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I want another chicken nugget!" shouts John Presley, with all the indignation a 5-year-old can muster. His mom, Theresa, 41, firmly says no, and instead hands her son a small Weight Watchers toffee crunch ice cream treat. He licks it contentedly, oblivious to the fact that for the last year he has been on a diet. "I tell him it's food that will make him strong," says his mom.

Not so long ago, a plump baby was a sign that parents were doing something right. In the U.S.—where, according to the Centers for Disease Control, 14 percent of kids 5 and younger are obese—that's no longer true. Some doctors are beginning to put children on diets if their body mass index—a ratio of height and weight—hits the 95th percentile or above, even if they're as young as 2. Dr. David Ludwig, founder of the Optimal Weight for Life (OWL) Clinic in Boston, one of a handful of programs catering to obese children, says so many kids are overweight that parents often fail to recognize warning signs for health problems such as diabetes in their kids. "Being heavy has become normalized," he says.

Experts urge caution before rushing small children into a diet. "These children are growing and need to be nourished to grow," says Marilyn Tanner-Blasiar, a spokeswoman for the American Dietetic Association. And discussing weight at such a young age could cause children to grow up with an unhealthy fixation on body image. "You don't want to make kids unhappy with their bodies," says Kelly Brownell, head of Yale University's Rudd Center for Food Policy and Obesity.

John was a normal 8 lbs. at birth, but with a steady diet of mac and cheese and two hours of daily TV, by the time he was 4 he stood 44 in. tall and weighed 84 lbs., the average weight of an 11-year-old, putting his BMI in the 98th percentile. "I'm a working mom and am always rushing around," explains Theresa, a coronary care nurse from Marshfield, Mass. "I just put premade food into the microwave." His doctor referred John to the OWL Clinic, where they discovered alarming signs of potential health problems, including a thickening of the skin around his neck that can be a precursor of diabetes.

With help from a nutritionist at the clinic, John's parents tossed the sugary cereals and Goldfish and put the whole family on a fruit, vegetable and high-protein diet. They also devised an exercise regimen for John that included daily walks and roughhousing with his 3-year-old brother James. To prevent an obsession with weight, John's doctor recommended that his parents avoid the word "diet" and talk about healthy living instead.

But sticking to a good eating plan can be a challenge for families. The Matlacks of Parma, Ohio, enrolled their then-3-year-old daughter Melody in a weight-loss program at Rainbow Babies & Children's Hospital in Cleveland when their pediatrician told them that her BMI was in the 98th percentile. The family immediately switched her to a diet rich in fruits and vegetables, enabling Melody to grow into a healthy height and weight over the next two years. But as Melody's dad, Richard, admits, it was more a lesson of "Do as I say, not as I do." Says Matlack, who struggles with his own weight: "When we eat dinner, the children will eat apple slices and I'll have a second helping."

As for John Presley, after a year on his diet he's dropped 1 lb. while growing 5 in.—a success story at any age. Though he still indulges in the occasional chicken nugget or slice of birthday cake, John has discovered he actually likes some healthy foods. "He never ate fruit before," says his mother. "But now he asks for cantaloupe and strawberries. And that makes me feel good, very good."

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