The modern American lifestyle, comprised primarily of sedentary activity and a diet high in calories, has produced an epidemic of alarming proportions in today's youth: obesity. Childhood obesity is a health concern of great magnitude. It demands significant attention from health care providers because of the adverse effects that obesity can have on health throughout the life span of those affected.
Overweight or obese?
There are no universal definitions for the classification of overweight and obesity in children. Obesity is most often determined by calculating body mass index (BMI): the ratio of weight in kilograms to the square of height in meters.
Since BMI does not distinguish between adiposity and increased lean body mass, clinicians should consider 
body habitus when using BMI to diagnose obesity. When used to classify obesity in children, the BMI has been correlated with comorbid, obesity-related conditions.1 A child with a BMI between the 85th and 95th percentile for age and gender is often considered overweight, while one with a BMI greater than the 95th percentile is considered obese.1
By this classification, an estimated 21% to 24% of children and adolescents in the United States are overweight, and 10% to 11% are obese.2 The number of overweight children has increased by an estimated 50% to 60% in just one generation.2 This is a crisis whose public health and economic implications are enormous. Preventing obesity is the key, and preventing childhood obesity begins with recognizing those children at greatest risk.
Risk factors for obesity
Obesity, the excess deposition of adipose mass, results when total energy consumption surpasses total energy expenditure. The imbalance of energy intake and expenditure resulting in weight gain is a complex process and is a consequence of the interplay among genetic, physiologic, metabolic, social, behavioral, and cultural factors.3
Genetic, physiologic, and metabolic factors A family history of obesity puts a person at risk for developing the condition. It had been thought that the association between obesity and family history stemmed from shared environmental factors. Recent research suggests, however, that there is a link between genetic makeup and obesity. Twin studies have shown that being overweight is a 65% to 75% inherited trait.4 If a young child has one obese biological parent, the odds ratio is roughly 3 for that child to be obese in adulthood; if both parents are obese, the odds ratio of obesity in adulthood is greater than 10.1 Genetics influence basal metabolic rate, feeding behavior, alterations in energy expenditure in response to overfeeding, lipoprotein lipase activity, and basal rate of lipolysis.4
Hormones and neurotransmitters such as growth hormone, leptin, ghrelin, neuropeptide Y, and melanocortin regulate satiety, hunger, lipogenesis, and lipolysis; these probably contribute to a person's risk for becoming obese, although their roles are not yet clearly understood. Further research is needed on the genetic basis of these substances and how they affect weight gain in order to develop more effective ways of treating obesity.
Some genetic and endocrine disorders play a role in dysregulation of energy expenditure versus intake, resulting 
in weight gain: Prader-Willi syndrome, Bardet-Biedl syndrome, Alström syndrome, hypothyroidism, and Cushing's syndrome all create a propensity toward obesity.
Social and behavioral factors The recent surge in childhood obesity cannot be explained by genetics alone.5 There have been societal changes over the past few decades that place American children at increased risk for being overweight and obese.
When the physiologic basis of weight gain is considered, the current lifestyle trends that place children at greatest risk for obesity are clear: decreased physical activity and increased caloric consumption. In the twin studies, environmental differences throughout childhood accounted for nearly 30% of the variance in body weight.6
Regular exercise is beneficial to overall health and weight control. It regulates weight by allowing direct energy expenditure during exercise, increasing the metabolic rate during and immediately after exercise, increasing lean body mass, and, in turn, raising the resting metabolic rate.3 When considering the benefits of exercise, clinicians should also bear in mind the unfavorable effects lack of regular exercise has on a child's health and well-being.
The amount of time children spend daily watching television and playing video games has increased over the past decade and is significantly correlated with the development of childhood obesity. In a national survey, 20% of children aged 8 to 16 years reported participating in two or fewer episodes of vigorous exercise per week, and more than 25% watched television for 4 or more hours daily.1 The average child watches 3 to 5 hours of television a day.6 According to the American Academy of Pediatrics (AAP), children who view television for more than 4 hours daily have significantly greater BMIs than their counterparts who watch television less than 2 hours a day. In addition, placing a television in a child's bedroom is a strong predictor of future overweight, even in young children.1
The urbanization of America also contributes to the problem. National transportation data suggest that walking and bicycling among children aged 5 to 15 years decreased 40% from 1977 to 1995.6 The recent reductions in mandatory school physical education classes further lowers physical activity. In fact, daily participation in high school physical education decreased from 42% to 29% between 1991 and 1999 alone.6
An enormous amount of television advertising for sugary cereals, fast foods, candy, and soda targets children. The increased consumption of these high-density foods further contributes to the epidemic of childhood obesity. The Third National Health and Nutrition Examination Survey (NHANES III) found that the rate of soda consumption among adolescent males increased almost 65% from 1989 to 1995.6 This is alarming given the findings that drinking soda correlates directly with obesity.
Several researchers have suggested that the diminishing number of families eating meals together in recent decades has played a role in increasing childhood obesity. Studies show that the absence of family meals is associated with reduced fruit and vegetable intake and increased consumption of fried food and soda.1