Body mass index (BMI) is an indirect measure of obesity based on the readily determined measures of height and weight. This report uses the term "obese" to refer to children and youth with BMIs equal to or greater than the 95th percentile of the age- and gender-specific BMI charts developed by the Centers for Disease Control and Prevention (CDC) (Kuczmarski et al., 2000). In most children, values at this level are known to indicate excess body fat, which itself is difficult to measure accurately in either clinical or population-based settings.
What constitutes "excess" is an amount of body fat (often expressed as a percentage of body mass) that is sufficient to cause adverse health consequences. The exact percentage of body fat at which adverse consequences occur can vary widely across individuals and the consequences themselves— ranging from low self-esteem or mild glucose intolerance to major depression or nephropathy—show considerable variation as well.
BMI—calculated as weight in kilograms divided by the square of height measured in meters (kg/m2)—is the recommended indicator of obesity-related risks in both children and adults. For adults, overweight is defined as a BMI between 25 and 29.9 kg/m2 and obesity is defined as a BMI equal to or greater than 30 kg/m2 (NHLBI, 1998). The BMI cut-off points were based on epidemiological data that show increasing mortality above a BMI of 25 kg/m2, with greater increases above 30 kg/m2 (NHLBI, 1998).
Because children's development varies with age, and because boys and girls develop at different rates, the use of BMI to assess body weight in children requires growth and gender considerations. Thus, BMI values for children and youth are specific to both age and gender (Barlow and Dietz, 1998; Dietz and Robinson, 1998).
The committee recognizes that it has been customary to use the term "overweight" instead of "obese" to refer to children with BMIs above the age- and gender-specific 95th percentiles (Himes and Dietz, 1994; Barlow and Dietz, 1998; DHHS, 2001a; Kuczmarski et al., 2002; AAP, 2003). Obese has often been considered to be a pejorative term, despite having a specific medical meaning. There have also been concerns about misclassi-fication, as BMI is only a surrogate measure of body fatness in children as in adults. Furthermore, children may experience functional impairment (physical or emotional) at different levels of body fatness.
However, the term "obese" more effectively conveys the seriousness, urgency, and medical nature of this concern than does the term "overweight," thereby reinforcing the importance of taking immediate action. Further, BMI in children correlates reasonably well to direct measures of body fatness (Mei et al., 2002), and high BMIs in children have been associated with many co-morbidities such as elevated blood pressure, insulin resistance, and increased lipids (Freedman et al., 2001). These are the same co-morbidities that often worsen in adult life and contribute to premature death from obesity.
The committee recognizes, however, that the term obese is probably not well suited for children younger than 2 years of age because the relationships among BMI, body fat, and morbidity are less clear at these ages.
Additionally, a high BMI in children younger than 2 years of age is less likely to persist than a high BMI in older children (Guo et al., 1994). BMI reference values are not established for children less than 2 years of age. Weight-for-length greater than the 95th percentile is used by CDC and the Special Supplemental Nutrition Program for Women, Infants, and Children to define overweight for children in this age group.
It is important that government agencies, researchers, health-care providers, insurers, and others agree on the same definition of childhood obesity. Although varying definitions have arisen from many uses of the term in public health, clinical medicine, insurance coverage, government programs and other settings, to the extent possible, there should be concurrence on definitions and terminology.
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