The onset of overweight often dates back to infancy and early childhood. It is most often the result of overfeeding, i.e. an energy supply that exceeds the child's ability to use it. Excess fat is mainly deposited on the abdomen, face and limbs. In families with a genetic predisposition to obesity, it can influence the perpetuation of overweight and obesity in adulthood. In children up to 6-7 years of age, there is uniform fatness throughout the body. After puberty, the distribution of subcutaneous tissue can vary according to gender and type of obesity. Overweight also occurs during adolescence.

In highly developed countries, obesity is estimated to affect 15-25% of children and adolescents, and the prevalence is steadily increasing. Other data from developed countries show that approximately 12% of children aged 8 to 11 years are overweight and 2 to 5% are obese. Among adolescents, 11% to 19% are overweight and 2% to 5% are obese.

The problem of nutrition in school-age children and adolescents boils down to 2 main issues:

• Coverage of the energy and nutrient requirements of the growing system with an intensification of the growth rate in 7-8 year old children and at puberty, with necessary differentiation according to sex.
• The correct implementation of nutrition for children and adolescents in the schools in which they attend.

Obesity is influenced by genetic and environmental factors. Hypotheses that address the genetic determinants of obesity are based on the concept of single gene mutations. These hypotheses have only been proven in relation to obesity associated with other genetically determined syndromes. Obesity is then a symptom and not a disorder. In this case, the influence of environmental factors is secondary.

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Obesity has been classified as a disease of civilisation. This is due to the scale of its occurrence and the number of complications such as diabetes, hypertension, bone and joint disease. In 2000, the Food and Nutrition Institute carried out a study on a group of children aged 1-18 years. In 607 girls and 609 boys, an increase in the number of children with excess body weight in the postnatal period was observed. Overweight tendencies in early childhood were found in 12.3% of boys and 22.7% of girls. Overweight was found in 12.3% of boys and 9.1% of girls, and obesity in 7.7% of boys and 11.4% of girls aged 1-3 years.

According to a recent study by the World Health Organisation, as many as 29% of Polish 11 adolescents are overweight and 18% of all children are obese. The increase in the prevalence of obesity indicates that environmental factors are one of the most important causes.