Obesity is common, affecting over 93 million adults and nearly 4 million children1; costs the U.S. healthcare system billions of dollars2,3; and puts both children and adults at risk for multiple health consequences including type 2 diabetes and coronary heart disease.4,5 With the high prevalence of obesity, associated consequences, and forecasted increases,6 preventing individuals from developing obesity and treating those who have it is the focus of many health initiatives across the country. Because obesity is a population-wide problem, this chapter examines the problem of obesity through a public health lens. Specifically, it focuses on the epidemiology of obesity including how it is defined, who is most affected, the health and economic effects of obesity; the modifiable risk factors for obesity; and strategies for prevention and treatment.
Although obesity is most simply defined as excess body fat, methods to assess obesity vary. Body mass index (BMI) is most often used as a screening tool to identify obesity in clinical and research settings and in public health surveillance to track obesity among populations over time. BMI is a person's weight in kilograms divided by their height in meters squared. A BMI at or above 30.0 kg/m2 in adults aged 20 years and older is defined as obesity by various expert bodies including the World Health Organization, the National Institutes of Health, and the Centers for Disease Control and Prevention.5,7–9 Because children are still growing, BMI for children and adolescents must be interpreted relative to their peers of the same sex and age. A BMI at or greater than the 95th percentile in children and adolescents aged 2–19 years is classified as obesity.10 In infants and children under 2 years of age, there is not a defined cutoff for obesity. Although no evidence-based guidelines for treating obesity in infancy exist, early recognition of a tendency toward obesity might appropriately trigger interventions to slow weight gain.11 As such, in the United States, World Health Organization growth charts are used to monitor high weight for length.11,12
Although not the focus of this chapter, the classification overweight is also commonly used to characterize weight status, particularly in concert with obesity. Overweight is defined as a BMI of 25.0–29.9 kg/m2 among adults and a BMI-for-age between the 85th percentile and <95th percentile among children and adolescents aged 2–19 years.9,10
Obesity can also be further categorized by levels of severity. Adults with a BMI 35.0–39.9 kg/m2 and those with a BMI at or above 40.0 kg/m2 are classified as having class II and class III obesity, respectively.7 Some debate exists on how to classify severe obesity in children and adolescents.13,14 To facilitate tracking changes in BMI status over time and interpretation of high-BMI values, some expert bodies suggest severe obesity in children and adolescents ...