A number of methods have been proposed to describe increases in body weight. The most widely used measurement is the BMI, calculated as weight/height2 (kg/m2). Among adult subjects overweight is defined as a BMI of 25 to 29.9 kg/m2, and obesity as a BMI of at least 30 kg/m2. Different ethnicities, such as Caucasians and Chinese, have highly contrasting distributions of body weight and height. Males and females differ in their BMI distribution. Age is an obvious modifier of body weight and height. Therefore, age- and sex-specific definitions for different ethnic groups must be applied when comparing effects in various study populations. Growth charts from the Centers for Disease Control and Prevention (CDC) include age- and sex-specific BMI reference values for children and adolescents aged 2 to 20 yr (8). However, BMI does not account for body frame and proportion of muscle mass. This limitation is particularly relevant in pediatric studies because of the effects of maturation and growth of lean muscle mass, fat mass, and hydration status. Furthermore, muscle mass increases with higher activity level, and fat mass values are higher among females than males and vary across ethnic groups (9). Therefore, other measures of body weight have been applied, such as assessment of body fat by skinfold thickness, dual-energy X-ray absorptiometry (DEXA) evaluation or bioelectrical impedance. However, large comparative studies on skinfold, DEXA, and bioelectrical impedance variables with BMI are missing to date.
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