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Article Excerpt
The body mass index (BMI) or the ratio of weight in kilograms to the square of height in meters, is widely used to determine the presence or absence of overweight and obesity in adults (National Institutes of Health, 1998). In children and adolescents, a BMI between the 85th and 95th percentiles for age and gender is defined as at risk for overweight, whereas a BMI above the 95th percentile for age and gender is defined as overweight (American Academy of Pediatrics, 2003). Over the past two decades, the prevalence of at risk for overweight and overweight youth has nearly doubled in the U.S. (Ogden, Flegal, Carroll, & Johnson, 2002). Some policy statements (American Academy of Pediatrics, 2003) consider overweight to be synonymous with obesity. However, obesity is an excess of body fat, and obesity in youth has been linked with an increased risk of Type 2 diabetes (Goran, 2001), glucose intolerance, hyperlipidemia (Dietz, 1998), and hypertension (Baskin, Ahluwalia, & Resnicow, 2001).
Although many consider BMI an acceptable clinical surrogate of body fatness, it does not differentiate between fat and fat-free tissues. Thus, children and adolescents with the same BMI may differ widely in body fat percentages. For instance, in a sample of 112 adolescent girls, 24 were at risk for an overweight BMI or had an overweight BMI, yet their body fat, as assessed by a four-component body density, total body water, and whole body bone mineral model, ranged from 16 to 42% (Wong, Stuff, Butte, O'Brien-Smith, & Ellis, 2000). In another study of 979 children and adolescents assessed by BMI and the three-component dual-energy x-ray absorptiometry (DXA) model, 90% of girls but only 71% of boys with an overweight BMI also had a DXA-derived body fat percentage above the 95th percentile for age and gender (Ellis, Abrams, & Wong, 1999). Therefore, the presence of either an at risk for overweight BMI or an overweight BMI is no guarantee of excess body fat in an individual child or adolescent (Ellis et al., 1999; Wong et al., 2000).
One low-cost alternative to BMI is assessing skinfold thickness to determine body composition in youth. Williams et al. (1992) used the sum of triceps and subscapular skinfold thicknesses to estimate body fat percentage in a sample of 3,320 Caucasian and African American youth ages 5-18 years. They reported that boys with body fat at or above 25% and girls with body fat at or above 30% were two to seven times as likely to have elevated blood pressures and adverse serum lipoprotein levels as those with lower body fat percentages (Williams et al., 1992). These standards, with an adjustment for girls to body fat at or above 32%, have been used for over a decade to identify individuals above the healthy fitness zone in the FITNESSGRAM[R] health-related test of youth fitness (Meredith & Welk, 2005). However, there are several limitations related to using skinfold testing in youth. Skinfold thickness assessments can be intrusive, may involve some disrobing and touching in sensitive areas, and may violate an individual's sense of modesty. Testers must be well trained to take accurate and reliable measurements, and the process takes a substantial amount of class time (National Association of Sport and Physical Education, 2005). Finally, no single equation is valid for all youth (Heyward & Wagner, 2004). However, the Slaughter et al. (1988) triceps and calf skinfold equation has a number of advantages in that the limb skinfold sites are easily accessible, the equation was validated against a four-component criterion estimate of body fat percentage, and the validation sample included Caucasian and African American children, adolescents, and young adults (Slaughter et al., 1988). Moreover, Wong et al. (2000) reported that the Slaughter et al. equation yielded body fat estimates that agreed closely ([less than or equal to] 2% body fat), on average, with a four-component criterion estimate of body fat in an independent sample of Caucasian and African American adolescents. However, the 95% limits of agreement indicated the Slaughter et al. equation resulted in individual body fat estimates that under- or overestimated the four-component criterion by 10% (Wong et al., 2000).
A potential alternative to both BMI and skinfold thicknesses is bioelectrical impedance analysis (BIA), which takes little time, is easy to administer, requires no special training, is noninvasive, requires no disrobing, and involves no threatening touching of youth in sensitive areas. Unfortunately, the cost of a whole body bioimpedance analyzer ($2,000-5,000) is much higher than a segmental bioimpedance analyzer (< $100). In addition, a whole body bioimpedance analyzer may be used to obtain raw electrical resistance values to use in published, sex-specific and cross-validated BIA equations foro estimating body composition from a four-component criterion in Caucasian and African American adolescents and adults ages 12-94 years (Sun et al., 2003). By contrast, a segmental bioimpedance analyzer can only be used to obtain predicted estimates of body composition using unpublished equations, commonly referred to as "proprietary" equations, developed and protected by the bioimpedance manufacturers. There are two main competing manufacturers of these low-cost segmental analyzers. With one analyzer...
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