Preschoolers who quickly pack on pounds may be at particular risk of becoming obese teenagers, a large new study finds.
Experts said the results point to a critical "window" in early childhood -- between the ages of 2 and 5 or 6 years -- that can set the stage for persistent obesity.
They also said the findings leave some questions unanswered. Why do some young children gain excessive amounts of body fat? And will altering their diets ultimately prevent obesity later on?
The study looked at weight-gain patterns among more than 51,000 German children.
Researchers found that more than half of obese teenagers had already become overweight or obese by age 5. And the strongest risk factor for teen obesity was faster-than-normal weight gain between the ages of 2 and 6.
The findings are "very important," said Dr. Michael Freemark, who wrote an editorial accompanying the study. Both were published Oct. 4 in the New England Journal of Medicine.
"This identifies a developmental window that's a powerful predictor of obesity in adolescence," said Freemark, a professor of pediatrics and chief of the Division of Pediatric Endocrinology at Duke University in Durham, N.C.
He explained what the normal trajectory of weight gain looks like early in life. First, babies pack on a lot of fat, which is healthy and necessary, Freemark said. By the time they are 1 year old, their body mass index (BMI) should be much higher than it was in early infancy. (BMI is a measure of weight in relation to height.)
After that, BMI declines as a child grows taller and sheds "baby fat" -- hitting a low around the age of 5 or 6. Then, Freemark said, there is a body-fat "rebound" where BMI gradually rises through the rest of childhood and the teen years.
According to Freemark, this study suggests that if the rebound happens early -- or if children never lose their baby fat -- the risk of teen obesity rises.
"What this doesn't tell us is why those children gained weight excessively during this developmental window," Freemark said.
There may be a role for factors earlier in life -- such as whether kids were breastfed or bottle-fed, or whether their moms had diabetes during pregnancy, according to Freemark.
"But it's probably related to a number of variables," he said.