Percentile measures how a child stacks up to others his age. So, a child in the 75th percentile for weight is presumably heavier than 75 percent of other children his age, since children are compared to one another. Therefore, by definition, 25 percent of kids should be in that category.
But with so many kids heavier then they used to be, the old weight distributions may not hold up, Datar said.
There were also fewer kids at the lower end of the weight spectrum. About 14 percent were in the lowest fourth for weight compared with 25 percent in earlier generations and 18 percent were in the second lower quartile compared with 25 percent in earlier generations.
The weight gain accelerated between kindergarten and third grade. The proportion of kids in the top quartile (75th percentile or above) was almost 48 percent by third grade, but weight gain leveled off after that.
Experts said the findings show that to make an impact on skyrocketing childhood obesity rates, programs to encourage better eating habits and more physical activity have to start very early, possibly even in preschool. Those programs also need to include kids who are normal weight.
"If you find your child is in the 75th percentile, it should be warning to you that your child is at higher risk of being an obese adult, and you need to start thinking about what your family is doing as far as eating habits, food intake and exercise," Datar said.
The reasons that America's kids are getting heavier overall aren't fully understood, but there are many possibilities, said Dr. Albert Rocchini, a professor of pediatrics at University of Michigan's C.S. Mott Children's Hospital.
These include the ready availability and convenience of high-fat, high-sugar and highly caloric snack and processed foods and less physical activity because of video games, TV and less outdoor play time. Many families rely more
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