The study, published recently online in JAMA Pediatrics, found that all classes of obesity in US children have increased over the last 14 years. Perhaps most troubling, the study found an upward trend in the more severe forms of obesity – those in which children have a body mass index (BMI) that is 120 to 140 percent higher than their peers.

"An increase in more severe forms of obesity in children is particularly troubling," said Asheley Cockrell Skinner lead author of the study and assistant professor of pediatrics in the UNC School of Medicine. "Extreme obesity is more clearly associated with heart disease and diabetes risk in children and adolescents, and is more difficult to treat."

These findings are based on a new analysis of data collected from 26,690 children ages 2-19 from 1999 to 2012 as part of the National Health and Nutrition Examination Survey (NHANES).

For the purposes of the study, "overweight" was defined as BMI greater than or equal to the 85th percentile for age and sex, and "obesity" was BMI greater than or equal to the 95th percentile. The more severe forms of obesity – Class 2 and Class 3 obesity – were defined as a BMI greater than 120 percent of the 95th percentile for Class 2 and greater than 140 percent of the 95th percentile for Class three. This means that a ten year old boy who is average height (four and half feet tall) and weighs 95 pounds would be considered obese, but would meet criteria for class two obesity at 115 pounds and class three obesity at about 130 pounds.

Using these definitions, the study found that 17.3 percent of US children ages 2-19 were obese in 2011-2012. At the same time, 5.9 percent met criteria for Class two obesity while 2.1 percent met criteria for Class three obesity.

These findings are in contrast to a recent report that showed a decline in obesity among young children in the last decade. Dr. Skinner explains the disparity: "Both our study and the prior one used the National Health and Nutrition Examination Survey. However, the earlier study examined only the last decade, while we make use of all available years - from 1999 to 2012. In 2003, there was an unusual uptick in obesity among young children, which led to the appearance of a significant decline. However, when we look at the bigger picture, that change is not there."

"One of the most important messages is whether we have an environment that allows for activity and encourages a healthy diet for all children, regardless of their weight."

Joseph A Skelton, of Wake Forest University Baptist Medical Center co-author of the study, concurs. He says, "We have made progress in public policy and healthy messages, but more must be done to help families lead healthier lives, day in and day out. The default in our country should not be unhealthy. We must support local changes in the environment, support each other as we try to live healthier lives, and support the health care changes occurring now that will make it easier for families to be healthy. We need to not only change the world, but change how we live in it."