What role does nutrition play in how we prevent obesity in children and adolescents?

A lot. Obesity in children or adolescents or adults is due to imbalance of intake and expenditure of calories. A simple view is that obesity is the result of taking in more calories than are expended. Caloric restriction and increase in physical activity are both important in controlling obesity.
It's half. Nutrition and exercise together are the way to preventing obesity in children (and adults). Kids should eat fresh, healthy, non-processed foods most of the time. Kids also must be active (really active, not just walking) for an hour a day. They need to drink water and avoid all sweet liquids (juice, pop, gatorade, chocolate milk, etc.).
EXTREMELY IMPORTANT. There is a large body of medical evidence that proves without a doubt the strong link between nutrition and health.

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What role does nutrition play in how we assess, treat, and prevent obesity in children?

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