Aug. 21, 2010 -- The heaviest American children and teens are only getting bigger, researchers found.
Overall measures of body mass index (BMI), waist circumference, and triceps skinfold thickness have increased over a 10-year period, but these increases were more marked in the heaviest 20 percent of children, May A. Beydoun and Dr. Youga Wang of Johns Hopkins reported in the International Journal of Pediatric Obesity.
"Heavier children and adolescents gained more adiposity, especially waist size," Wang said in a statement.
The increases in obesity were also more pronounced in black girls and other ethnic groups.
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For their analysis, the researchers looked at data from the National Health and Nutrition Examination Surveys (NHANES) from 1988-1994 and 1999-2004. To assess adiposity, they looked not only at the common measure of obesity known as body mass index (BMI), but also waist circumference and triceps skinfold thickness.
Beydoun and Wang found that the mean 10-year increases in fatness were statistically significant, and were higher in older groups, in some sex-specific ethnic groups (i.e. black girls), and in the more obese groups.
Over the 10 years, the increase in mean BMI was 0.64 kg/m2 in boys and 0.60 kg/m2 in girls.
Waist circumference rose "substantially," Beydoun and Wang wrote, by a mean of nearly an inch among boys and and more than an inch among girls.
Triceps skinfold thickness increased especially among boys -- by a mean of 1.18 mm -- but also went up in girls, by 0.81 mm.
Racial and sex disparities in mean BMI became wider over time, particularly when comparing black and white girls, the researchers said. For instance, black girls had mean increases of 1.34 kg/m2 compared with 0.42 kg/m2 increases for white girls.
Mean BMI among white girls grew from 19 kg/m2 to 19.7 kg/m2, compared with an increase from 19.8 kg/m2 to 21 kg/m2 among black girls, the researchers said.
Shifts were also more dramatic in adolescents than in 6- to 11-year-olds, they found, with the older group gaining a mean 0.90 kg/m2 compared with 0.56 kg/m2 for the younger group.
And there were larger increases across all four measurements of adiposity for kids in the heaviest quintiles compared with the lowest.
The thinnest white adolescent girls, for example, had a mean BMI increase of 0.03 kg/m2 and waist circumference increase of 0.61 inches, but their heavier counterparts had mean BMI increases of 1.38 kg/m2 and waist circumference of 1.46 inches.
Skinny white adolescent boys lost a mean 0.43 kg/m2 over that time, while the heaviest ones gained a mean 1.74 kg/m2.
The thin boys' waists grew only 0.9 inches over the 10 years, compared with 3.19 inches for the heaviest boys.
Research has shown that increases in waist circumference poses greater health risks than elevated BMI.
"Solely examining the changes in the prevalence of overweight and obesity based on fixed BMI cut points could not gain such important insights regarding shifts in the obesity epidemic," Wang said.
Thus, she said, "waist circumference is a better predictor of future health risks, such as for type 2 diabetes and heart disease in adults."
The researchers suggested that the age disparities may be explained by lifestyle differences, including higher intake of energy-dense foods and sedentary behavior.
The study was limited by its cross-sectional nature, but the researchers concluded that "more vigorous efforts should be made to understand the underlying causes" of the increases in obesity and the discrepancies between groups.