Health Literacy, Oral diseases, and contributing pathways: A LifeLines Cohort Study

The Global Burden of Disease Study (2019) estimated that 3.5 billion people worldwide live with dental conditions. (1) The most prevalent dental conditions were untreated dental caries, severe periodontitis, edentulism, and severe tooth loss (having between 1 and 9 remaining teeth). (2) Moreover, various studies have shown an association between dental diseases and systemic health conditions, showing the need for a strategic preventive approach. (3) Research has shown that health literacy (HL) is an important determinant of preventive oral health behaviours and oral health status.(4,5) Hence, one of the comprehensive intervention to improve oral health, should focus on investigating the associations between HL, oral health status and behaviours. This can provide insight into how to develop strategic approaches for promoting oral health and preventive care.
Health Literacy
Health literacy (HL) is defined as “the degree to which people are able to access, understand, appraise and communicate information to engage with the demands of different health contexts in order to promote and maintain good health across the life-course”.(6) Data from large, nationally representative samples documented 36% of U.S. and 47% of EU adults have limited HL.(7,8,9) In the Netherlands, 28.7% of the population was found to have limited health literacy.(5) HL is a crucial factor for understanding health information which is related to maintenance of overall health on the long term.(9)  Limited HL is associated with increased emergency services, poorer health outcomes, and limited use of health services, leading to increasing costs  for health services.(9,10) Thus, as stated by the WHO, HL plays a key role to health promotion, as it is considered an observable, measurable and modifiable factor.(9) 
Health Literacy and Oral conditions
Until now, very few studies have specifically assessed the association of HL with oral conditions and oral health behaviour.(11,12) Two systematic reviews concluded that the evidence on the association between oral health literacy (OHL) and oral conditions in adults is inconclusive.(4,14) These reviews  reported a weak association between lower levels of parental OHL and dental caries in primary teeth. An increased OHL was significantly associated with a higher number of teeth present in adults, but it was not associated with periodontal status. The most common limitation was that more than two-thirds of the studies were conducted with non-representative or non-probabilistic samples (4,14).  As such, these mixed results were due to issues in the generalizability of results as all sample was recruited from the clinical setting, small sample size, and cross-sectional design.(13,14) Since, there is a knowledge gap in previous studies, future research studies using large longitudinal cohorts with representative samples is needed to overcome these limitations. Furthermore, previous studies have stated the need for evidence on indirect pathways (mediators) that may link OHL to oral conditions.(5,11- 14) Greater knowledge of how health literacy impacts oral health, makes it possible to focus interventions on intermediate outcomes, in order to mitigate the adverse effects of low health literacy on (oral) health outcomes.
Mediating pathways
The first mediating  pathway between health literacy and oral health outcomes could be via oral health behaviours like brushing frequency, flossing, diet, frequency of cariogenic food items and acidic drinks, number of eating moments, smoking behaviour, all of which are known to be linked to oral conditions and health literacy. (5,15-17,20) Past evidence suggests that difficulty in maintaining health behaviours may partly explain why low HL results in poor health outcomes. (10,12,19) The poorer health outcomes in low health literates could be explained by lack of awareness of the importance and of performing these health behaviours.(6,20) Therefore, oral health behaviour (brushing, flossing, diet, smoking) could be a mediating pathway between health literacy and oral health outcomes. Several studies found that poor oral health behaviours are associated with poor oral health outcomes (15-17,20), but there is a lack of evidence on the mediating role of oral health behaviours in the relationship between oral health literacy and oral health outcomes; only one cross-sectional study has assessed the mediating pathway of oral health behaviour with OHL and oral health status. (5) This study concluded that HL exerted an indirect effect on parental oral health behaviour, with knowledge, self-efficacy, and perceived barriers being the primary constructs linking HL to behaviour.
Moreover, the Dutch Guidelines for a Healthy Diet 2016, do not have specific guidelines for sugar consumption, sticky (hard to brush easily) food intake (such as sweets, or caramel pop-corn), and high acidic drinks in relation to dental caries, but advised limited consumption in the guidelines. Hence, additional research is needed to determine if guidelines for diet need to be more specific in order to prevent dental diseases. This project might be a stepping stone in terms of association of dietary patterns with dental diseases.
Another mediating pathway between HL and oral diseases could be the utilisation of dental services (dental attendance, access to dental care, delayed treatment due to expenses and supplementary dental-insurance). (20-22) Previous evidence have stated that individuals with low health literacy have increased emergency department (ED) utilization, sporadic dental attendance, as well as unmet dental needs. (20). The estimated costs resulting from low health literacy in general healthcare range between $106 billion and $238 billion annually.(20) In addition only few studies have highlighted the reasons for poor oral health and unmet dental needs in low health literate patients, which are specified as the absence of dental insurance or insufficient coverage, inability to find a dentist who accepts their insurance, and limited dental care utilization.(23–25) 
Need for Research
Previous studies have shown that there is a need for research into the mechanisms that link HL to oral outcomes. Identifying these mediators would be useful for strategizing a targeted approach towards oral health promotion.(10) In addition, to our knowledge no population-based cohort study evaluating HL, oral health behaviour, dental care utilisation, and oral conditions has yet been conducted in Europe. Therefore, this study will examine the potential mediating pathways and association between HL and oral health outcomes via oral health behaviour and dental care utilisation, through a population-based longitudinal cohort. Furthermore, this study will provide insights on the association of supplementary dental insurance and oral outcomes, which is useful for tailoring educational interventions that may help to reduce disparities regarding dental attendance. Hence, it can be stated that this study will provide more clarity on the inconclusive findings about the association between oral health literacy and oral outcomes. The findings of this study are pivotal for oral health promotion and reducing oral health inequalities amongst individuals with low health literacy and are of importance to revert the ‘dental treatment delivery’ model to the ‘preventive care’ model. (26) This will help to reduce utilization of emergency dental treatment such as tooth extraction, and help in early diagnosis and treatment of preventable dental diseases.