Active Voice: Inactivity Causes Excess Body Fat — Or Is the Reverse True?
By Kathleen F. Janz, Ed.D., FACSM, and Soyang Kwon, Ph.D.

Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.

Kathleen F. Janz, Ed.D., FACSM, is Professor in the Department of Health and Human Physiology at the University of Iowa in Iowa City. Her research interests include quantification of physical activity and fitness and examining the influences of these factors on health status, particularly in children. Soyang Kwon, Ph.D., is a post-doctoral scholar in the Department of Health Studies at the University of Chicago. Her research focus is the epidemiology of physical activity. In the Mar. 2011 issue of Medicine & Science in Sports & Exercise® (MSSE), Dr. Janz, Dr. Kwon and their coauthors published a related research paper entitled “Effects of Adiposity on Physical Activity in Childhood: Iowa Bone Development Study.”

Given the health consequences for individuals and the economic burden of health care for society, the lack of physical activity and increased prevalence of obesity are major areas of interest for researchers, health practitioners and the public. There is substantial evidence that both the lack of physical activity and the rise in obesity contribute to poor health, independently of and synergistically with each other.

Physiologically, it makes sense that the lack of physical activity contributes to adiposity accumulation. Many epidemiologic studies have also shown the effect of inactivity on obesity phenotype. However, the role of adiposity on physical activity participation (reverse causation hypothesis) has been investigated less often. Understanding the potential effects of adiposity levels on physical activity has significant implications for designing physical activity intervention programs, even those not specifically aimed at preventing or reducing obesity. The reverse causation hypothesis implies a positive feedback loop, where becoming obese leads to physical inactivity which leads to becoming more obese – a pattern that may continue throughout life. The consequences of this loop could be particularly devastating for children due to the strong tracking of adiposity and the critical formation of attitudes, values and motor skills that takes place during childhood.

How can adiposity level negatively influence physical activity behaviors in children – especially given that the general population, including most children, is aware of the detrimental health consequences of obesity as well as the weight loss potential afforded by physical activity? (One might assume that being obese would be an excellent motivator for high levels of physical activity.) The reality is that obesity and physical activity behaviors exist in a complex bio-behavioral context. There is accumulating support for obesity-induced biomechanical and physiological changes and psycho-societal influences of obesity, which in turn, could act to suppress physical activity behaviors. For example, there is some evidence that obesity leads to musculoskeletal changes, decreased mobility, modification of gait patterns, changes in energy expenditure and, perhaps most importantly, decreased affect during moderate-intensity movement. That is, exercise doesn’t feel the same if you are obese. Obesity has also been shown to decrease physical activity self-efficacy. In addition, obese children are more likely to be bullied and perceived as less athletic. These pieces of evidence support a positive feedback model. However, future studies are needed to directly address this issue, which to date, has rarely been tested.

In the past, feasibility issues, such as cost and participant burden, have forced epidemiologists to use self-reported physical activity surveys and body mass index (BMI) data to examine physical activity and adiposity relationships. This approach is prone to measurement error. With the reduced cost of activity monitoring systems and increased accessibility of DXA-scanning, significantly greater objectivity is currently available. However, even with better methods for quantifying associations, it will be difficult to separate the influence of adiposity on physical activity from the influence of physical activity on adiposity.

Yet in the end, understanding barriers to physical activity participation among various populations will be critical to developing physical activity treatment programs targeting the nearly 20 percent of U.S. children and majority of U.S. adults who are already overweight or obese. If the reciprocal relationship between physical activity and adiposity exists, physical activity promotion from early childhood, prior to excess fat accumulation, will also be critical to truly prevent childhood obesity.