Background: Type 2 diabetes mellitus (T2DM) is a significant health problem that imposes a large global economic and social burden on individuals, families and healthcare systems. It is the main cause of kidney insufficiency, blindness and non-traumatic amputations. T2DM is also a major risk factor for cardiovascular disease, which is the leading cause of mortality in people with T2DM. As T2DM is a metabolic disease, there is a large body of research around diet and diabetes. Human intervention studies have evaluated dietary components and dietary patterns for managing blood glucose levels in people with T2DM who have lost the ability to manage levels homeostatically. Through this body of work, there is clear evidence that sustained periods of high diet quality have positive effects on glycaemic control. However, surprisingly little is known about the diet quality of people with T2DM who are not participating in intervention studies, which is majority of the population with T2DM. Evaluating the usual dietary intake of certain groups is an important research activity that can help influence public health initiatives, inform dietary education materials and contribute to the development of targeted nutritional interventions. Improved, tailored care may lead to higher diet quality in people with T2DM, consequently reducing diabetes-related complications and healthcare costs. To enhance understandings of the natural course in adults’ diet quality with T2DM, the aims of this PhD were to: (i) synthesise evidence regarding the dietary intake of people diagnosed with T2DM; (ii) investigate how diet quality changes after diagnosis in people newly diagnosed with T2DM and (iii) identify the factors (demographic, physical and psychosocial) associated with changes in diet quality in people newly diagnosed with T2DM. These aims were addressed in three phases of research. Phase 1: A systematic literature review was conducted to investigate the dietary intake (by food group) in people with T2DM and compare these intakes to national and international dietary guidelines. Eleven studies met the inclusion criteria and were included in the final review. Findings (published in Diabetes Research and Clinical Practice, 2018) indicated that majority of people with T2DM were consuming less than the recommended serves of fruit, vegetables, grains and dairy and were meeting or exceeding the recommended serves for meat/meat alternatives. However, included studies only reported cross-sectional data and did not control for length of time with diabetes among participants. While cross-sectional analyses provide a snapshot of diet quality, there was no quantitative data on how people with T2DM change their diet quality over time, particularly from the point of diagnosis. This meant it was difficult to determine whether diet quality remains fixed once an individual is diagnosed or whether there are periods of marked dietary change. These findings went on to inform Phase 2 of this thesis. Phase 2: The systematic review findings informed the protocol for an observational, case-series study titled, “How does Diet change with a Diagnosis of Diabetes – The 3D study” (published in Nutrients, 2019). The 3D study was conducted in Australia between 2018-2020 and recruited 225 people newly diagnosed with T2DM (in the 6-months prior to recruitment) and monitored their dietary intake and demographic, health and psychosocial characteristics over 12-months. Diet quality was assessed using the Dietary Approaches to Stop Hypertension (DASH) dietary pattern scoring tool. The project was led by the candidate as the substantive work towards this thesis. Phase 3: Phase 3 involved the analysis of short and long-term outcomes from the 3D study (one paper published in Nutrition and Dietetics, 2020 and another under consideration with Public Health Nutrition) as well as a process evaluation (Chapter 8). The objective of the short-term outcome paper was to quantify short-term improvements in diet quality and to identify factors associated with improvements after T2DM diagnosis among participants in the 3D study. Participants were categorised into two groups: those who improved their DASH score by 3 or more points between baseline and 3-months and those who did not. Factors associated with change in DASH scores were clinically and statistically evaluated. Diet quality changes after diagnosis did not appear to be associated with demographic characteristics but were associated with some health factors. It was concluded that strategies targeted at better supporting smokers, those with low physical activity levels and men with a higher body mass index (BMI) are required. The second outcome paper (currently under consideration with Public Health Nutrition) aimed to examine how the diet quality of individuals newly diagnosed with T2DM changed over the 12-month 3D study period and to identify factors associated with diet quality changes. To assess changes in DASH, energy, fruit and vegetable intake over time, repeated measure analyses of variance (ANOVA) were used. Multivariate repeated measures models were used to investigate whether these dietary changes were associated with demographic, health or physical characteristics. Participants were categorised into diet quality change profiles based on early (3-month) and long-term (12-months) changes to DASH score. Results showed that diet quality improvements after a diagnosis of T2DM are not maintained to 12-months. The development of cost‐effective, real-world interventions to achieve sustained diet quality change early after diagnosis are warranted. The process evaluation of the 3D study aimed to: (i) evaluate participant perceptions of the processes underpinning the 3D study, (ii) explore participant experiences of the study to give a deeper understanding of its effects, (iii) investigate if involvement in the 3D study may have impacted diet quality and/or physical activity levels over the observation period and (iv) seek reasons for drop out. Results found the design and implementation of the 3D study to be sound. Participants had favourable perceptions towards the processes underpinning the 3D study and positive experiences related to their participation. Participation served as a reminder to some participants to practice healthy diet and exercise behaviours prior to the scheduled interview time, but this did not appear to greatly impact the study results. The research in this thesis contributed important, novel findings relevant to the dietary intake of an at-risk group, providing a solid foundation for future work in this area that was not previously available. The research findings suggest that the majority of people with T2DM do not make significant sustained improvements to their diet quality in the 12-months after diagnosis. In the short term, (3-months after diagnosis) some health factors were associated with diet quality change including having a lower BMI, being a non-smoker and having higher physical activity levels at baseline. These findings will be able to be used to test novel interventions of state and non-government organisations to help reduce the impact of diabetes, improve patients' quality of life and help decrease health care costs attributed to diabetes-related complications.