“The leanest and fittest kids are the ones who have the most vigorous activity for longer periods of time,” says Dr. Bernard Gutin, exercise physiologist at the Medical College of Georgia and lead author on the study published in the April American Journal of Clinical Nutrition. “We could not show that the amount of minutes spent in moderate activity – such as walking to school – was related to how fat they were, although it did have some impact on cardiovascular fitness. The implication is that you need to do vigorous activity if you want to be lean.”

Researchers used an accelerometer that discriminates between light, moderate and vigorous physical activity to monitor the usual physical activity of 421 black and white adolescents for five days. Cardiovascular fitness was measured by a treadmill test that went progressively faster at an increasing incline. The most fit took in more oxygen at a lower heart rate. Dual-energy X-ray absorptiometry, which measures bone, fat and fat-free soft tissue (mostly muscle) was used to determine body fat.

Adolescents averaged just five minutes per day of vigorous physical activity across the five days, with black males having the highest average at 8.6 minutes and black females the lowest at 2.8 minutes. They averaged nearly 40 minutes of moderate activity daily. Not surprisingly, boys had less body fat than girls, says Dr. Gutin. White and black females averaged 29.8 percent body fat and 30 percent respectively. Teenage girls should target at or below 25 percent, based on the accurate DXA measurement; over 30 percent body fat is considered obese.

“… (T)hese data suggest that general exercise recommendations for adolescents should encourage vigorous physical activity,” study authors write. However, to help ensure success, obese, unfit children should start with moderate physical activity and an improved diet, then progress to more vigorous activity as they become leaner and fitter.

Researchers noted that children who inherit a predisposition to be unfit or obese may be less likely to engage in vigorous physical activity. However, studies show that cycle can be moved in a positive direction with controlled physical training.

“Based on this study, what you have on average is below an hour and for girls, way below," Dr. Gutin says. Most of the adolescents in this study – and likely generally – need to double their amount of physical activity, he says.

Parents play an important role by influencing how children balance their time, he says. “We still have the same demands for schoolwork. The body has the same requirements for fitness, but all these other things have intruded, television-watching and screen time being the main one. Something has to give.”

He encourages parents to put children in environments where they can be active and safe, such as after-school and weekend programs. “They can do their schoolwork when they get home; they will just not be able to watch as much television.”

Dr. Gutin noted that the findings regarding the need for vigorous activity likely did not translate to sedentary adults whose waistlines and fitness levels would benefit from as little as a daily half-hour walk at a moderate pace.

Co-authors include Dr. Paule Barbeau, MCG exercise physiologist; Dr. Zenong Yin, former MCG exercise scientist now at the University of Texas, San Antonio; and Matthew C. Humphries, MCG research manager.

Note: If no author is given, the source is cited instead.

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