Kids who are overweight in kindergarten often become obese by the time they're in middle school, a new study finds.

In the study, children who were overweight at age 5 were four times more likely than normal-weight children to become obese by age 14.

In fact, nearly half of the kids who became obese between kindergarten and eighth grade were already overweight by age 5. [10 Ways to Promote Kids' Healthy Eating Habits]

"Something is getting set in those first five years of life which seems to carry forward," and drive the development of obesity, said study researcher Dr. Venkat Narayan, a professor of epidemiology and medicine at Emory University's School of Public Health. The study cannot determine what those factors are, but they could involve early diet, physical activity patterns, or the mother's diet during pregnancy, Narayan said.

The findings suggest that focusing obesity-prevention efforts on children who are overweight by age 5 may be a way to target the children who are most susceptible to becoming obese, the researchers write in the Jan. 30 issue of The New England Journal of Medicine.

"We need to promote the idea of health weight during the first five years of life," said Narayan, who noted that obesity in young children can be overlooked.

The percentage of U.S. children who are obese has more than doubled over the last three decades, according to the Centers for Disease Control and Prevention.

Many studies on childhood obesity have examined the overall prevalence of obesity, but few studies have looked at the development of new obesity cases over time, the researchers said.

The new study analyzed information from more than 7,700 U.S. children who were in kindergarten in 1998 and were followed until eighth grade.

In kindergarten, about 15 percent of children were overweight, and about 12 percent were obese. (Overweight children have a body mass index in the 85th percentile of kids their age, and obese children have a BMI in the 95th percentile.) By eighth grade, 17 percent of the kids were overweight, and nearly 21 percent were obese.

Between ages 5 and 14, about 12 percent of all of the children in the study became obese. But about 32 percent of children who were overweight in kindergarten became obese by age 14, compared with just 8 percent of children who were normal weight in kindergarten.

The study also found that more than a third of children who were large at birth (more than 8,8 pounds) became obese between ages 5 and 14. Children who were both large at birth and overweight in kindergarten had the highest risk of becoming obese by age 14, the study found. The findings also highlight the importance of research into factors that lead children to become overweight or obese early in life, the researchers said.

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