ATLANTA, Georgia (CNN) -- Adrian McHargh grew up active and skinny in Kingston, Jamaica. An enthusiastic swimmer, he had the pristine waters of the Caribbean for a playground until two years ago, when he and his family moved to America.
Before coming to America, Adrian McHargh played on the beach and ate healthy foods.
"I would always cook a healthy dinner," the 13-year-old's mother, Visha Siew-Narine, says about mealtime on the island. "When he came here, I think the fascination of having all this food that we couldn't afford in Jamaica, or that wasn't really available, that was kind of new to him."
In suburban Atlanta, things changed fast. The new foods were cheap, unhealthy and hard to resist: toaster waffles, corn dogs and the chocolate syrup Adrian liked so much he guzzled it straight from the bottle.
"I would eat a lot because I was bored," he says. "I was sitting on the couch just watching TV, not playing a lot."
Within months, Adrian was in trouble. He'd packed on 30 pounds. Doctors said he had hypertension, high cholesterol and was at high risk for type 2 diabetes.
Pat Crawford, a co-director of the Center for Weight and Health at the University of California, Berkeley, says cases like Adrian's are typical.
"We really would like to encourage immigrant families to continue the kinds of eating that they ate in their country of origin because our studies show that the longer they've been in this country the more likely that their children are going to get fat," Crawford says.
The concern isn't only for the children of immigrants.
Since 1980, the number of overweight teenagers has more than tripled. The Centers for Disease Control and Prevention reports the heaviest children, those considered to be obese, are increasingly being diagnosed with type 2 diabetes. Two decades ago, type 2 diabetes was known as "adult-onset diabetes." Quiz your knowledge on what foods contain real fruit »
Dr. Seema Csukas, a pediatrician at Children's Healthcare of Atlanta, says overweight children as young as 5 and 6 years old are developing heart and joint problems.
"It's not only the adolescents -- it's the very young kids," Csukas says. "We've seen 3- and 4-year-olds who are in the overweight category, so it's very sad to think that that's even possible, but it's a reality today."
Adrian's diagnosis alarmed his mother, who signed him up for a free 12-week exercise, diet and nutrition program developed by Csukas' hospital to help children like Adrian and their families do better.
"You hear all these horror stories about people who are diabetic," she says. "Grown people, adults, who've had wounds that didn't heal and had to get a leg or, you know, a hand amputated. I don't want that for him, not at all."
For three months, Adrian worked with a fitness coach and a nutritionist to reduce his body mass index, or BMI. For exercise, the program prescribed more play outside and a dumbbell routine to add muscle. To change his diet: an end to the junk food. See how Adrian is getting control of his weight »
Adrian's mother, who like her son and daughter, Marisa, admits she bought into the fascination of all the new foods available to her in America, says she's stopped buying so many sweets.
"I just wanted to make them happy," she says. "I'm buying more eggs and more yogurt. More organic foods -- organic peanut butter. I'm doing a lot of vegetables -- fresh vegetables. Carrots. Potatoes. Broccoli."
Adrian, who loves the water, was playing in it again -- hurling water balloons at his friends from his front porch. His mother says she's seen changes in her son. More muscle. Less flab. Improved self-esteem.
"They used to make fun of me at school, but I don't feel bad any more," Adrian says.
His doctors have also seen improvement. After a few weeks on the program, Adrian's cholesterol was normal. In August, he started his eighth-grade year with a clean bill of health.
"I've made super changes," he says. "I'm excited, especially for school because everybody says I've changed. They said, 'Adrian, you've lost a lot of weight.'" E-mail to a friend
John Bonifield is an associate producer with CNN Medical News.