Fat chance for physical activity

Commentary The enormous burden of ill health caused by overweight and obesity is well understood, particularly in developed countries like the United States, which have seen massive increases in prevalence over the last few decades [1]. Most recently the Global Burden of Diseases, Injuries, and Risk Factors Study 2010 reported that high body mass index (BMI) is now the third-leading cause of ill health in the United States, responsible for more than one-third of ischemic heart disease, half of osteoarthritis, and three-quarters of diabetes [2]. The consequences for the health, economics, and well-being of individuals and the community are enormous [3]. At its most basic level, obesity is easily understood as a problem of energy balance–if energy intake from food exceeds energy expenditure from physical activity, weight gain ensues. Likewise, it follows that if energy intake can be reduced or physical activity increased, weight loss should follow. Less clear has been the importance of each to the causation of the obesity epidemic and the potential for interventions targeting diet versus physical activity to reverse it. The paper by Dwyer-Lindgren et al. in Population Health Metrics [4] provides important new insight into where the United States should focus its efforts over the next few years. The key observation in the report by Dwyer-Lindgren is the apparently paradoxical association between changes in physical activity and obesity–average levels of physical activity in the United States appear to have risen, but so too have obesity rates. The most likely explanation for this finding is that the rise in average physical activity levels has been outstripped by the excess of energy provided by the food supply. The message is plain – the primary driver of the obesity epidemic in the United States is now the food supply, and interventions targeting physical activity are not going to resolve it. So, while physical activity will benefit the health of the population even if it is not accompanied by weight loss [5],