The decline of childhood obesity rates seen in a couple of recent studies may be nothing more than an illusion, according to a new study published Monday in JAMA Pediatrics.

The researchers looked at data from more than 26,000 children age 2 to 19 in the United States who participated in the National Health and Nutrition Examination Survey. They found that rates of overweight and obese children have been trending upward since 1999, with significant increases seen recently in the number of severely obese children.

Severe childhood obesity rates have more than doubled since 1999, according to the study. In 1999-2000, less than 1% of children fell into the Class 3 obesity category - meaning they had a body mass index 140% higher than their peers. In 2011-2012, 2.1% of children were in the same category. An additional 5.9% met the criteria for Class 2 obesity.



"I think there's certain kids who are at greatest risk for obesity," said lead study author Asheley Skinner, an associate professor of pediatrics at the University of North Carolina School of Medicine. "When you put them in an environment like this one... they're more likely to gain a whole lot of weight. That's part of what's going on."

The risks associated with that extra weight are scary.

Obese children are more likely to have high cholesterol, high blood pressure and type 2 diabetes later in life, according to the Centers for Disease Control and Prevention. They're also at risk for bone and joint problems, sleep apnea and psychological problems due to poor self-esteem. Studies show that obese children and adolescents are likely to remain obese as adults.

A separate study published in the journal Pediatrics this week estimates an obese child will incur anywhere from $12,000 to $19,000 in additional medical costs throughout his or her lifetime compared to a normal weight child.

"What we worry about is not that they're going to be unhealthy now, but if they have unhealthy behaviors that are going to follow them into adulthood," Skinner said.

Body mass index for children is calculated a bit differently than it is for adults. While children's height and weight are still used, whether they are "normal," "overweight" or "obese" is determined by a percentile. Most parents are familiar with percentiles, as they are commonly used to chart children's growth in the United States.

A healthy weight child falls between the 5th and the 85th percentiles. An overweight child is the 85th to 95th percentile; an obese child is above the 95th percentile.

Researchers categorize the severity of obesity in levels or "classes." For example, a 10-year-old boy of average height - about 4.5 feet tall - would be obese at 95 pounds, Class 2 obese at 115 pounds, and Class 3 obese at 135 pounds.

Skinner and her colleague, Dr. Joseph Skelton, found that all classes of childhood obesity have increased over the last 14 years. The most significant increases were seen in the severe obesity classes, elementary school girls and adolescent boys. When broken down by race, obesity rates were highest for Hispanic and white girls, and African-American boys.

A few weeks ago, another JAMA study concluded obesity rates in preschool-age children have decreased significantly over the past decade. The study authors used data from the same survey as Skinner, but came up with different results. Why?

The key word is "decade," Skinner says.

The researchers for that study only looked at data from 2003-2004 to 2011-2012. Their starting point - 2003-2004 - is a "blip" in the data, Skinner says, where the rates shot up and then came right back down. Beginning with that year skews the overall pattern, making it look like a decrease. But Skinner, who looked at a longer period, found a slight upward trend.

The overall takeaway from this survey data is that too many children are still overweight or obese, Skinner says, despite education pushes and policy changes.

"It's frustrating to put so much effort into (fighting childhood obesity) and not get any apparent results," she said. "We really need to think about this as a larger, cultural issue."