One in eight children in the U.S. is obese when they enter kindergarten, with the ratio increasing through the elementary school years, a nationwide study found.

By eighth grade, one in five U.S. students is obese and another 17 percent are overweight, according to the study published in the New England Journal of Medicine. Most of the increase takes place before fifth grade, giving researchers key information about the obesity epidemic among American children and the ages at which they are most vulnerable.

Obesity has more than doubled in children and tripled in adolescents from the early 1980s, though researchers say the rates may now have plateaued. The condition puts people at risk for diseases such as Type 2 diabetes, stroke and arthritis.

The study released yesterday found that 5-year-olds carrying extra weight for their age were four times more likely to become obese during the elementary school years. Almost half of those who developed obesity were overweight when school started.  In the study, a child was considered overweight if they were above the 85th percentile for weight and obese if they were above the 95th percentile as calculated by the U.S. Centers for Disease Control and Prevention.

The study was funded by the National Institute of Child Health and Human Development. The researchers analyzed data from 21,260 kindergartners enrolled in the Early Childhood Longitudinal Study in 1998. More than 9,000 were followed through the eighth grade, providing a representative sample of the 3.8 million children the same age across the U.S.

“We speculate that obesity-prevention efforts that are focused on children who are overweight by the age of 5 years may be a way to target the children who are most susceptible to becoming obese during later childhood and adolescence,” said the researchers led by Solveig Cunningham of Emory University’s department of global health in Atlanta. First lady Michelle Obama has focused on childhood obesity, urging food companies to make healthier products and encouraging more exercise with her “Let’s Move” campaign.