Abstract
Background: Dietary modification is a cornerstone of cardiometabolic disease management. Given the substantial burden of cardiometabolic disease in Aotearoa New Zealand and globally, evidence-based dietary strategies are essential. However, translating any dietary approach into effective, acceptable, and accessible real-world practice remains a significant challenge.
Aims: The overarching aim of this thesis was to examine the effectiveness, acceptability, and real-world applicability of dietary interventions as tools for cardiometabolic disease management in Aotearoa New Zealand. Three separate randomised trials were conducted to collectively address this aim, each applying a mixed-methods approach.
Methods: In the Dunedin DiRECT randomised controlled trial (Chapters 4 and 5), n=40 patients with obesity; type 2 diabetes (T2D) or prediabetes; and a desire to lose weight at a kaupapa Māori primary care provider were randomised to 12 months of (i) dietitian-supported usual care, or (ii) three months of total diet replacement followed by nine months of food reintroduction and supported weight loss maintenance (DiRECT). The primary quantitative outcome was weight loss at three and 12 months (Chapter 4). In the Healthy Heart Study randomised controlled trial (Chapters 6 and 7), n=309 participants recovering from an acute coronary event were randomised to 12 weeks of (i) usual care only, or (ii) free, medically tailored grocery deliveries high in either dietary fibre or healthy fats. Participants’ dietary intakes were analysed to assess whether changes in intake were sustained at three and 12 months after the interventions had ended (Chapter 6). The GluCoast randomised crossover trial (Chapters 8 and 9) considered two dietary strategies in n=31 participants with T2D: (i) twice-daily partial diet replacement (PDR), and (ii) time-restricted eating (TRE) within a nine-hour eating window ending by 7 pm. The primary quantitative outcome was time-in-tight-range (% time between 3.9–7.8 mmol/L), a metric of glycaemia measured using continuous glucose monitoring (Chapter 8). In each of the three trials, semi-structured interviews were conducted and analysed using reflexive thematic analysis to explore participants’ experiences of the dietary interventions (Chapters 5, 7, and 9).
Results: In the Dunedin DiRECT trial, the DiRECT intervention resulted in significantly greater weight loss than with dietitian-supported usual care at three months (–6.1 kg [95% CI –10.2, –2.0]) and 12 months (–3.8 kg [–7.6, –0.1]). Qualitative findings highlighted that DiRECT was an acceptable weight loss strategy in participants with T2D or prediabetes with strong cultural emphases on food and shared eating. Individualised support was perceived as central to success in both interventions, while cultural eating norms posed potential barriers to sustained weight loss. In the Healthy Heart Study, participants who received groceries high in healthy fats had greater improvements in monounsaturated (1.89% total energy intake [95% CI 0.88, 2.91]) and polyunsaturated fat (1.17% total energy intake [0.51, 1.83]) intake at three-month but not 12-month follow-up compared with usual care. Improvements in dietary fibre intake among participants who received high fibre groceries were not sustained beyond the active intervention phase when compared with usual care. Qualitative results highlighted that medically tailored groceries enhanced opportunity for dietary change but did not directly improve capability or motivation. Together, findings indicated that sustained changes in dietary intake following medically tailored grocery provision may require ongoing environmental or behavioural reinforcement. In the GluCoast trial, the primary glycaemic outcome of time-in-tight range did not differ significantly between interventions (TRE vs PDR: 4.5% [95% CI –0.5, 9.6]). TRE resulted in a 5.0% (0.6, 9.3) greater time-in-range (% time between 3.9–10.0 mmol/L) than with PDR, however. PDR more effectively reduced energy intake than TRE, with longer-term implications for weight loss and glycaemic outcomes if sustained. Acceptability was shaped by how well intervention demands (changing what versus changing when to eat) fitted individual contexts. PDR was seen as easier to adopt for short-term weight loss, while TRE was considered more disruptive, but potentially more sustainable long-term.
Conclusions: Each of the dietary interventions evaluated across these three trials demonstrated (potential) effectiveness, acceptability, and real-world applicability for cardiometabolic disease management. However, sustained dietary change and improvements in cardiometabolic health outcomes required ongoing structural support. Person-centred care responsive to individual, social, and cultural contexts was consistently identified as an essential facilitator of meaningful change. Alongside greater health system investment, the design and delivery of future dietary interventions should consider these factors to ensure dietary support for cardiometabolic disease management is equitable and accessible in Aotearoa New Zealand.