Dropping the Ball: Weight and Activity in America's Youth

Childhood obesity continues to rise in America. "In the past 20 years, there has been a twofold increase in the rate of obesity in children. Today, it is estimated that approximately 15% of children between the ages of 6-19 yrs are overweight," says Dr. Simon Marshall, author of a recent American Journal of Preventative Medicine review that looks into some studies on childhood fatness and activity. Society has looked in every possible direction to find the culprit for this phenomenon. Some have posited that schools do not teach nutrition, others believe parents should ensure their kids are involved in a lot of physical activity. "It is tempting to blame computer games, TV, and fast food, but relationships to body fat are unclear and definitely complex," says Dr. Marshall. When it comes to a topic as complex as the health of children it can be hard to find a reliable and informative scientific method for investigating trends. Adding to this difficulty is the fact that many of the variables involved in physical activity can be hard to measure. This recent review by Dr. Marshall at San Diego State University looks at two studies which are taking a scientific approach to unraveling the link between childhood activity and obesity.

Physical activity early in life can have a huge impact on the habits of children as they grow, but this may not be the only benefit of an active childhood. Some studies suggest that activity before the age of 5 somehow conditions the body's metabolic rates. In this way vigorous activity may serve a protective function against excessive fat gain later in life. The editorial highlights a recent study by Janz and colleagues at the University of Iowa; this study measured activity and fat for 333 children at ages 5, 8 and 11. Using the information garnered at age 5 as a baseline the study determined that early exercise, during the preschool ages of 2-5, had more effect on fat levels than exercise habits during age 6-11. For example, 10 minutes of moderate to vigorous exercise a day at age 5 correlated to .2 kg less fat mass at ages 8 and 11, while 10 minutes of moderate to vigorous exercise between the ages of 8 and 11 only correlated to .1 kg less fat mass (1). This result may be surprising, but other studies have also suggested that it is the exercise a person gets in their formative years that will determine their risk for obesity in late childhood and adolescence(2). This information is especially critical when combined with known information about activity trends in teens.
Activity levels undergo a sharp decline for most people during adolescence (3). Many studies have been aimed at determining why this drop occurs. Unfortunately, many of the studies have relied on open-ended questions to determine why teens discontinue many forms of physical activity. The data from these studies can be hard to sort out and cannot be quantified. They have not been able to answer questions about why this trend exists. A recent study by Belanger and colleagues in Montreal, set out to take a more systematic approach to solving this riddle and analyzed the self-reported data of 1300 Canadian adolescents. The participants answered questions about their activity levels every three months for five years. The researchers then determined the intensity of activities and compared this information to the likelihood that an activity will be dropped or picked up. The study found that walking was the most likely activity to be picked up or continued by most teens. This is possibly due to the fact that walking also serves the purpose of transportation for many young teens. Team sports with vigorous levels of activity were more likely to be dropped than individual activities with moderate levels of activity. This study does not explain why there is such a drastic drop-off of activity in teens, but it does help elucidate which activities are most likely to be continued and which are most likely to be dropped. Characterizing the phenomenon is the first step in eventually determining the cause of activity drop-off (3).
Together these studies show how effective a controlled, well-defined study can be in helping people understand and eradicate childhood obesity. Efforts can be made to target critical times in a child's life and to encourage sustainable activity as childhood progresses. As more is understood about the decisions that result in childhood obesity society can better combat underage weight issues.

Written for mysdscience.com by Jenn Rust

To read the review click here.

1. K. Janz, S. Kwon and E.M. Letuchy et al., Sustained effect of early physical activity on body fat mass in older children, Am J Prev Med 37 (2009), pp. 35–40.
2.L.L. Moore, D. Gao and M.L. Bradlee et al., Does early physical activity predict body fat change throughout childhood?, Prev Med 37 (2003), pp. 10–17
3.M. Belanger, K. Gray-Donald, J. O'Loughlin, G. Paradis and J. Hanley, When adolescents drop the ball: sustainability of physical activity in youth, Am J Prev Med 37 (2009), pp. 41–49

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