Active Voice: The Most Important Activity Behavior to Combat Childhood Obesity?

By Kathleen F. Janz, Ed.D., FACSM, and Piroska Boros, M.S.

Kathleen F. Janz, Ed.D., FACSM Piroska Boros, M.S.
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 a physical activity epidemiologist in the Department of Health and Human Physiology at the University of Iowa in Iowa City, Iowa. Her research addresses everyday physical activity and musculoskeletal and metabolic health outcomes during the developmental years.

Piroska Boros, M.S., is completing her Ph.D. with Dr. Janz. Her research focus relates to the effects of intramuscular adiposity on muscle function and bone health. She is a student member of ACSM.

This commentary presents Dr. Janz’ and Ms. Boros’ views on the topic related to a research article they and Iowa Bone Development Study colleagues authored. Their article appears in the October 2017 issue of
Medicine & Science in Sports & Exercise® (MSSE).

Physical activity and sedentary behaviors are highly relevant to the excess prevalence of childhood obesity, which, in the US, affects one in every six children or adolescents. However, it is unclear what intensity and amount of physical activity are associated with adiposity during the growing years. Even less is known about the relationship between sedentary behaviors and adiposity. To help address this research gap, as we reported in the October 2017 issue of MSSE, we (Iowa bone development study investigators) followed 450 youth for up to 14 years (from ages 5 to 19 years) and examined the intensity and volume of physical activity that most influenced fatness. We also considered the effects of total sedentary time and TV viewing on adiposity. Repeatedly measuring the same youth as they grew was a key feature of our study since it allowed us to create individual growth models of changes in adiposity. This strategy was particularly helpful for avoiding the known attenuation of effects typically associated with cross-sectional or longitudinal studies that include only baseline and follow-up measures. Throughout the study, with up to seven clinical exams, we used an objective measure of physical activity and sedentary time (ActiGraph accelerometer), as well as an objective standardized measure of adiposity (dual-energy x-ray absorptiometry). In addition, we repeatedly collected questionnaire data about the participants’ TV viewing. To the best of our knowledge, this is the first study to follow a cohort from childhood to young adulthood using this level of precision.

Regardless of age or gender, our findings indicated that the best objective physical activity predictor of total body fatness was moderate- to vigorous-intensity physical activity (MVPA). Sedentary time, as measured by the accelerometer, did not predict fatness, once MVPA entered our growth models. In addition, light-intensity physical activity was not predictive of adiposity for females; in contrast, it was a positive predictor of fatness for males, i.e., the more light-intensity physical activity, the greater the level of adiposity! This latter finding surprised us since conventional wisdom suggests that any energy expenditure, even at a low level, helps to combat obesity. TV viewing, presumably a subset of sedentary time, was a strong predictor of fatness; thus, the greater the TV viewing, the greater the adiposity. That TV viewing effect was independent of MVPA. This finding suggests that two separate pathways explain how physical activity and TV viewing influence adiposity. Of course, TV viewing, as an obesogenic behavior, is more influential than simply time spent at a low energy expenditure. Therefore, we believe researchers should stop using TV viewing as a proxy for total sedentary time.

Our study indicates that researchers and practitioners should pay special attention to other obesogenic behaviors that occur during TV viewing, such as reduced sleep time and increased time devoted to meals and snacking. In the end, the findings suggest that, on average, a 10-min per day increase in MVPA would decrease absolute levels of adiposity by 1.7 percent in children and adolescents while a one hour per day increase in TV viewing would increase absolute levels of adiposity by two percent. These dose-response influences would be expected to have clinical and community significance in combating obesity. The TV influences that directly contribute to excessive fatness in youth may even be slightly greater that the opposing (favorable) influences of MVPA on fatness. Taken together, these results may translate to the design of more effective and targeted interventions.