An estimated 537 million adults globally have diabetes (of which over 90% have type 2 diabetes), contributing substantial impact to their health and future risk of disease. Excessive sedentary time – too much sitting – has now been identified as a distinct behavioural target for the management of type 2 diabetes. Glycaemic control is a key management consideration for diabetes and for reducing the risk of cardiometabolic diseases and diabetes complications. However, the associations of sedentary time with glycaemic control in people with type 2 diabetes, and the potential benefits of replacing sedentary time with alternatives (e.g., with standing or stepping), along with the impacts of COVID-19 on lifestyle behaviours are unclear. Such information is necessary to help inform the development of randomised controlled trials aiming to reduce sedentary behaviour in people with type 2 diabetes to improve glycaemic control.
The overarching aim of this thesis is to better understand the role of sedentary time and its alternatives in people with type 2 diabetes. Specific objectives are to:
- Understand associations of sedentary behaviour and its main alternatives – standing and stepping, with glycaemic control and related cardiometabolic risk markers in those with and at risk of type 2 diabetes
- Determine how relationships of glycaemic control and related cardiometabolic risk markers with sedentary behaviour and main alternatives may differ between those with or at risk of type 2 diabetes.
- Describe the development of a multicomponent sedentary behaviour reduction intervention in adults with type 2 diabetes.
- Investigate the longer-term changes in sedentary behaviour and physical activity in people with type 2 diabetes and explore the impact of COVID-19 lockdown restrictions on these behaviours.
Data for this thesis were drawn from three studies: the Australian Diabetes, Obesity and Lifestyle (AusDiab) study, the Maastricht Study, and the OPTIMISE Your Health Study. Across two countries (Australia, Netherlands), 3,047 participants either with type 2 diabetes, at higher risk of type 2 diabetes, or with normal glucose metabolism were investigated. Key measures of interest were glycaemic control (glycosylated haemoglobin; HbA1c), fasting and OGTT blood draws (plasma glucose and insulin), and other related cardiometabolic biomarkers relevant to diabetes management (e.g., lipids, triglycerides, waist circumference, insulin sensitivity). In all studies, free-living sedentary time and activity were objectively assessed with device measures including the research grade activPALTM inclinometer and a consumer wearable FitbitTM, which facilitated the measurement of longer-term observation periods. Activity data collected were analysed with a range of different statistical methods including compositional data analysis.
The epidemiological findings demonstrated that compositions of both waking and the full 24hr time with greater proportions of sitting time were associated detrimentally with indices of glycaemic control and related cardiometabolic risk markers, with direct implications for diabetes management. Compositions with greater proportions of sitting, and less standing had detrimental associations only in those at higher risk of diabetes (≥5.7% HbA1c), suggesting that interventions may be of greater benefit to those with more-impaired glycaemic control. These findings were further corroborated in a cohort of people with type 2 diabetes, with findings suggesting that greater proportions of standing or stepping in lieu of sitting time was associated with greater glycaemic control benefits. Importantly though, only variations in sitting and stepping time in a daily composition were associated with glycaemic and insulin sensitivity outcomes (fasting and 2-hour post-load glucose, HbA1c, and insulin sensitivity) in people with type 2 diabetes, suggesting that stepping time should ideally be replaced by sitting time for more favourable changes in diabetes-related outcomes. Potential offsets in time spent sitting by time spent standing and stepping are presented, with an indication of progressively beneficial associations with diabetes-related outcomes up until a particular threshold.
Longitudinal findings suggest that the COVID-19 lockdown restrictions were associated with significant changes to physical activity and sedentary behaviours in office workers with type 2 diabetes. Overall, intervention trial participants monitored for months preceding and during the lockdown restrictions reduced their light and fairly active intensity minutes and increased their sedentary minutes and sedentary bout duration. Some participants maintained or improved their longitudinal pattern of behaviours despite the restrictions, suggesting there were a range of behavioural changes elicited by the lockdown restrictions. Future studies could investigate the contributing factors behind the variation in responses. These factors may have relevance and implications for adherence to the intervention in the randomised control trial (the OPTIMISE Your Health study) described in this thesis. In future, the analytical approaches devised and used in the studies reported in this thesis can test outcomes of this trial and determine how sedentary behaviour may be addressed to improve glycaemic control in those living with type 2 diabetes.
This thesis expands current knowledge on sedentary behaviour within free-living contexts and in people at risk of and with type 2 diabetes. Favourable time-use compositions associated with beneficial glycaemic control and related cardiometabolic risk markers have been identified using novel methodology. These compositions require corroboration by more robust study designs, including by the sedentary behaviour intervention trial described. Further, the COVID-19 pandemic provided a novel opportunity to explore longer-term physical activity and sedentary behaviours, the findings of which suggested increased sedentary behaviours and decreased active behaviours in participants with type 2 diabetes following lockdown restrictions. This suggests addressing high levels of sedentary behaviour among people with type 2 diabetes is an even more urgent public health priority during pandemic times.