A new study finds that much of a child's "weight fate" is set by age 5, and that nearly half of kids who became obese by the eighth grade were already overweight when they started kindergarten.
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.
The study's findings do not mean that it's too late for schools to act, but their best tactic may be to focus on kids who are overweight and try to encourage exercise and healthy eating, Cunningham said.
The work also shows the need for parents, doctors, preschools and even day care centers to be involved, said Dr. Stephen Daniels, a University of Colorado pediatrician and a spokesman for the American Heart Association.
Parents who are concerned about a child's weight should talk with their child's doctor, because it may be hard to tell what is normal at various ages and appearances can be misleading. In children, obesity and overweight are defined by how a child ranks on growth charts that compare them to other kids the same age and gender. Kids at or above the 85th percentile are considered overweight, and obese at the 95th percentile or above.
No child should be placed on a diet without a doctor's advice, the federal Centers for Disease Control and Prevention advises. To help keep kids healthy, balance the calories a child gets from food and beverages with how much exercise he or she gets to allow enough for normal growth - some weight gain is normal, the CDC says.
"You can change your fate by things that you do early in life," with more exercise and eating a healthy diet, Daniels said. "Once it occurs, obesity is really hard to treat. So the idea is we should really work hard to prevent it."