Fitness versus fatness: Disentangling their effects on disease outcomes and estimating the population burden of disease

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Overweight and physical inactivity are two of the major risk factors for cardiovascular disease and diabetes. With increasing population levels of overweight, governments are increasingly advocating public health measures aimed at increasing physical activity levels or otherwise decreasing weight. There has been much research concerning which of these factors is the key prognostic factor for adverse health outcomes, but an ongoing lack of clarity of research findings has led to uncertainty as to the direction of recommendations for preventive health strategies and population lifestyle changes. In addition, the risks of overweight, in particular, have been accused of being exaggerated in both the scientific and lay literature. This is often due to the difficulty of dealing appropriately with time varying confounders which are also intermediate factors (such as hypertension). To date, no studies have performed careful longitudinal modelling of the joint effects of physical inactivity and overweight on cardiovascular events and diabetes while taking into account the effects of factors, such as hypertension or atherosclerosis, that influence both physical activity and overweight as well as disease, and simultaneously are consequences of these risk factors. Standard statistical methods are known to produce biased estimates in these situations but we will apply more recently developed statistical techniques to provide much improved estimation of these effects. After the statistical modelling stage, we will model the burden of diabetes, cardiovascular disease and mortality associated with given levels of overweight and physical inactivity. These will be combined with population levels of overweight and inactivity to identify the fraction of the current and future burden of disease attributable to these risk factors.