At the individual level, childhood obesity is the result of an imbalance between the calories a child consumes as food and beverages and the calories a child uses to support normal growth and development, metabolism, and physical activity. In other words, obesity results when a child consumes more calories than the child uses. The imbalance between calories consumed and calories used can result from the influences and interactions of a number of factors, including genetic, behavioral, and environmental factors. It is the interactions among these factors – rather than any single factor – that is thought to cause obesity.

Genetic Factors

Studies indicate that certain genetic characteristics may increase an individual’s susceptibility to excess body weight. However, this genetic susceptibility may need to exist in conjunction with contributing environmental and behavioral factors (such as a high-calorie food supply and minimal physical activity) to have a significant effect on weight. Genetic factors alone can play a role in specific cases of obesity. For example, obesity is a clinical feature for rare genetic disorders such as Prader-Willi syndrome. However, the rapid rise in the rates of overweight and obesity in the general population in recent years cannot be attributed solely to genetic factors. The genetic characteristics of the human population have not changed in the last three decades, but the prevalence of obesity has tripled among school-aged children during that time.

Behavioral Factors

Because the factors that contribute to childhood obesity interact with each other, it is not possible to specify one behavior as the “cause” of obesity. However, certain behaviors can be identified as potentially contributing to an energy imbalance and, consequently, to obesity.

  • Energy intake: Evidence is limited on specific foods or dietary patterns that contribute to excessive energy intake in children and teens. However, large portion sizes for food and beverages, eating meals away from home, frequent snacking on energy-dense foods and consuming beverages with added sugar are often hypothesized as contributing to excess energy intake of children and teens. In the area of consuming sugar-sweetened drinks, evidence is growing to suggest an association with weight gain in children and adolescents. Consuming sugar-sweetened drinks may be associated with obesity because these drinks are high in calories. Children may not compensate at meals for the calories they have consumed in sugar-sweetened drinks, although this may vary by age. Also, liquid forms of energy may be less satiating than solid forms and lead to higher caloric intake.
  • Physical activity: Participating in physical activity is important for children and teens as it may have beneficial effects not only on body weight, but also on blood pressure and bone strength. Physically active children are also more likely to remain physically active throughout adolescence and possibly into adulthood.

    Children may be spending less time engaged in physical activity during school. Daily participation in school physical education among adolescents dropped 14 percentage points over the last 13 years — from 42% in 1991 to 28% in 2003. In addition, less than one-third (28%) of high school students meet currently recommended levels of physical activity.
  • Sedentary behavior: Children spend a considerable amount of time with media. One study found that time spent watching TV, videos, DVDs, and movies averaged slightly over 3 hours per day among children aged 8–18 years. Several studies have found a positive association between the time spent viewing television and increased prevalence of obesity in children. Media use, and specifically television viewing, may
    • displace time children spend in physical activities,
    • contribute to increased energy consumption through excessive snacking and eating meals in front of the TV,
    • influence children to make unhealthy food choices through exposure to food advertisements, and
    • lower children's metabolic rate.

Environmental Factors

Home, child care, school, and community environments can influence children’s behaviors related to food intake and physical activity.

  • Within the home: Parent-child interactions and the home environment can affect the behaviors of children and youth related to calorie intake and physical activity. Parents are role models for their children who are likely to develop habits similar to their parents.
  • Within child care: Almost 80% of children aged 5 years and younger with working mothers are in child care for 40 hours a week on average. Child care providers are sharing responsibility with parents for children during important developmental years. Child care can be a setting in which healthy eating and physical activity habits are developed.
  • Within schools: Because the majority of young people aged 5–17 years are enrolled in schools and because of the amount of time that children spend at school each day, schools provide an ideal setting for teaching children and teens to adopt healthy eating and physical activity behaviors. According to the Institute of Medicine (IOM), schools and school districts are, increasingly, implementing innovative programs that focus on improving the nutrition and increasing physical activity of students.
  • Within the community: The built environment within communities influences access to physical activity opportunities and access to affordable and healthy foods. For example, a lack of sidewalks, safe bike paths, and parks in neighborhoods can discourage children from walking or biking to school as well as from participating in physical activity. Additionally, lack of access to affordable, healthy food choices in neighborhood food markets can be a barrier to purchasing healthy foods.