One of the biggest misconceptions is a person's weight is only about calories in, calories out, says Dr. Jack Yanovski, head of the Growth and Obesity Clinic at the National Institutes of Health. For example, if you see someone who is heavy, "Yes, of course they have to be eating more than they burn, but that's at a very simplistic level," Yanovski says.
Yanovski says genetics plays a critical role.
"We know that there are a lot of processes going on in the body that promote digestion, that decides how quickly you eat, how quickly your stomach feels full, how quickly your brain receives that signal, how much the energy will be put into fat cells, how much will be used to burn, all those things can be affected by different genetic variants, and the treatments for each of those will probably be different," he says.
"If we take that same person, born of two heavy parents, transplant him into another family, where he's adopted by two lean parents, that person would still be obese as an adult. And the estimate we have of how likely that is to be is like 60 to 80 percent," he says.
Researchers differ on the importance of what some call our "genetic burden." Depending on whom you speak to, experts cite figures ranging from 40 to 90 percent for what percentage of obesity can be attributed to hereditary factors.
But experts do agree on one thing: Your genes don't have to be your destiny. Yanovski estimates fewer than 100 people in the world have such severe gene defects that they cannot lose weight with diet and exercise.
Yanovski says research about childhood obesity is still in its infancy. In 2010 the NIH published a study showing teenage boys going through puberty ate almost 2,000 calories on average at a buffet meal. Before that, researchers just knew it was a lot. Now, they're taking it a step further, studying exactly which foods teens choose and why.
Jaci Zocca coordinates a research study with teenagers for Yanovski. She lays out a buffet including cold meats, cheeses and fruit, as well as peanut butter and jelly, candy and jelly beans, even some chicken nuggets.
Teenagers fast for 10 hours before they come here and listen to a recorded message: "Please eat until you are no longer hungry..."
Yanovski says some teens eat 14 sandwiches.
"The whole buffet has 12,000 calories. We have had some folks who've eaten 5- and 6,000 calories," he says.
After they're full, teens are shown a five-minute video clip -- either one that's sad or one of birds flying. Then they're given another buffet of ice cream, candy and chips. Researchers study whether certain teens react to the emotional triggers by eating even though they're full.
Yanovski also studies how teens exercise. He says doctors often tell parents to focus on maintaining their child's weight. But studies conducted at NIH show these children actually have to shed pounds, not just maintain their existing weights.
"It's not enough just to say, 'Oh, they'll be fine. Let them grow into their weight.' There are some exceptions, and those children are very lucky, but for the vast majority of kids, being heavy at a young age is going to predict being heavy in adolescence and in adulthood," Yanovski says.
Sara Bleich is an assistant professor in the Department of Health Policy and Management at Johns Hopkins University. She says changing teen behavior is difficult. For one, they're young and aren't thinking of long-term health consequences. Also, there's a time lag between eating something and putting on weight.
"We often tend to err toward, 'I'll just have that cookie today and worry about my skinny jeans tomorrow!'" Bleich says.
"For the vast majority of kids, being heavy at a young age is going to predict being heavy in adolescence and in adulthood."
Another problem may be understanding calorie counts,so Bleich says just labeling food and drinks is not enough. She conducted an experiment with teenagers at corner stores in Baltimore. A coke machine in the store would have a bright sign with one of three messages. Teens would see only one message at a time.
"The first message would say, 'Did you know a bottle of soda has about 250 calories?' which is directly consistent with what you're seeing now in restaurants," she says. "The second message would say, 'Did you know that a bottle of soda, to burn it off would take about 50 minutes of running?'And the third was making it as a percent of daily value, so, 'A bottle of soda is about 10 percent of your daily intake,'" she says.
Bleich says her hypothesis is that teens in the second group would make better choices because it's practical information they understand, instead of an abstract concept, such as calories.
"People think if you do a little bit of exercise, you're burning off a lot of calories, and that is not the case," she says.
Researchers are tackling childhood obesity from various angles: how adjusting the price in vending machines can influence student choices, how the placement of foods in school cafeterias can change selections, and how food packaging affects children's preferences.
But Professor Maureen Black with the University of Maryland School of Medicine says it's not just about changing the menu and increasing physical education. It's about what she calls "altering the social norm."
"Meaning that it is cool to eat healthy things, and it is not cool to be eating donuts," she says. "We try to change the ethos of the school, that it's cool to take care of yourself," she says.
Black's strategy involved airing several "healthy" messages before classes, having older girls mentor younger ones and introducing activity clubs. But changing the way people think takes time.
Dawn Witherspoon, a post-doctoral fellow who works with Black, found girls who were overweight weren't concerned about their body size and had healthy self esteem, until they became obese. She says that's challenging.
"How do we intervene if they don't think it's a problem? That's the part that I think is a little scary," Witherspoon says. "If overweight becomes the new normal, if people don't see it as a problem, then how to you make a change?"
Experts agree that early intervention is best. Dr. Alan Schuldiner is with the University of Maryland School of Medicine. He studies the DNA of the Amish population in Lancaster, Penn., hoping their fairly homogeneous gene pool will provide some clues about obesity. While it's rare to find an obese Amish child because they are so active, "Amish adults are just as obese as the general population," he says.
That's partly because as the adults reach their 50s, their physical activity decreases after their children take over farming, according to Schuldiner. Also, Amish women have many more children. He says this finding has implications in the broader conversation about obesity.
"Some pediatricians suggest that if we could cure or prevent childhood obesity, we would solve the adult obesity problem. And I think that the Amish are telling us that's probably not true," Schuldiner says.
In other words, maintaining a healthy weight is a lifelong challenge.
This is part two of a five-part series on childhood obesity in America from WAMU 88.5 News.