centration, lean and fat tissue, bone mineral via DXA) change with age and are influenced by gender, race/ethnicity, and biological maturation status (Daniels et al., 1997; Malina, 1996, 2005; Malina et al., 2004). Elements of body composition, especially bone mineral content, also may be influenced by regular physical activity (Strong et al., 2005).

Body Mass Index (BMI)

BMI is an indicator of weight-for-height. As discussed earlier, in contrast to height and weight, which increase with age during childhood, BMI declines from infancy through early childhood and reaches its lowest point at about age 5-6.

BMI is reasonably well correlated with fat mass and percent body fat in heterogeneous samples of youth, but has limitations (Goran et al., 1995); it also is related to fat-free mass. Among youth aged 8-18 in the Fels Longitudinal Study, age-specific correlations between BMI and components of body composition ranged from 0.37 to 0.78 for percent body fat, 0.67 to 0.90 for fat mass, and 0.39 to 0.72 for fat-free mass in girls, and from 0.64 to 0.85 for percent body fat, 0.83 to 0.94 for fat mass, and 0.25 to 0.78 for fat-free mass in boys (Maynard et al., 2001). When chronological age was statistically controlled in five samples of boys and girls aged 8-18, correlations for BMI were a bit lower: percent body fat, 0.28 to 0.61; fat mass, 0.46 to 0.81; and fat-free mass, 0.27 to 0.64 (Malina and Katzmarzyk, 1999). Correlations for fat mass and fat-free mass were similar in four of the five samples, but those for BMI and percent body fat were variable. In a nationally representative sample of American children aged 2-19 in NHANES III, BMI was better than other anthropometric indicators (Rohrer index and weight-for-height) in predicting underweight and overweight when percent body fat or total fat mass based on DXA was the criterion measure (Mei et al., 2002).

Nevertheless, youth with the same BMI can differ considerably in fat mass and percent body fat, so care is essential when interpreting BMI as an indicator of fatness in youth. BMI is, more appropriately, an indicator of heaviness and, indirectly, of adiposity; at the extremes of heaviness, BMI is probably a reasonable indicator of fatness in general population surveys, but its limitations must be recognized (Pietrobelli et al., 1998).

Limited evidence supports higher intra- and interobserver reliability for BMI and waist circumference than for skinfold thicknesses (Artero et al., 2011). Beyond the debate about what the measurement of BMI actually represents (body composition, body fat, body weight, etc.), the association between BMI and health markers justifies its use among school-aged children as a means of tracking health status.

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