Definition Etiology and Assessment

A Pietrobelli, Verona University Medical School, Verona, Italy

© 2005 Elsevier Ltd. All rights reserved.

Obesity is a situation of excess body fat accumulation, and a clinical diagnosis of obesity should be based on an accurate direct or indirect measure of total body fat. The most widely used measurement to define obesity in adults is body mass index (BMI; weight in kg/height in m2). It is a predictor of body fat from a population perspective, but it has limitations on an individual level and is only a proxy measurement of body fat. BMI shows significant variations during childhood; thus, age- and gender-specific reference standards must be used, and in adolescents the pubertal status should also be evaluated. An expert committee convened by the International Obesity Task Force (IOTF) in 1999 determined that although BMI is not an ideal measure of adiposity, it has been validated against other measures of body fat and may therefore be used to define overweight and obesity in children and adolescents. Because it is not clear at which BMI level adverse health risk factors increase in children, the group recommended cutoffs based on age-specific values that project to the adult cutoffs of 25 kg/m2 for overweight and 30 kg/m2 for obesity. Using data from six different reference population (Great Britain, Brazil, The Netherlands, Hong Kong, Singapore, and the United States), Cole and colleagues derived centile curves that passed through the points of 25 and 30 kg/m2 at age 18 years. Table 1 is useful for epidemiological research because children and adolescents can be categorized as non-overweight, overweight, and obese using a single standard tool.

There are differences in body composition across adult ethic groups, with one study of whites and Asians showing a difference of 2 or 3 BMI units in adults with the same body composition. It has been found that African American, Mexican American, and Mohawk Indian children carry more central fat than white children. Several studies have compared the US NHANES criteria for defining overweight or obesity using age- and gender-specific 85th and 95th percentile cutoffs with those of the Centers for Disease Control and Prevention (CDC) using similar percentile cutoffs and the IOTF alternative set of cutoffs based on centiles passing through BMI 25 and 30 at age 18 years. Using the NHANES III data, the different methods (i.e., NHANES/WHO, CDC, and IOTF) give approximately similar results but with some discrepancies, especially among younger children.

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