Jan. 30 (Bloomberg) — One in eight children in the United States is obese when they enter kindergarten, with the ratio increasing through the elementary school years, a nationwide study found.

By eighth grade, 1 in 5 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. 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.