Because direct measures of body fat are neither feasible nor available for nationwide assessments of the prevalence of obesity, the National Health and Nutrition Examination Surveys (NHANES),1 conducted by the National Center for Health Statistics, have been using body mass index (BMI) as a surrogate measure for body fatness. The prevalence of childhood and
1NHANES is a series of cross-sectional, nationally representative examination surveys that became a continuous survey in 1999. Previous surveys include NHANES III (conducted from 1988 to 1994), NHANES II (conducted from 1976 to 1980), NHANES I (conducted from 1971 to 1974), the National Health Examination Survey (NHES) cycle 3 (conducted from 1966 to 1970), and the NHES cycle 2 (conducted from 1963 to 1965).
adolescent obesity is equated to the proportion of those who are in the upper end of the BMI distribution—specifically, at or above the age- and gender-specific 95th percentile of the Centers for Disease Control and Prevention's (CDC's) BMI charts for children and youth aged 2 through 19 years2 (Kuczmarski et al., 2000) (see Chapter 3 for a more extensive discussion about the use of terms for childhood overweight and childhood obesity).
If BMI is normally distributed and survey-specific percentile distributions are presented, then by definition, 5 percent of children in each survey will be above the 95th percentile BMI of the survey sample. Thus, reports based on the survey-specific BMI percentiles would always designate 5 percent of children as obese and would fail to detect any true increasing prevalence of obesity across surveys. The CDC therefore developed a revised growth reference in 2000 that established the age- and gender-specific 95th percentile of BMI. The growth reference data were based on BMI distributions from national surveys between 1963 and 1980 for children aged 6 to 19 years, and between 1971 and 1994 for children aged 2 through 5 years (Kuczmarski et al., 2002; Ogden et al., 2002b). There are no BMI-for-age references or accepted definitions for children younger than 2 years of age. However, the Special Supplemental Nutrition Program for Women, Infants and Children (WIC) has defined the term overweight for children under 2 years who are at or above the 95th percentile of weight-for-length and uses this standard for determining WIC program eligibility (Ogden et al., 2002a).
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