmel and Kossioni42,Reference Kossioni43) and discourage the consumption of specific food types such as fruits, seeds and raw vegetables(Reference Kossioni43–Reference Savoca, Arcury, Leng, Chen, Bell and Anderson45) that are important components of the MDi. Bearing in mind the data scarcity on the association between adherence to MDi and oral health, and especially the lack of any relevant systematic reviews, it was necessary to summarise the existing knowledge in this field and, on the basis of the results, direct further research.
Therefore, the aim of this study was to investigate the association between adherence to MDi and oral health in adult populations.
Methods
Protocol and registration
The present scoping review adhered to Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) guidelines(Reference Tricco, Lillie, Zarin, O’Brien, Colquhoun and Levac46). The search process is also reported following the PRISMA-ScR guidelines of 2018. The study protocol is registered in Open Science Framework (https://osf.io/vxbnh/).
Research question and eligibility criteria
The Population, Exposure, Outcomes (PEO) strategy was used to formulate the principal research questions: (1) Does better oral health (E) enable adult individuals (P) to better adhere to MDi (O)? and (2) Does better adherence to MDi (E) enable adult individuals (P) to have better oral health (O)?
In the scoping review, specific eligibility criteria were employed to ensure the relevance and consistency of included studies. Specifically, only research focusing on adult individuals was considered eligible, with studies involving children being excluded. Furthermore, only research exploring the MDi as a whole dietary pattern was included, while studies examining variations, such as ‘MDi style’ diets, ‘alternative MDi’, ‘healthy diet with Mediterranean components’, etc. were excluded. In addition, studies investigating oral/dental health in relation to the MDi were included. Notably, review papers and meeting abstracts were excluded from consideration to maintain the focus on primary research studies.
Information sources and search strategy
The content of three databases, Clarivate Analytics’ Web of Science (including Web of Science Core Collection – WoS, Korean Journal Database – KCI, SciELO Citation Index – SCIELO, Preprint Citation Index, Grants Index, ProQuest™ Dissertations & Theses Citation Index), Scopus and PubMed (including MEDLINE), was searched until 6 January 2024, without language, date or any other restrictions. The complete search strategy, including controlled vocabulary (Medical Subject Headings – MeSH, https://www.ncbi.nlm.nih.gov/mesh), free keywords, search operators (AND, OR, NEAR, W) and truncation (*, $) used according to the searched database, is detailed in Supplementary Table 1. In the pursuit of relevant unpublished manuscripts, conference papers, doctoral dissertations and other grey literature, additional searches were conducted in resources such as OpenGrey (http://www.opengrey.eu) and Google Scholar™ (first 100 returns), as well as other available digital repositories such as the Networked Digital Library of Theses and Dissertations (http://www.ndltd.org), Open Access Theses and Dissertations (https://oatd.org), DART-Europe E-theses Portal – DEEP (https://www.dart-europe.org/basic-search.php), and Opening access to UK theses – EThOS (https://ethos.bl.uk). Furthermore, snowballing and screening the reference lists of included studies and relevant previously published reviews were performed using citation indexes (Web of Science and Scopus). During the drafting phase of the paper in March 2024, a complete search of the primary databases was repeated, which led to the identification of three new relevant trials that have been included in this systematic review. The search results were imported into the Rayyan environment(Reference Ouzzani, Hammady, Fedorowicz and Elmagarmid47) for duplicate removal, initially performed using Rayyan’s duplicate identification strategy, and then manually.
Study selection
The screening and study selection process comprised two phases, and two independent reviewers (A.P. and J.J.) assessed the titles and abstracts of the retrieved records. Articles not meeting the inclusion criteria were excluded, and the full texts of the initially selected papers were accessed to determine their eligibility for inclusion. During the second stage of study selection, two independent reviewers (A.P. and A.M.L.) critically evaluated full texts of studies considered possibly relevant during the initial screening stage. Lists of relevant studies were compared, and any disagreements regarding the eligibility of specific studies were resolved through discussion with a third reviewer (A.K.).
Data extraction and analysis
The two reviewers (A.P. and E.P.) independently performed data extraction using a pre-established form (Microsoft Excel™, Microsoft Corporation, USA). Information from each study included in the final review was collected, encompassing the first author’s name, journal, year of publication, study design type, setting, study population characteristics, implemented MDi questionnaire, oral health factor investigated and key results. In case of any disagreements or uncertainties, a third reviewer (A.K.) was consulted to resolve them through discussion.
To comprehensively assess all aspects and identify variations in study characteristics and outcomes, the collected data were consolidated into evidence tables. These tables served as a descriptive summary, enabling the revelation of similarities and differences between studies, as well as determining their suitability for further synthesis or comparison methods. The adopted approach for data synthesis was the narrative synthesis method. The data were descriptively synthesised on the basis of whether they exhibited significant, positive, negative or null effects, as reported by the authors of included studies.
Results
Study selection
The sequence and details of the literature search are depicted in Fig. 1, the PRISMA flowchart. Initially, 1127 studies were identified through the primary search, and 409 were excluded due to duplicates. Following the screening titles and abstracts, according to exclusion criteria, an additional 695 studies were excluded. In addition, by reviewing the references of relevant papers, one more study was found. Ultimately, twenty-four studies remained and were subjected to full-text evaluation. After carefully reviewing the full texts, twenty studies were included in the current scoping review. The list of excluded studies, along with the reasons or their exclusion, is provided in Supplementary Table 2. Thereafter, the studies were divided into two groups depending on the direction of causality between the MDi and oral health as indicated by the aims and content. The first group consisted of studies that examined the impact of oral health on adherence to the MDi (Table 1), and the second group consisted of studies that aimed to show how MDi adherence affects oral health (Table 2).
Figure 1. Prisma 2020 flow diagram.
From: Page MJ, McKenzie JE, Bossuyt PM, Boutron I, Hoffmann TC, Mulrow CD, et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ 2021;372:n71. doi: 10.1136/bmj.n71. For more information, visit: http://www.prisma-statement.org/
Table 1. Studies retrieved on the primary effect of oral health indicators on the level of adherence to MDi
Table 2. Studies retrieved on the primary effect of adherence to MDi on oral health indicators
Does better oral health enable individuals to better adhere to MDi?
The first group of studies consisted of only three research papers that primarily investigated oral health factors affecting adherence to MDi (Table 1). These studies varied in design, participants’ characteristics and methodology. One was carried out in a Mediterranean country (Greece)(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38) and the other two(Reference Logan, McEvoy, McKenna, Kee, Linden and Woodside48,Reference Lohse and Masters49) in non-Mediterranean ones (Northern Ireland and the United States). The MDi scales varied among studies. Bousiou et al. used the Mediterranean diet Index (MDI_BNC4H), Lohse et al.(Reference Lohse and Masters49) the 14-item Mediterranean Diet Questionnaire and Logan et al.(Reference Logan, McEvoy, McKenna, Kee, Linden and Woodside48) the Mediterranean Diet Score (MDS). Two investigations included older participants(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38,Reference Logan, McEvoy, McKenna, Kee, Linden and Woodside48) and one wider age groups(Reference Lohse and Masters49). Two studies were cross-sectional(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38,Reference Lohse and Masters49) and one prospective(Reference Logan, McEvoy, McKenna, Kee, Linden and Woodside48). Two studies included both interviews and oral examination(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38,Reference Logan, McEvoy, McKenna, Kee, Linden and Woodside48) and one(Reference Lohse and Masters49) only an online interview.
All studies revealed significant associations between various oral health parameters and adherence to MDi with large variation in the investigated factors and the outcomes. The number of teeth that was investigated in all studies revealed inconclusive findings. Logan et al. (Reference Logan, McEvoy, McKenna, Kee, Linden and Woodside48) showed that having more than twenty-one teeth resulted in overall better future adherence to MDi and higher intake of fruit, vegetables and nuts compared with other dental status categories (with fewer teeth and with dentures). However, Bousiou et al. (Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38) and Lohse et al.(Reference Lohse and Masters49) did not find any statistically significant associations between number of teeth and adherence to MDi scales. Bousiou et al.(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38) in the multivariate statistical analysis found that objectively recorded masticatory performance was the most important oral factor that was marginally significantly associated with adherence to MDi. Also, Bousiou et al.(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38) did not find any significant association between adherence to MDI_BNC4H and rehabilitation with removable dentures. Logan et al.(Reference Logan, McEvoy, McKenna, Kee, Linden and Woodside48) found that those with twenty-one to twenty-eight teeth but without dentures had significantly higher MSD scores compared with all other dental status groups with fewer than twenty teeth even when removable dentures were present. Some indicators of periodontal disease (PD) that were examined did not reveal any significant association with better adherence to MDi. Tooth mobility objectively recorded(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38) and loose teeth and bleeding gums subjectively reported(Reference Lohse and Masters49) did not reveal any significant associations with the related MDi scales’ scores.
Regarding oral hygiene behaviour and dental check-up regularity, the data were also inconclusive. Lohse et al.(Reference Lohse and Masters49) found that participants who followed MDi were more likely to have visited a dentist in the past year than those who did not. However, Bousiou et al.(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38) did not find any significant associations between adherence to MDi and dental visitation habits or frequency of oral hygiene.
Does better adherence to MDi enable individuals to have better oral health?
Seventeen studies were identified with the primary aim to investigate the effect of the level of adherence to MDi on various oral parameters (Table 2). Eight studies(Reference Radić, Vučković, Gelemanović, Roguljić, Orešković and Kovačević50–Reference Sáenz-Ravello, Matamala, Cisternas, Gamonal, Hernández and Santos57) investigated the association of adherence to MDi with various PD indicators, three(Reference Laiola, De Filippis, Vitaglione and Ercolini58–Reference Louro, Simões, Penetra, Carreira, Castelo and Luis60) with saliva indicators, one with Sjögren syndrome(Reference Machowicz, Hall, de Pablo, Rauz, Richards and Higham61) and five(Reference Filomeno, Bosetti, Garavello, Levi, Galeone, Negri and La Vecchia62–Reference Saraiya, Bradshaw, Meyer, Lund, Slade and Olshan65) with UADT cancers. Of those studies, nine(Reference Radić, Vučković, Gelemanović, Roguljić, Orešković and Kovačević50,Reference Marruganti, Traversi, Gaeta, Ferrari Cagidiaco, Parrini, Discepoli and Grandini53,Reference Iwasaki, Ennibi, Bouziane, Erraji, Lakhdar and Rhissassi55,Reference Laiola, De Filippis, Vitaglione and Ercolini58,Reference Shaalan, Lee, Feart, Garcia-Esquinas, Gomez-Cabrero and Lopez-Garcia59,Reference Filomeno, Bosetti, Garavello, Levi, Galeone, Negri and La Vecchia62–Reference Saraiya, Bradshaw, Meyer, Lund, Slade and Olshan65) were conducted in Mediterranean countries. The MDi scales applied were variant and included the Mediterranean Diet Adherence Screener (MEDAS)(Reference Wu, He, Chen, Yu, Wu and Yang52,Reference Marruganti, Traversi, Gaeta, Ferrari Cagidiaco, Parrini, Discepoli and Grandini53,Reference Iwasaki, Ennibi, Bouziane, Erraji, Lakhdar and Rhissassi55,Reference Bartha, Exner, Basrai, Bischoff, Schweikert and Adolph56,Reference Shaalan, Lee, Feart, Garcia-Esquinas, Gomez-Cabrero and Lopez-Garcia59,Reference Louro, Simões, Penetra, Carreira, Castelo and Luis60) , the Italian Mediterranean index score (ItMedIndex)(Reference Bartha, Exner, Basrai, Bischoff, Schweikert and Adolph56), the Mediterranean Diet Score (MDS)(Reference Iwasaki, Ennibi, Bouziane, Erraji, Lakhdar and Rhissassi55,Reference Machowicz, Hall, de Pablo, Rauz, Richards and Higham61–Reference Saraiya, Bradshaw, Meyer, Lund, Slade and Olshan65) , the aMed(Reference Altun, Walther, Borof, Petersen, Lieske and Kasapoudis51), the Mediterranean Diet Index(Reference Sáenz-Ravello, Matamala, Cisternas, Gamonal, Hernández and Santos57), the Mediterranean Diet Serving Score (MDSS)(Reference Lohse and Masters49), the Mediterranean Dietary Pattern Adherence Index (MDP)(Reference Filomeno, Bosetti, Garavello, Levi, Galeone, Negri and La Vecchia62) and the Mediterranean Adequacy Index (MAI)(Reference Filomeno, Bosetti, Garavello, Levi, Galeone, Negri and La Vecchia62). The study participants were adults of various age groups.
The oral factor most frequently investigated was PD using various indicators such as bleeding on probing (BOP), probing pocket depth (PPD), gingival recession (GR), clinical attachment level (CAL), periodontal inflamed surface area (PISA), self-reported gingival health status, etc. All studies reported a beneficial effect of better adherence to MDi on periodontal health.
Five cross-sectional studies concluded that better adherence to MDi is associated with less frequent PD occurrence(Reference Radić, Vučković, Gelemanović, Roguljić, Orešković and Kovačević50–Reference Marruganti, Traversi, Gaeta, Ferrari Cagidiaco, Parrini, Discepoli and Grandini53,Reference Sáenz-Ravello, Matamala, Cisternas, Gamonal, Hernández and Santos57) . Altun et al.(Reference Altun, Walther, Borof, Petersen, Lieske and Kasapoudis51) revealed a significant association between higher adherence to the MDi and lower odds of PD, specifically plaque index and bleeding on probing. Moreover, the odds of stage III/IV periodontitis were found to be six times higher in subjects with low MDi adherence compared with those with high adherence(Reference Marruganti, Traversi, Gaeta, Ferrari Cagidiaco, Parrini, Discepoli and Grandini53). For each point of increase at the Mediterranean Diet Index scale, there were 18% greater odds of self-reporting very good or good gingival health among Chilean adults(Reference Sáenz-Ravello, Matamala, Cisternas, Gamonal, Hernández and Santos57). Radic et al.(Reference Radić, Vučković, Gelemanović, Roguljić, Orešković and Kovačević50) showed that better adherence to MDi was associated with better periodontal status in kidney transplant recipients. Similarly, Wu et al.(Reference Wu, He, Chen, Yu, Wu and Yang52) found that better MDi adherence was negatively associated with important PD parameters such as pocket depth and clinical attachment loss(Reference Wu, He, Chen, Yu, Wu and Yang52).
In an interventional study, gingival inflammatory parameters (bleeding on probing, gingival index and periodontal inflamed surface area) were significantly reduced by adherence to a 6-week MDi dietary programme even when the plaque index remained constant(Reference Bartha, Exner, Schweikert, Woelber, Vach and Meyer54). The mechanism of this association was further explained by the effect of MDi on reducing serum omega-6 levels, through a mechanism affecting gingival inflammatory parameters(Reference Sáenz-Ravello, Matamala, Cisternas, Gamonal, Hernández and Santos57). Possible mechanisms of the positive effect of the MDi on PD lie in the possibility of changing the composition of saliva and the bacterial biofilm, hence contributing to the host defence immunomodulation(Reference Laiola, De Filippis, Vitaglione and Ercolini58). Laiola et al.(Reference Laiola, De Filippis, Vitaglione and Ercolini58) investigated the changes in the salivary microbial composition in overweight and obese subjects after following individually tailored MDi intervention for 8 weeks and did not find any changes in the overall microbiota composition but a significant decrease in the red bacterial complex that may be implicated in PD. Increased MDi adherence was found to lead to a significant decrease in the salivary concentration of periodontopathogenic microorganisms such as Porphyromonas gingivalis, Prevotella intermedia and Treponema denticola (Reference Laiola, De Filippis, Vitaglione and Ercolini58) together with the increase of Streptococcus cristatus levels which has ability to act antagonistically to P. gingivalis. Moreover, P. intermedia and T. denticola are members of the ‘red bacterial complex’, which appear later in biofilm development and represent progressive PD(Reference Laiola, De Filippis, Vitaglione and Ercolini58).
Shaalan et al.(Reference Shaalan, Lee, Feart, Garcia-Esquinas, Gomez-Cabrero and Lopez-Garcia59) in their study in patients with diabetes type II and obesity and controls found that saliva microbiota was associated with the level of adherence to MDi but not to a significant level. However, they showed specific changes in patients associated with the frequency of consumption of individual MDi items (sugar snacks, fish/shellfish and nuts). Iwasaki et al.(Reference Iwasaki, Ennibi, Bouziane, Erraji, Lakhdar and Rhissassi55) reported a negative but not statistically significant association between better adherence to MDi and periodontitis among university students in Morocco, but their sample was very young (mean age 20·2 years) and only 6·6% had periodontitis. Nonetheless, olive oil consumption, which can be considered the fundamental component of the MDi, showed a significant inverse association with periodontitis(Reference Iwasaki, Ennibi, Bouziane, Erraji, Lakhdar and Rhissassi55).
Four case–control studies(Reference Filomeno, Bosetti, Garavello, Levi, Galeone, Negri and La Vecchia62–Reference Saraiya, Bradshaw, Meyer, Lund, Slade and Olshan65) and one cross-sectional study(Reference Saraiya, Bradshaw, Meyer, Lund, Slade and Olshan65) investigated the association of adherence to MDi with UADT cancer, and all agreed on a strong protective effect of MDi. The decrease in UADT cancer risk was 30% for a two-unit increase and 41% for a three-unit increase in MDS(Reference Samoli, Lagiou, Nikolopoulos, Lagogiannis, Barbouni and Lefantzis64). Besides the cancer incidence, better adherence to MDi prior to cancer diagnosis affected 5-year mortality(Reference Saraiya, Bradshaw, Meyer, Lund, Slade and Olshan65).
One study(Reference Machowicz, Hall, de Pablo, Rauz, Richards and Higham61) investigated the association of MDi with Sjögren syndrome and found that higher adherence to a MDi was associated with a lower likelihood of developing primary Sjögren syndrome.
Discussion
This scoping review revealed twenty studies investigating the association between adherence to MDi and various oral health indicators that were divided into two groups based on the primary direction of this association. The inclusion criteria included only publications that investigated the effect of full MDi scales’ scores and not their individual components because the findings are more reliable, particularly when sample sizes are small(Reference Samoli, Lagiou, Nikolopoulos, Lagogiannis, Barbouni and Lefantzis64). Only three studies investigated the effect of oral factors on adherence to MDi, and safe conclusions cannot be drawn. On the other hand, seventeen studies explored the effect of the level of adherence to MDi on oral health indicators, mainly PD and UADT cancer, showing a systematic negative association between better adherence to MDi and prevalence of disease. It should be noted that the reviewed papers revealed increased inconsistencies in the methodology applied regarding both oral and MDi indicators, posing difficulties in comparisons. Various indexes and scores for quantifying MDi adherence were implemented, using either cutoff values classifying participants as high, moderate or low adherers to MDi or various scoring systems to credit or penalise individuals according to their level of adherence to each MDi component.
The effect of oral factors on food choices and nutrient intake is ambiguous, as found in the present study regarding the effect of oral indicators on adherence to MDi. Some studies conducted in non-Mediterranean countries have shown that older adults with very few teeth (zero to ten) were less likely to choose dark-green or orange vegetables and whole grains and were more likely to consume more calories from solid fat, alcohol and added sugar compared with those with eleven and more teeth(Reference Savoca, Arcury, Leng, Chen, Bell and Anderson45,Reference Zhu and Hollis66) . Furthermore, deficiency of posterior functional units (opposing pair of natural or fixed prosthetic teeth) has been related to avoidance of vegetables and fibres and higher consumption of sugar-rich food(Reference Kossioni and Bellou67,Reference Quandt, Chen, Bell, Savoca, Anderson and Leng68) . Removable denture wearers showed less dietary variety and were often discouraged from eating hard food, mostly fruit and vegetables, compared with dentate older adults(Reference Yoshida, Kikutani, Yoshikawa, Tsuga, Kimura and Akagawa69–Reference Jauhiainen, Männistö, Ylöstalo, Vehkalahti, Nordblad, Turunen and Suominen71). Several studies did not find any significant improvement in food intake after removable dentures provision to edentulous people,(Reference Tada and Miura72) whereas others have shown that those satisfied with their removable dentures received significantly higher amounts of vegetables in contrast to those unsatisfied(Reference Tada and Miura72,Reference Lin, Chen, Lee, Yang and Chou73) . A systematic review of the association of mastication with food and nutrient intake in older people found that studies reporting no associations were performed in developed countries where food is often processed to become softer and manageable by individuals with poor dental status(Reference Tada and Miura72).
The effect of local culture on food choice can be revealed from studies performed in Greece, a Mediterranean country, in comparison with other European countries. Greek studies have shown that tooth loss and denture use did not have a significant impact on different food types consumption(Reference Quandt, Chen, Bell, Savoca, Anderson and Leng68,Reference Anastassiadou and Heath74) . Older Greeks, with dental or chewing deficiencies, still consumed basic elements of MDi, such as cereals, fruits and vegetables, either by selecting those with softer texture or by preparing them to be easier to chew. For example, they ate oranges, grapes, tangerines and melons, cut harder fruits such as apples into small pieces, softened bread crust and rusks in water or milk or minced the meat. Regarding vegetables, ‘Ladera’ (stewed vegetables in olive oil, easy to chew) is the common way for their consumption together with boiling them and eating as a salad with the addition of raw olive oil. Meanwhile, English complete denture wearers were more likely to avoid vegetables, grains and other healthy but harder foods and settled for softer, sugary foods(Reference Millwood and Heath75). These studies revealed an increased cultural effect on food preparation pattens enabling or impeding older people with poor dental status and\or chewing difficulties eating basic elements of MDi, such as cereals, raw vegetables and fruits. Bousiou et al.(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38) in an older community-dwelling sample living in Athens, Greece, concluded that the most important oral predictor for better adherence to MDi was good masticatory performance(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38). The number and location of teeth is not the only predictor of good masticatory performance, that is, a complex function affected by several factors, including tooth mobility, neuromuscular coordination, intraoral sensitivity, jaw-closing muscle force, tongue function, saliva quality and quantity, general medical condition or ageing(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38,Reference Bousiou, Konstantopoulou, Polychronopoulou, Halazonetis, Schimmel and Kossioni42) .
Moreover, apart from chewing process, hyposalivation, swallowing problems or oral neuromuscular dysfunction may affect food choices(Reference Kossioni43). Regarding PD, the reviewed studies(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38,Reference Lohse and Masters49) did not reveal any statistically significant association with adherence to MDi, but PD was not comprehensively measured using current validated clinical methodology and further investigation is needed. The above findings reveal the complex nature of food selection patterns, associated with various medical, oral, psychological, religious, socioeconomic and cultural factors(Reference Bousiou, Konstantopoulou, Martimianaki, Peppa, Trichopoulou and Polychronopoulou38,Reference Kossioni43) and the need to further investigate effective strategies to promote healthy dietary patterns such as MDi.
The findings of the opposite direction of the association between adherence to MDi and oral health were clearer and revealed a general tendency for a protective dietary effect. The moderate to strong association between better MDi adherence and lower PD prevalence and severity is not surprising considering current evidence of the effect of certain ‘healthy foods’, integral to MDi, on the aetiopathogenesis of PD. PD is a biofilm-mediated inflammation located in the periodontal tissue in response to the increased presence of bacteria in dental plaque. When an imbalance is created between periodontopathogenic bacteria and the host’s defence mechanisms, inflammation is triggered progressively. This imbalance is subject to the influence of several external factors, including nutrition(Reference Millwood and Heath75,Reference Albandar76) . It has been shown that the consumption of at least five portions a day of vegetables (especially dark-green and yellow ones) such as spinach, broccoli, yellow peppers, cabbage and onions, fruits rich in vitamin C such as black currants, grapefruit, oranges and fruits rich in dietary fibres such as bananas, apples and plums can prevent the development of an aggressive form of PD and, thus, tooth loss(Reference Kinane, Stathopoulou and Papapanou77). Intake of larger quantities of salad, fruit/vegetables, poultry, seafood, water, whole grains and lower intake of red/processed meat showed a positive association with lower clinical attachment loss and, consequently, less chances of a more severe form of PD(Reference Skoczek-Rubińska, Bajerska and Menclewicz78–Reference Salazar, Laniado, Mossavar-Rahmani, Borrell, Qi, Sotres-Alvarez and Morse80). In particular, whole-grain cereals are considered to have a prominent effect in the prevention of PD, mostly in lower interdental clinical attachment loss(Reference Marruganti, Traversi, Gaeta, Ferrari Cagidiaco, Parrini, Discepoli and Grandini53). Whole-grain consumption showed an association with lower systemic inflammatory markers, such as C-reactive protein, and decreased insulin resistance(Reference Nielsen, Trak-Fellermeier, Joshipura and Dye81). Consequently, when insulin sensitivity is preserved, the production of glycation end products, oxidative stress and, accordingly, the release of cytokines is reduced, which has a positive effect on the periodontal tissue(Reference Jenkins, Axelsen, Kendall, Augustin, Vuksan and Smith82). Catechins and ellagic acids found in fruits, vegetables and nuts can inhibit the growth of Prevotella intermedia which has a significant role in the pathogenesis of PD(Reference Esposito, Marfella, Ciotola, Di Palo, Giugliano and Giugliano83). In addition, it is considered that phytochemicals, including vitamin C (ascorbic acid), vitamin E (α-tocopherol), vitamin A, β-carotene and coenzyme Q-10, and minerals provided by fruits, vegetables, nuts and whole grains in the MDi are effective for maintaining periodontal homeostasis(Reference Veloso, Abrão, Martins, Bronzato, Gomes, Higino and Sampaio84). Olive oil is a key protective factor for PD, and in some studies, it was shown to be the only effective ingredient(Reference Iwasaki, Ennibi, Bouziane, Erraji, Lakhdar and Rhissassi55,Reference Hujoel and Lingström85) . An olive oil-enriched diet was found to protect mononuclear phagocyte system function(Reference Rasperini, Pellegrini, Sugai, Mauro, Fiocchi, Corvi Mora and Dellavia86), and phagocyte abnormalities are among the key mechanisms in the pathogenesis of PD(Reference Millwood and Heath75).
A significant protective effect of MDi was also recorded for UADT cancers(Reference Machowicz, Hall, de Pablo, Rauz, Richards and Higham61–Reference Giraldi, Panic, Cadoni, Boccia and Leoncini63,Reference Saraiya, Bradshaw, Meyer, Lund, Slade and Olshan65) . The positive influence of MDi on infectious, inflammation and cancerogenic processes is based on its potential in modifying oxidative stress(Reference Garnacho-Montero, Ortiz-Leyba, Garnacho-Montero, Garcia-Garmendia, Pérez-Paredes and Moyano-Del Estad87). The bioactive components from MDi that mainly act as antioxidants are polyphenols, vitamin C, vitamin E and β-carotene(Reference Veloso, Abrão, Martins, Bronzato, Gomes, Higino and Sampaio84–Reference Rasperini, Pellegrini, Sugai, Mauro, Fiocchi, Corvi Mora and Dellavia86) Dietary polyphenols play an important role in preventing the disequilibrium between oxidative stress and antioxidant activities in the oral cavity, thereby preventing periodontal tissue destruction. Polyphenol intake was positively associated with S-type cystatins levels in saliva in a study by Louro et al.(Reference Louro, Simões, Penetra, Carreira, Castelo and Luis60) who investigated the association of MDi with saliva composition. Besides antioxidant capacities, polyphenols also carry anti-inflammatory properties by interfering with a number of proinflammatory pathways and inhibits them, which results in a reduction of the synthesis and release of proinflammatory cytokines such as interleukin (IL)-1β, IL-6, IL-8 and tumour necrosis factor (TNF)(Reference Itsiopoulos, Mayr and Thomas88–Reference Zhang and Tsao90). These positive effects of polyphenols have been demonstrated in other low-grade inflammatory diseases such as cardiovascular and endocrine diseases as well as metabolic and immune disorders(Reference Billingsley and Carbone91,Reference Scalbert and Williamson92) . In addition to their anticancerogenic effect, phytochemicals that are involved in cellular differentiation and proliferation are also involved in the synthesis and repair of DNA and in the inhibition of the formation of carcinogenic chemicals in inflammatory responses(Reference Billingsley and Carbone91,Reference Medina-Remón, Tresserra-Rimbau, Pons, Tur, Martorell and Ros93–Reference Mentella, Scaldaferri, Ricci, Gasbarrini and Miggiano95) . High adherence to MDi has been associated with a lower risk of pancreatic, colorectum, breast, aerodigestive tract, prostate and liver cancer(Reference Ciancarelli, Massimo, Amicis and Ciancarelli96,Reference Bosetti, Turati, Dal Pont, Ferraroni, Polesel and Negri97) . Besides polyphenols, omega-3 fatty acids from fish and nuts can affect cancer cell proliferation, angiogenesis and metastasis, slowing down cancer development(98). Also, animal studies have shown that extra virgin olive oil may induce cancer cell apoptosis and minimise DNA damage(Reference Castelló, Boldo, Pérez-Gómez, Lope, Altzibar and Martín99).
Machowicz et al.(Reference Machowicz, Hall, de Pablo, Rauz, Richards and Higham61) concluded that adherence to the MDi was associated with a lower likelihood of having primary Sjögren syndrome (pSS). Increasing the MDi score by just one unit was associated with a 19% decrease in the odds of having pSS. This positive effect was explained in terms of the fish intake since the MDS fish domain was inversely associated with pSS in both univariate and multivariate analysis. Fish intake has also been previously reported to reduce the risk of other autoimmune disorders such as rheumatoid arthritis(Reference Escrich, Moral and Solanas100,Reference De Pablo, Romaguera, Fisk, Calder, Quirke and Cartwright101) .
Clarification of the association between oral health and adherence to MDi may lead to large public health benefits. The World Health Organization (WHO) has raised awareness of the importance of good oral health in essential functions such as eating, breathing and speaking, and its contribution to overall health and quality of life, and developed the Global Strategy and Action Plan on Oral Health (2023–2030) stressing the need for a life-long, multi-sectoral approach (WHO 2024)(102). The high rates of oral disease worldwide, particularly among the most vulnerable members of society, pose major challenges to health and social care systems(103). PD, a major cause of tooth loss in middle and older age groups, peaks around 55 years of age and remains highly prevalent among older dentate people. Higher prevalence is expected in the near future owing to larger numbers of older individuals and longer lives(103). Oral cancer is closely associated with tobacco use and still has high mortality and high prevalence with large variation in relation to region, countries, sex and age(103).
It should be noticed that most oral diseases are largely preventable, if early diagnosed, with unhealthy diet, rich in free sugars and poor in fruits and vegetables, being a significant risk factor. Oral health shares common risk factors with other non-communicable diseases (NCD) and may benefit from common preventive strategies in research agendas and policy development. Based on WHO recommendations, enabling actions on the prevention and control of NCD, such as oral health, include the development and implementation of national nutrient-and food-based dietary guidelines and nutrient profile models for different applications(104). A healthy lifestyle requires a reduction in the intake of free sugars and increase in the consumption of legumes, fruits and vegetables, which are basic components of MDi(104). It is a significant opportunity that MDi has progressively become globally well known even in non-Mediterranean countries, because of wider availability of olive oil, fresh fruits and vegetables, whole grains, legumes, seeds and nuts in many world regions(Reference Zupo, Castellana, Piscitelli, Crupi, Desantis and Greco3,Reference Yang, Farioli, Korre and Kales6) . However, a decrease in MDi adherence has recently been observed in Mediterranean countries, due to modern stressful lifestyles prompting people to consume fast and sugary foods(Reference Mazza, Ferro, Pujia, Mare, Maurotti, Montalcini and Pujia30).
Therefore, effective strategies need to be developed and implemented at national levels to promote diet literacy and encourage adoption of MDi. Food education campaigns may play an important role in the nutritional behaviour change, and MDi adherence should be encouraged worldwide, aiming to promote healthy ageing. Mass media and targeted client communication messaging services with mobile phones such as the BeHe@lthy BeMobile WHO applications may promote healthy behaviours through propagating balanced information. Exposure to information delivered by mass media was associated with a greater adherence to both Mediterranean diet and Mediterranean diet-like eating pattern in a representative large sample of a general adult Italian population(Reference Hernández-Ruiz, García-Villanova, Guerra Hernández, Amiano, Azpiri and Molina-Montes105).
Nutrition education for all healthcare providers, including dental professionals, and multidisciplinary collaboration may improve both oral and general health as studies have shown improvement in nutritional status of dental patients when accompanying dental rehabilitation with nutritional advice(Reference Bradbury, Thomason, Jepson, Walls, Allen and Moynihan106).
Study limitations
The present review had some limitations. Although non-English language publications were not excluded, many of them may be missing from the databases that were searched. Most articles analysed had a cross-sectional design, and clear causative effects between adherence to MDi and oral health indicators cannot be concluded. Moreover, there was large variability in study design, population recorded and MDi scales applied, precluding direct comparison between findings. Finally, this study included only articles where the total MDi scale scores were considered and the specific effect of different MDi elements could not be identified.
Implications for future research
The high prevalence of oral diseases and their negative impact on general health and quality of life necessitate enhancement of knowledge about the benefits of MDi to population oral health. More interventional studies are necessary to clarify the protective effect of MDi on oral health and the related mechanisms. The individual components of MDi should also be investigated to identify their specific protective role in the prevention and control of oral diseases. Regarding PD, further studies should contribute to the elucidation of the mechanism of action in cytokines, the phagocyte system and the overall inflammatory process. Apart from PD and UADT cancer, other components of oral health and function should be investigated in relation to adherence to MDi such as dental decay, masticatory performance, oral dryness and bite force.
Since evidence on the impact of oral health on MDi adherence is not clear, further studies should be conducted taking into consideration the complexity of sociomedical and oral factors involved in food selection. Interventional studies should investigate the effect of different types of prosthodontic rehabilitation on food selection and the best strategies to improve nutritional habits of fully and partially edentulous individuals. The role of a multidisciplinary care team in improving adherence to MDi could also be further studied.
Multi-centre studies in different Mediterranean and non-Mediterranean countries would be particularly useful to elucidate the effect of combining environmental and oral health factors on MDi adherence.
Finally, further studies should be conducted on effective interventions to educate the public and the healthcare providers on the benefits of MDi on oral health and NCD prevention and control and drive changes in food selection behaviour. As required by WHO, research findings should be translated into practice aiming at country-, location- and population group-specific evidence-based clinical practice guidelines(104).
Conclusions
The findings in this scoping review revealed systematic negative associations between better adherence to MDi and prevalence of PD and UADT cancer. As a healthy diet, MDi is a significant tool for prevention and control of NCD, and its specific effect and mechanisms in prevention and control of oral diseases require further investigation.
Few studies investigated the effect of oral factors on adherence to MDi, with inconclusive findings. It would be important to further elucidate the effect of various oral health indicators, such as number of teeth, prosthodontic rehabilitation, masticatory performance, severe periodontal disease, biting force, tongue force and oral dryness, on adherence to MDi and inform dental professionals and other members of the multidisciplinary healthcare team on more effective oral rehabilitation and nutritional improvement strategies.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/S0954422424000337.
Authorship
A.P.: conceptualisation, investigation, formal analysis, data interpretation, writing – original draft; J.J.: conceptualisation, investigation, data interpretation, search protocol implementation, formal analysis, manuscript writing – draft; A.T.: investigation, data interpretation, writing – review and editing; E.P.: investigation, data interpretation, writing – review and editing; K.K.: investigation, data interpretation, writing – review and editing; I.S.: investigation, data interpretation, writing –review and editing; A.M.L.: investigation, study protocol implementation, data interpretation, corresponding, writing – draft; A.K.: conceptualisation, search protocol supervision, data interpretation, manuscript review and editing. All authors have read and agreed to this version of manuscript(Reference Sahyoun and Krall107–Reference Casas, Sacanella and Estruch109). Fully open access was granted by the agreement between National Library of Serbia and Cambridge University Press.
Financial support
If accepted and have an open access publication we should insert here funding from Heak-Link.
Competing interests
The authors report no conflicts of interest.
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