Physical activity is known to play a role in the prevention of obesity and related cardiometabolic conditions. However, in order to develop targeted public health interventions, it is first necessary to understand how levels of physical activity vary in different populations, along with the determinants of physical activity and its association with intermediate disease traits.

This thesis presents the results of descriptive studies of physical activity in three population cohorts: the UK-based National Diet and Nutrition Survey, the Cambridgeshire-based Fenland Study, and the Russian-based Know Your Heart Study, along with an ecological analysis of 35 population estimates of physical activity energy expenditure from populations as diverse as Arctic Inuit to the Maasai of Kenya. In summary, the analysis shows that physical varies by age, sex, BMI and location.

The thesis additionally presents the results of two aetiological studies, one examining the associations between energy intake and macronutrient composition as the exposure, and physical activity energy expenditure as the outcome, and the other examining the joint associations of physical activity volume and intensity as the exposure with body-fatness as the outcome. Those analyses show that volume, be it intake or expenditure, is more strongly associated with the outcome than the underlying macronutrient composition or intensity.

Overall, the body of work presented in this thesis represents a meaningful addition to the physical activity literature and adds specific additional information about the determinants of, and associations with, physical activity.