Obesity is caused by a chronic imbalance between energy supplied with food and energy expended. In the greater number of obese children and adolescents, the excess energy supplied is staggeringly low (less than 100 kcal per day). It follows that it must take a long period of risk factors for a child or adolescent with no obesity problem to become obese.

In addition to the immediate and distant psychosomatic complications, overweight and obesity in children and adolescents, may be a risk factor for obesity in adulthood. Studies on the likelihood of adult obesity have found that among children in infancy (1 to 3 years of age) diagnosed as overweight, 19% will be overweight in adulthood. 26% of these individuals will remain obese adults.

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Adverse changes in lifestyle (including diet, physical activity) and health behaviour (increasing external influences - economic, social, cultural factors, advertising) are responsible for the rapidly growing epidemic of obesity in children and adolescents in many countries.

Research in many countries shows that, in parallel with the increasing availability of energy-dense foods, there is a diversion of youth activity towards sedentary lifestyles and a decrease in physical activity.

The propensity to obesity in children and adolescents is primarily genetic (family). Children of obese parents are more likely to be overweight and at risk of future lifestyle diseases. The risk of girls and boys developing obesity increases if one or both parents are obese. The probability that an obese teenager will also be overweight as an adult is 70 per cent, and when one parent is also obese, the risk increases to 80 per cent.