Populations all over the world are experiencing rapid increases in the prevalence of obesity and diabetes. To date, the public health response to the emerging obesity epidemic has been almost totally ineffective. As a first line of response, professional bodies and government organizations have issued prevention guidelines, all of which include recommendations on levels of activity energy expenditure (AEE). However, even if fully implemented, it is not clear that the current recommendations on AEE would impact the trend in age-related weight gain. In fact, there is virtually no direct evidence that can be brought to bear on the question of whether the obesity epidemic has resulted primarily or even partially from society-wide declines in habitual physical activity (PA).