As with adult obesity, any factors that cause energy intake to be greater than the energy expended can lead to obesity. The possible causes of childhood obesity include:

• Rare genetic factors
• Poor diet
• Physical inactivity

Genetic factors

There are a number of rare genetic conditions that lead to obesity including Prader–Willi, Cohen and Bardet–Biedl syndromes. In 1997, interest arose in the role of the hormone leptin. However, the number of children worldwide who have presented with a mutation in the gene encoding leptin is extremely small.

Undoubtedly, in some cases genes do play a role in the aetiology of obesity. However, the rate at which the prevalence of childhood obesity has increased indicates that this trend is unlikely to be underlined primarily by genetic factors.

Diet and physical activity

The way in which we have shaped our society is without question reflected in the health of today’s children. Large food corporations have long recognised the benefits of marketing aimed at children, who have become consumers in their own right. A trip around any major supermarket will reveal that many foods have become segregated (based on their packaging) into child-specific and adult-specific foods. Cartoon characters and celebrities are used to increase recognition and undoubtedly the appeal of certain foods.

In addition, much physical activity has been removed from the daily lives of children. The time spent in active play has been replaced by more sedentary pursuits, such as watching television and playing computer games.

• The number of primary school children who walk to and from school has fallen from 62% in 1989/91 to 56% today ^₅
• Participation in school sport (>2 hours per week) decreased from 46% in 1994 to 33% in 1999 _⁵
• Watching television is the most popular sedentary activity for children of all ages, with over a quarter of 11–16-year-olds watching more than 4 hours a day ^₆
• Activity levels for teenage girls are particularly low with 64% of 15-year-old girls being classified as ‘inactive’ ^₆

Risk factors

Risk factors for the development of childhood obesity have not been well researched in the UK. Potential factors include:

• Parental obesity
• Time spent in inactive pursuits such as television viewing
• Low socio-economic status