Unhealthy diets and physical inactivity are important modifiable risk factors for premature mortality and major chronic diseases, such as cardiovascular disease (CVD) and type 2 diabetes (diabetes). However, few large-scale studies have examined the combined influence of overall diet quality and physical activity on longevity and the risk of clinically-diagnosed CVD and diabetes in adults recruited from the general population. Of popular opinion, is the notion that the health risks from poor diets can be compensated by physical activity; i.e. that one can “outrun a bad diet”. The food industry has often sought to sway public opinion towards the belief that dietary indulgences can be enjoyed as long as one engages in sufficient physical activity. However, it is unclear if the risk of premature mortality and cardiometabolic disease from unhealthy diets can be offset by physical activity, and vice versa. Therefore, the overall theme of this dissertation is to examine the epidemiological associations of diet quality and physical activity on the risk of mortality, CVD and diabetes. Specific objectives are to:

1. Study the independent associations of overall diet quality and physical activity on the primary outcomes of all-cause mortality, cardiovascular disease, and diabetes.

2. Examine the dose-response associations between physical activity and the risk of mortality and cardiovascular disease, including more of the evidence base by employing comprehensive exposure dose-harmonisation methods.

3. Examine the interactions between diet and physical activity on the health outcomes: a. If the risk of mortality and cardiometabolic disease from unhealthy diets can be offset by physical activity, and vice versa. (i.e. compensation) b. If diet and physical activity can work synergistically to further lower the risk of mortality and cardiometabolic disease, beyond the independent effect from each exposure alone (i.e. synergism)

4. Quantify the population health impacts of diet and physical activity, in terms of preventable events (deaths and disease cases) associated with both exposures, as well as the population attributable fractions (PAFs) of poor diet and inactivity contextualised with other major lifestyle risk factors.

5. Examine the long-term trajectories of physical activity (increases, decreases, and maintenance) on the risk of mortality.

These investigations can bring further clarity to the public health messaging around diet and physical activity for improving longevity and the prevention of cardiovascular disease and diabetes