Dental caries is a highly prevalent health issue across all age groups worldwide and is the most common chronic childhood disease in New Zealand. Concurrently, obesity and related chronic metabolic diseases are the most challenging public health problems of modern times. A common dietary behaviour – the high and frequent consumption of processed fermentable carbohydrates – is implicated in dental caries, and chronic diet-related disease. This body of work explores this challenge in New Zealand, particularly regarding the scarcity of evidence of the efficacy of dietary behaviour change interventions for the prevention of dental caries. The thesis profiles seven studies: the first three studies addressed epidemiological factors of dental caries in New Zealand children, the role of processed sugar- and starch-containing carbohydrate foods in dental caries, and the evidence regarding dietary factors that are implicated in both dental caries and other diet-related chronic disease, and the extent to which this evidence is reflected in dietary recommendations. The remaining four formative research studies provided original contributions in the development of a novel method for evaluating nutrition knowledge, the assessment of such knowledge in disparate groups of children and adults for each of general and oral health, and a qualitative investigation into beliefs about, and barriers to healthy eating.

The first of these studies investigated potential inequities in caries distribution using a retrospective review of dental records of five-year-old Auckland schoolchildren. The findings from this epidemiological study showed that children of Māori, and Pacific descent experienced significantly higher burdens of disease than children of New Zealand European or other descent. Children experiencing the highest levels of socio-economic deprivation had a nearly three-fold greater dental caries experience than children experiencing the least deprivation.

The relationship between high dietary intakes of sugar and dental caries is well established; however, there is debate about the caries risk associated with the intake of other processed fermentable carbohydrates. The aim of the second study, the systematic review, was to investigate the relationship between the consumption of processed sugar- and starch-containing foods and dental caries. In a systematic review of prospective studies, statistically significant associations were observed between high between-meal consumption of processed sugar- and starch-containing fermentable carbohydrates and dental caries. There were mixed findings regarding total consumption of processed sugar and starch-containing foods, owing to confounding factors including the simultaneous consumption of caries-protective foods at mealtimes. Most importantly, there were no interventions that evaluated the efficacy of dietary change interventions on dental caries; this represents a significant gap in the literature on the prevention of dental caries, a chronic disease associated with dietary behaviour.

Finally, the aim of the third study, the narrative appraisal, was to incorporate the evidence regarding dietary factors in dental caries and other diet-related chronic disease and assess the extent to which this evidence is reflected in dietary recommendations. Substantial disconnects were identified between the evidence of dietary factors associated with dental caries and poor general health and dietary guideline recommendations, which underpin advice provided in health promotion initiatives.

The findings from these three studies indicate that current strategies for caries prevention are insufficient to address the burden of dental caries, and the observed large inequities in disease distribution. Advice provided in health promotion initiatives can also undermine progress in prevention, through the advocacy of foods that are harmful to dental health. In addition, there is a lack of integration of the medical and dental professions in their work towards the prevention of chronic diet-related disease. Crucially, a key gap identified in the literature was the absence of interventions that have addressed the simultaneous prevention of both dental caries and general health using dietary behaviour change. The epidemiological and biological evidence by which a poor-quality diet causes harm in oral and general health provides justification for designing dietary change interventions. The design of such interventions requires the measurement and understanding of the current knowledge and beliefs of the target populations, and of health practitioners who instigate such interventions. There may also be a range of barriers to engaging in behaviour change and in implementing strategies for change at multiple levels; these also require consideration in the design of potential interventions.

These factors gave rise to a shift in the research direction of this thesis to a set of four formative studies yielding quantitative and qualitative data that were undertaken in the Taupō community in the North Island of New Zealand. The principal objectives of these formative studies were to measure and evaluate the beliefs of dietary behaviours and healthiness of foods in disparate groups of children and adults. Furthermore, the alignment of the beliefs and knowledge with dietary guideline advice for oral and general health was also assessed. Given that knowledge alone may be insufficient to change behaviours, the aim of the qualitative work was to gain insights of the barriers that children and adults experience to healthy eating.

The first of these formative studies was the development and validation of a set of novel card sorting exercises as a method by which nutrition knowledge of dietary behaviours and foods for oral and general health could be measured in children and adults. The card sorting exercises were valid, reliable, and acceptable for assessing beliefs and knowledge in these groups.

In the next two studies, the card sorting exercises were used to evaluate beliefs and knowledge in children, parents, and health professionals and to compare the results between these disparate groups for each of i. general health and ii. oral health. Reducing sugar intake, and eating fresh fruit and vegetables were classified as very important for general health, alongside exercising, getting outside, and getting adequate sleep. For oral health, two components were evaluated i. behaviours for oral health and ii. foods for oral health. Oral hygiene behaviours were categorised by nearly all participants as very important for oral health; conversely, dietary behaviours were categorised by fewer participants as very important for oral health. In addition, although the participants correctly judged high-sugar foods to be unhealthy for oral health, there were varied levels of understanding observed in children and adults of both the healthiness of animal produce, and the detrimental effects of processed fermentable carbohydrate foods in oral health. The categorisations of foods as healthy and unhealthy aligned with both the beliefs about dietary behaviours, and recommendations in dietary guidelines, including foods for which there are caveats around consumption. These findings suggest that information provision through health promotion initiatives is successful in achieving knowledge acquisition in adults and children.

The final component of the formative research was the qualitative investigation into the beliefs about, and the barriers children and adults experience, in achieving healthy eating goals. A socio-ecological model of healthy eating for families was developed; themes of healthy eating included knowledge, rules around consumption of foods, preferences and enjoyment, and tradition and culture. Barriers experienced by all groups to healthy eating were social factors relating to other people, and the food environment characterised by the omnipresence of highly processed foods.

In summary, although dental caries is a significant public health challenge, there is a lack of evidence on the efficacy of interventions for caries prevention. Interventions should be a priority given the current paucity of evidence of firstly, the role of a poor-quality diet in dental caries, and secondly, the effectiveness of interventions that focus on reducing processed fermentable carbohydrate intake to decrease the risk of dental caries. The findings in this thesis regarding the common dietary factors implicated in poor oral and general health, and the formative research to evaluate nutrition knowledge and barriers to healthy eating should be used to inform the development of interventions for the prevention of chronic disease. Dietary behaviour is a complex interplay of perceptions of healthy eating, and factors beyond individual knowledge that influence dietary choices; these barriers exist within multiple socio-ecological levels. The prevention of dental caries and chronic metabolic diseases requires the input of both the dental and medical professions, and multi-level integrated approaches to improve population health.