June 1, 2012 (San Francisco, California) — How much of a child's body is fat and how much is lean depends more on physical activity than on diet, researchers reported here at the American College of Sports Medicine 59th Annual Meeting.
A study of 734 middleclass Australian children of average weight showed that those with a higher ratio of fat to lean were more active, but actually consumed fewer calories. "The kids who are fatter eat less and they exercise a hell of a lot more," said Richard D. Telford, PhD, an exercise physiologist at the Australian National University in Canberra.
Whether diet or exercise matters more in body composition has stimulated vigorous debate, with leading experts taking opposite positions, Dr. Telford said.
To help answer the question, Dr. Telford and his colleagues examined suburban children whose socioeconomic status was similar to the Australian average. They took measurements when the children were 8, 10, and 12 years of age.
They used a variety of instruments to measure the health of the children. Dual-energy x-ray absorptiometry was used to obtain body fat percentage; 7-day pedometers were used to obtain average daily physical activity; 7-day accelerometers were used to obtain average daily moderate to vigorous physical activity (in the final year only); and parent- or teacher-assisted 2-day records were used to measure total daily energy, fat, carbohydrate, and sugar intake.
They also recorded the stage of puberty, using Tanner staging.
Dr. Telford showed a graph of percent body fat and Physical Activity Index scores (approximately equal to the square root of the number of steps per day); increased body fat correlated with increased physical activity in a straight line. The correlation was statistically significant for boys and girls (P < .001 for both).
On another graph, more moderate to vigorous physical activity corresponded to a lower percentage of body fat. Once again, the finding was significant (P < .001 for boys and P < .01 for girls).
"If we looked at any of the age groups separately, we got exactly the same results," said Dr. Telford.
To give an example, Dr. Telford said that an average 12-year-old boy in the study would have half a percentage point less body fat if he took 2100 steps per day more than another boy of the same age (e.g., 24.5% vs 25.0% body fat).
Dr. Telford and colleagues also found a correlation between diet and body fat for boys, but it was the opposite of what they expected. The fatter boys consumed less sugar, less fat, fewer carbohydrate calories, and fewer total calories than the leaner ones.
These correlations in boys were only statistically significant for carbohydrates (P = 0.1), but they approached significance for sugar (P < .06) and total calories (P = .05).
The researchers found no significant relation between diet and body composition in girls.
"I don't want to underestimate the importance of diet," said Dr. Telford. "That would be foolish."
But in this population, physical activity clearly makes a bigger difference to body composition than diet, he concluded.
Session moderator John M. Jakicic, PhD, an exercise physiologist from the University of Pittsburgh in Pennsylvania, asked whether the percentage of fat was the most important measurement to target when trying to improve children's health.
"I think it might be the total amount of fat that's critical," he said. He isn't ready to give up using body mass index either. "The more body weight you carry, the more physical problems you have."
Other research has shown the importance of diet on these measures of health, he said.
Finally, he pointed out that public health advocates are already focused on activity in children.
"Physical activity is an important target," he said. "How to get the lowest-activity kids to be more active is the greatest challenge."
Dr. Telford and Dr. Jakicic have disclosed no relevant financial relationships.
American College of Sports Medicine (ACSM) 59th Annual Meeting: Abstract 656. Presented May 31, 2012.
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