The U.S. population is growing—and no one has to tell you that a lot of that growth is happening at the waist.

Two thirds of adults are overweight or obese, according to the U.S. Centers for Disease Control and Prevention. That extra baggage heightens the risk for heart disease, stroke and type 2 diabetes. But less is known about the 17 percent of children and teens who meet the more arbitrary criteria for childhood obesity, a proportion that has tripled over the past three decades.

The good news is that the epidemic may be abating, according to a report published May 6 in The Lancet. The prevalence of childhood obesity has reached something of a plateau since 2001.  

But without firm guidelines on how much extra weight is too much or how best to tackle it in growing kids, the problem of obesity in the young remains a complex challenge, researchers say. "There's still so much unknown about childhood obesity," says the report's senior author Sue Kimm, an expert on childhood obesity from the University of New Mexico School of Medicine in Albuquerque.

Experts often point to the body-mass index, or BMI, as a convenient measure of excess weight; the index is calculated by dividing body weight in kilograms by the square of height in meters. "In adults, a BMI of 25 kg/m² is considered overweight and 30 kg/m² is obese, based on evidence. If your BMI is greater than 25 kg/m², you begin to have problems such as elevated blood pressure, abnormal blood lipid levels, insulin resistance, and these increase with increasing BMI."

Insurance companies such as Metropolitan Life have carefully gathered such evidence since the 1940s to identify higher-risk clients. The National Institutes of Health expanded this information base by pooling large longitudinal studies such as the Framingham Study for these BMI cutoffs.

"But in children, we don’t have such evidence-based information," Kimm says. "We don't know how much is too much."

The U.S. Centers for Disease Control and Prevention defines obese children as being in the 95th percentile of children the same age and gender. Studies suggest that obese children are prone to developing metabolic and cardiovascular diseases, just like obese adults. But they might also suffer from vitamin and mineral deficiencies and musculoskeletal problems because of the burden of excess weight on their developing bodies.

Prevention is considered the best approach to tackling obesity, especially in young people, according to the report. On the surface it seems obvious: teach kids to eat less and exercise more. But in practice, preventing vulnerable kids from putting on the pounds has proved a major challenge. When tested in clinical trials, many prevention strategies have failed.

"Our caloric recommendations may be too generous," Kimm says. Many "weight-loss diets" may be energy neutral or even lead to weight gain in sedentary female teens, she says. According to a 2003 report in Science , an energy deficit of at least 250 calories per day is necessary to stave off further weight gain in overweight children. That's equivalent to consuming only four fifths of the usual calorie intake or putting in an extra two hours of walking every day.

Weight-loss surgery should be considered only in the most severely obese of children, according to the report. "If gastric bypass is done, it’s usually in teenage girls or boys—that’s that time of peak bone-mass formation," Kimm says. That's when children should be trying to build stronger bones, as this period is their last chance to do so, she explains.

For most cases, though, smarter food choices are the key. Beyond better food labeling to help parents choose, some advocate more government intervention, such as limiting the availability of energy-dense snacks by removing school vending machines. But the problem often lies in the home: a 2002 report in Obesity Research found that snacks in the home constituted 69 percent of a child's calories (only 2 percent came from vending-machine snacks). “We should be teaching children better behaviors," Kimm says. "It comes down to how we live, how we eat and how we move."