WEDNESDAY, Oct. 3, 2018 (HealthDay News) -- 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 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.
Dr. Antje Korner, the senior researcher on the study, agreed.
The roots of obesity are "complex," and involve a mix of genes and environmental influences, said Korner, of the University of Leipzig's Center for Pediatric Research in Germany.
What's important, said Korner, is that parents and pediatricians are aware of this "critical" age window where obesity risk manifests itself.
When children that age are overweight, she said, it should not be dismissed as baby fat that will go away.
In the United States, 1 in 5 children and teens is overweight, according to the U.S. Centers for Disease Control and Prevention. This sets them up for future health problems such as diabetes and heart disease.
The current study was based on 51,505 kids whose BMI was measured repeatedly during childhood and adolescence.
In most cases, the study found, normal-weight teens had been normal-weight their whole lives. But the picture was different for obese teens: Before age 2 they usually had a normal BMI, but 53 percent had become overweight or obese by age 5.
And children with an early "acceleration" in BMI during the preschool years were at heightened risk of teen obesity. Their odds were 40 percent higher, versus kids whose BMI held steady during preschool.
For now, Freemark said, it's not clear whether changing those youngsters' diets will ward off teen obesity.
But he encouraged parents to take their kids to routine checkups, to see whether their weight gain is on track. If it's excessive, Freemark said, parents might need nutrition counseling.
He also pointed to some general diet rules that are "reasonable for any child" -- such as avoiding sugary drinks, sweets and fast food, and limiting fried foods and starches.
The Nemours Foundation has more on childhood obesity.