by Kenneth W. Krause.
[Notable New Media]
Kenneth W. Krause is a contributing editor and “Science Watch” columnist for the Skeptical Inquirer. Formerly a contributing editor and books columnist for the Humanist, Kenneth contributes frequently to Skeptic as well. He can be contracted at email@example.com.
Is physical activity, including structured exercise, an effective strategy in the battle against overweight and obesity? In part one of this article, I reviewed evidence suggesting that, despite popular media misinformation, most people who maintain weight loss do so through a combination of diet and physical activity. Nor is it true, as many have reported, that exercisers completely compensate for energy expenditure through increased sedentary time or energy intake.
The National Academies’ Institute of Medicine recently gathered preeminent experts in several relevant fields to summarize the current science exploring “the impact of physical activity in the prevention and treatment of overweight and obesity” (IOM 2015). Here, I discuss the panel’s conclusions relating to children specifically.
The two most common study designs used to examine children’s health are the cross-sectional and prospective longitudinal models. While the former measures the explanatory variable (physical activity, in this case) and the outcome (adiposity) at the same time, the latter measures those variables on multiple occasions. While neither “proves” cause and effect, the longitudinal design is especially capable of supporting inferences that compliment randomized controlled trials by providing important information about real-world patterns.
Panelist Kathleen Janz, professor of health and human physiology and associate director of the University of Iowa Obesity Research and Education Initiative, focused initially on the Iowa Bone Development Study (IBDS), a sixteen-year longitudinal program with which she has been intimately involved since its inception (Kwon et al. 2013, 2015). The IBDS was one of the first to use an accelerometer to more accurately measure physical activity. Janz’s team also used dual-energy X-ray absorptiometry (DXA) to sort body composition into lean, fat, and bone tissues, and to distinguish between visceral and subcutaneous fat.
In the IBDS, Janz and her team followed 500 children from the age of five and, to date, have conducted at least eight clinical exams of each child. Defining obesity as 32 percent body fat in girls and 25 percent in boys, twelve percent of study participants were obese from the beginning. Unfortunately, another ten percent had joined them by the age of nineteen.
So which variables were found to be potentially explanatory? Total sedentary time did not matter, according to Janz. But television viewing time (a subset of total sedentary time) and moderate-to-vigorous physical activity (MVPA) did. Janz explained her findings in the context of a typical eleven-year-old study participant. Averaging every variable other than MVPA, her group discovered a 7.5 difference in adiposity between females with high and low levels of MVPA, and a five percent distinction in males. Averaging every variable except TV time, they revealed a five percent difference in adiposity between girls who watched a great deal of TV and very little TV, and a 9.3 percent difference in boys. When averaging all variables except both MVPA and TV, Janz’s team found an 11.8 percent difference in female adiposity and a whopping 21.3 percent difference in males.
In a recent cross-sectional study of more than 6000 children aged nine to eleven residing at twelve different locations across the world, another group of researchers came to a similar conclusion (Katzmarzyk et al. 2015). The best predictor of reduced obesity, they found, was MVPA, rather than either sedentary time or vigorous-intensity physical activity. More specifically, 55 minutes of daily MVPA was the most reliable predictor of lower obesity rates.
But might inferred causality run in the opposite direction as well? In other words, does higher adiposity predict less physical activity? If so, one would be forced to question the logic underlying the Health at Every Size (HAES) movement, as well as the increasingly popular claim that physical fitness, but not excess adiposity, is the more accurate predictor of superior health outcomes.
Indeed, one accelerometer study showed that, while MVPA at age seven did not predict decreased body fat between the ages of seven and ten, body fat percentage at age seven did in fact predict decreased MVPA between ages seven and ten (Metcalf et al. 2011). More specifically, a ten percent increase in adiposity at age seven was associated with four fewer minutes per day of MVPA at age ten. According to Janz, a “bidirectional relationship” between physical activity and adiposity might signal the existence of a “positive feedback loop.”
Summarizing the data from the IBDS, Janz instructed that children who consistently engaged in at least 45 minutes of MVPA every day “were 60 percent less likely to end up obese at the age of nineteen than children whose level of MVPA decreased as they aged.” This evidence, she concluded, supports the current national guidelines emphasizing at least 60 minutes of MVPA per day and two hours or less of television.
IOM (Institute of Medicine). 2015. Physical activity: moving toward obesity solutions: workshop summary. Washington, D.C.: The National Academies Press.
Katzmarzyk, P.T., T.V. Barreira, S.T., S.T. Broyles, et al. 2015. Physical activity, sedentary time, and obesity in an international sample of children. Medicine & Science in Sports & Exercise 47(10):2062-2069.
Kwon, S., K.F. Janz, T.L. Burns, et al. 2011. Effects of adiposity on physical activity in childhood: Iowa Bone Development Study. Medicine & Science in Sports & Exercise 43(3):443-448.
Kwon, S., K.F. Janz, E.M. Letuchy, et al. 2015. Developmental trajectories of physical activity, sport, and television viewing during childhood to young adulthood: Iowa Bone Development Study. JAMA: Pediatrics 169(7):666-672.
Metcalf, B.S., J. Hosking, A.N. Jeffery, et al. 2011. Fatness leads to inactivity, but inactivity does not lead to fatness: a longitudinal study in children (EarlyBird 45). Archives of Disease in Childhood 96(10):942-947.