Managing glycaemic control in individuals with type 2 diabetes mellitus: Assessing the influence of exercise timing

Exercise is a cornerstone of type 2 diabetes (T2D) management and is of paramount importance. Past research has investigated various elements of exercise (frequency, intensity, time, etc.) for optimal glycaemic control in those with T2D. However, the physical activity (PA) levels of those with T2D are still not improving (i.e., time constraints, fear of exercise, etc.). The diurnal timing of exercise is a recent topic of interest, and may be an important exercise variable to manipulate in the hope of improving the management of T2D. Within this thesis, a systematic review (Chapter 2) was undertaken to understand the effect of exercise timing on both acute and chronic glycaemic control measures in individuals with T2D. The aim of the systematic review was to review the literature associated with diurnal exercise timing and glycaemic control in individuals with T2D. Systematic searches of online data bases for randomized controlled trials published in English were performed from inception to September 2021. The screening of both the title and abstract, and the full-text of all attained sources were carried out by three authors and resolved by a fourth member of the team. Thereafter, three authors independently utilized the Cochrane Collaboration Risk of Bias Assessment Tool Version 2 (RoB2) and The Cochrane Collaboration Data Collection Form for the completion of study quality and data extraction of articles, respectively. A qualitative synthesis was performed on the included studies, with results summarized in tables. Twenty-seven studies (752 participants: 23 crossover and 4 parallel-groups) were included in the final qualitative synthesis. Greater improvements in average glucose concentrations were observed when exercise was performed in the morning compared to evening exercise; however, similar improvements in glucose area under the curve (AUC) were seen for both morning and evening exercise. Based on large study design heterogeneity, the observations were largely based on indirect comparisons given the limited number of studies that directly investigated the timing of exercise. Therefore, although current evidence indicates that there may be an important relationship between diurnal exercise timing and glycaemic control in individuals with T2D, it remains inconclusive whether this link is a viable mean for glycaemic control management; however, future low risk trials are warranted to help distinguish a possible relationship. The systematic review indicated that the effect of diurnal exercise timing on glycaemic control remains inconclusive, and as such, a Randomised Controlled Trial (RCT) was conducted to investigate the effects of morning vs evening exercise. The study (Chapter 3) pursued to determine the impact of diurnal exercise timing on i) fasting glycaemic control and insulin resistance measures; ii) postprandial glycaemic control and; iii) measures of body anthropometrics/composition and physical activity measures in overweight individuals with T2D enrolled into a 12-week supervised multi-modal exercise training program (3 days per week; each session 4 resistance-based exercises for 3 sets of 8-12 repetitions and 30 minutes walking). Secondarily, the study sought to explore potential mechanisms associated with improved glycaemic control following exercise training in those with T2D; more specifically, through intracellular glucose metabolism pathways. The conducted intervention improved measures of glycaemic control, body composition, body anthropometrics, cardiorespiratory fitness and maximal strength in individuals with T2D; however, this was irrespective of diurnal exercise timing. Although sRAGE, methylglyoxal and L-lactate did not demonstrate a change in their systemic concentrations, further work adopting intracellular measures of these markers will be required to discard their mechanistic involvement. The existing evidence and recommendations support the manipulation of exercise intensity, type and volume (duration and frequency) to help manage glycaemia in individuals with T2D; however, findings from this study suggest that diurnal timing may not be a considerable factor in a holistic management program. In conclusion, the findings presented in this thesis provide evidence of tactically manipulating the diurnal timing of exercise on glycaemic control in overweight individuals with T2D. Despite this manipulation of diurnal exercise timing not resulting in any additional benefits of glycaemic control, this thesis provides reason that the current exercise guidelines are an effective tool for i) improving glycaemic control; ii) improving cardiovascular health and physical activity and; iii) improving body anthropometric and body compositive measures in individuals with T2D. Results suggesting that the frequency of exercise performance could be a vital component for the management of T2D