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Thesis (PhD(Health Sciences))--University of South Australia, 2018. Includes bibliographical references (pages 114-120) Dietary recommendations have changed over the past six years with current Australian guidelines acknowledging for the first time that there are a variety of suitable eating patterns and that nutrition advice should be based on personal circumstances. Specifically, low-fat, higher-protein (HP) diets, where protein has replaced some carbohydrate, have been shown to be effective, particularly with concurrent exercise training for weight loss, lowering glycated haemoglobin (% HbA1c) and cardiometabolic health risk factors in energy-restriction in T2D. However, it is not known whether these improvements remain during energy-balance when weight loss is sustained. Furthermore, there is a lack of intervention studies investigating HP diets on food cravings, psychological wellbeing and cognitive function in T2D despite these parameters being associated with hyperglycaemia. The randomised controlled trial underpinning this thesis was designed to address those knowledge gaps.

Background Although higher-protein diets (HP) can assist with weight loss and glycemic control, their effect on psychological wellbeing has not been established. The objective of this study was to compare the effects of a HP and a higher-carbohydrate diet (HC), combined with regular exercise, on psychological wellbeing both during weight loss (WL) and weight maintenance phases (WM). Methods In a parallel RCT, 61 adults with T2D (mean ± SD: BMI 34.3 ± 5.1 kg/m2, aged 55 ± 8 years) consumed a HP diet (29% protein, 34% carbohydrate, 31% fat) or an isocaloric HC diet (21%:48%:24%), with moderate intensity exercise, for 12 weeks of WL and 12 weeks of WM. Secondary data evaluating psychological wellbeing was assessed using: Problems Areas in Diabetes (PAID); Diabetes-39 Quality of Life (D-39); Short Form Health Survey (SF-36); Perceived Stress Scale-10 (PSS-10) and the Leeds Sleep Evaluation Questionnaire (LSEQ) at Weeks 0, 12 and 24 and evaluated with mixed models analysis. Results Independent of diet, improvements for PAID; D-39 diabetes control; D-39 severity of diabetes; SF-36 physical functioning and SF-36 general health were found following WL (d = 0.30 to 0.69, P ≤ 0.04 for all) which remained after 12 weeks of WM. SF-36 vitality improved more in the HP group (group x time interaction P = 0.03). Associations were seen between HbA1c and D-39 severity of diabetes rating (r = 0.30, P = 0.01) and SF-36 mental health (r = − 0.32, P = 0.003) and between weight loss and PAID (r = 0.30, P = 0.01). Conclusion Several improvements in diabetes-related and general psychological wellbeing were seen similarly for both diets following weight loss and a reduction in HbA1c with most of these improvements remaining when weight loss was sustained for 12 weeks. A HP diet may provide additional increases in vitality. Trial registration The trial was prospectively registered with the Australian New Zealand Clinical Trials Registry (ACTRN 12613000008729) on 4 January 2013.