The prevalence of weight problems has long been known — about a third of U.S. kids are overweight or obese. But surprisingly little is known about which kids will develop obesity, and at what age.
Researchers think there may be a window of opportunity to prevent it, and ''we keep pushing our critical window earlier and earlier on,'' said Solveig Cunningham, a scientist at Emory University. ''A lot of the risk of obesity seems to be set, to some extent, really early in life.''
She led the new study, which was published in this week's New England Journal of Medicine and paid for by the federal government.
It tracked a nationwide sample of more than 7,700 children through grade school. When they started kindergarten, 12 percent were obese and 15 percent were overweight. By eighth grade, 21 percent were obese and 17 percent were overweight.
Besides how common obesity was at various ages, researchers focused on the 6,807 children who were not obese when the study started, at kindergarten entry. Here are some things they found:
WHO BECAME OBESE: Between ages 5 and 14, nearly 12 percent of children developed obesity — 10 percent of girls and nearly 14 percent of boys.
Nearly half of kids who started kindergarten overweight became obese teens. Overweight 5-year-olds were four times as likely as normal-weight children to become obese (32 percent versus 8 percent).
GRADE LEVELS: Most of the shift occurred in the younger grades. During the kindergarten year, about 5 percent of kids who had not been obese at the start became that way by the end. The greatest increase in the prevalence of obesity was between first and third grades; it changed little from ages 11 to 14.
RACE: From kindergarten through eighth grade, the prevalence of obesity increased by 65 percent among whites, 50 percent among Hispanics, almost 120 percent among blacks and more than 40 percent among others — Asians, Pacific Islanders and Native Americans and mixed-race children.
By eighth grade, 17 percent of black children had become obese, compared to 14 percent of Hispanics and 10 percent of whites and children of other races.
INCOME: Obesity was least common among children from the wealthiest families and most prevalent among kids in the next-to-lowest income category. The highest rate of children developing obesity during the study years was among middle-income families.
BIRTHWEIGHT: At all ages, obesity was more common among children who weighed a lot at birth — roughly 9 pounds or more. About 36 percent of kids who became obese during grade school had been large at birth.
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