Children who are overweight in kindergarten have four times the risk of becoming obese by eighth grade, researchers reported Wednesday – in just one of the ways they said that the risk of becoming overweight or obese could start even before birth.
Put another way: "Half of childhood obesity occurred among children who had become overweight during the preschool years," the scientists wrote in the New England Journal of Medicine.
And as kids got older, their chances of becoming obese fell.
The researchers set out to track not the prevalence, but the incidence of childhood obesity, about which they said little is known. That is, they wanted to know the rate at which new cases of obesity occurred in children ages 5 to 14 in an effort to get at effective interventions.
"Emerging from the finding that a substantial component of childhood obesity is established by the age of 5 years are questions about how early the trajectory to obesity begins and about the relative roles of early-life home and preschool environments, intrauterine factors and genetic predisposition," the researchers wrote.
In an accompanying editorial, Steven Gortmaker of the Harvard School of Public Health and Elsie Taveras of Massachusetts General Hospital wrote that the study should lead to some ideas about interventions to prevent obesity, with evidence pointing to ideas "that focus on children's environments and that aim to alter early life systems" as probably the most effective.
The researchers, from Emory University, looked at 7,738 children who went to kindergarten in 1998 and were studied over the years through the Early Childhood Longitudinal Study. The results, they said, were consistent with other national studies.
When they started kindergarten, 12.4% of the children were obese; an additional 14.9% were overweight. At eighth grade, 20.8% were obese, 17% overweight. The annual incidence of obesity decreased from 5.4% in kindergarten to 1.7% from fifth through eighth grade, the study showed.
Measured using body mass index – the weight in kilograms divided by the square of height in meters – children at the 50th percentile at age 5 had a 6% probability of being obese by age 14. That probability rose to 25% for 5-year-olds at the 85th percentile and to 47% at the 95th percentile.The prevalence of obesity increased to 20.8% by eighth grade (about 14 years). The researchers found no significant increases from ages 11 to 14.
They looked at some characteristics of the children who were overweight or obese as well.
Though they found no difference between children with a low birth weight and those with a normal birth weight, they found that babies born with a high birth weight (more than 4,000 grams, or about 8.8 pounds) had a significantly higher prevalence of obesity.
Wealthy children – those from the wealthiest 20% of families – had a lower prevalence of obesity in kindergarten than the other children. The highest prevalence was among the children in the second-poorest 20% -- with nearly 26% of them becoming obese by eighth grade.
Among all the children, the highest increase in the prevalence of obesity was in first to third grades, from 13% to 18.6%. Prevalence was higher among Latino children and starting in third grade among African American children.
But although the prevalence of obesity rose with age, the incidence declined, the researchers wrote. From fall to spring of kindergarten, it was 5.4%, and fell to 1.4% among girls and 1.9% among boys per year at fifth grade.