After writing my last post on exercise, I came across this article, which has some very important implications for young people. Again, it shows how complex the question “who needs exercise?” is and how important it is to specify “for whom?” and “exactly what do you mean by exercise?”
by Bernard Gutin, PhD
My colleagues and I at the Medical College of Georgia investigated the relationship among diet, physical activity and body composition in 661 African American and white adolescents ages 14 to 18.1 We hypothesized that fatter youths would have higher levels of energy intake and lower levels of both moderate and vigorous physical activity.
To our surprise, we found that higher levels of percent body fat were associated with lower levels of energy intake and lower levels of vigorous (but not moderate) physical activity. Youths who did the most vigorous physical activity and consumed the most calories were the leanest. Those who did no vigorous physical activity had a percent body fat of 28.6 and consumed 1744 calories a day, while those who did at least 1 hour of vigorous physical activity each day had a percent body fat of 19.4 and consumed 2203 calories a day. See Relationship of Vigorous Physical Activity, Caloric Intake and Percent Body Fat.
Although moderate physical activity, such as brisk walking, burns calories, we found that lower percent body fat was linked to greater amounts of vigorous, but not to moderate, physical activity. Vigorous activity includes sports, games and dance activities such as running, swimming, soccer and aerobic dancing. These activities impart a significant “mechanical load,” which means they work your body’s muscles and bones. This type of activity stimulates stem cells to differentiate into bone and muscle rather than fat.2 A healthy body composition in youths requires both a large amount of vigorous physical activity and ingestion of sufficient calories and nutrients to support this tissue-building process.
This idea is further supported by experimental studies that looked at the effect of mostly vigorous physical activity, without restriction of calories, on body composition. Research on youths with varying levels of fatness and fitness found moderate physical activity to be ineffective in preventing obesity, so we conducted studies using 300 to 400 minutes a week of mostly vigorous physical activity and found positive effects on body composition, including reduction of visceral adipose tissue (the fat around abdominal organs). Within the intervention groups, those youths who participated regularly and maintained the highest heart rates during the physical activity sessions showed the greatest decreases in percent body fat and the greatest increases in bone density.3,4
Youths who are obese and unfit can benefit from exercise of relatively low intensity and duration. For example, in obese youths, studies using 155 to 180 minutes per week of physical activity at moderate to high intensity produced favorable reductions of percent body fat and visceral adipose tissue and increases in bone density and aerobic fitness.5 As children improve in fitness, they should be encouraged to progress to higher amounts and intensities of physical activity.
An expert consensus panel has suggested that youths engage in at least 420 minutes a week (about 60 minutes a day) of moderate to vigorous physical activity.6 The research reviewed here suggests that greater emphasis should be given to vigorous rather than moderate physical activity.
Taken together, these findings suggest that a paradigm shift is needed to improve the effectiveness of pediatric obesity prevention interventions. It is well known that eating a nutritious diet supports the development of muscles and bone and other aspects of proper growth and development, as well as good health. However, limiting energy intake runs counter to the biologic demands of growth, which require adequate calories and nutrients. When youths engage in adequate amounts of vigorous physical activity, calories and nutrients are preferentially directed to the production of lean tissue (muscle and bone) rather than fat. So, insuring a high quality diet and plenty of exercise, rather than calorie restriction, is the model to pursue to prevent obesity and improve body composition.
* A longer version of this editorial will soon appear as a Perspective article in the journal Obesity.
Bernard (Bob) Gutin, PhD, is Adjunct Professor of Nutrition at University of North Carolina in Chapel Hill and Professor Emeritus at Teachers College of Columbia University and the Medical College of Georgia.