Definition and Incidence of Obesity

Obesity among children and adolescents is an increasing problem in the United States and the rest of the world, even in locations previously prone to malnutrition. Obesity refers to excess body fat, not excess body weight. Previous indicators of obesity, such as the weight for age and weight to height ratio did not adequately adjust for height or were prone to false-positive descriptions of obesity, respectively. Obesity is now defined with a measure that provides a better reflection of the amount of body fat than previous measures. The new measure is age-specific value of body mass index (BMI), which is calculated as follows: Weight in kilograms/Height² in meters.

Body mass index for age is shown in Table 24-45, and new height and weight charts composed by the United States National Center for Health Statistics are available at: (http://www.cdc.gov/nchs/about/major/nhanes/growthcharts/charts.htm).

More accurate measurements of body composition than BMI are used in clinical research and epidemiologic studies. Dual-energy x-ray absorptiometry (DEXA) is used to determine bone mineral content and full body fat.

Computed tomography or MR imaging best shows the distribution of body fat, especially the amount of intraabdominal adipose tissue or visceral fat. Visceral fat is considered to be the most metabolically active fat depot and to be responsible for many of the comorbidities of obesity among adults.

This relation has not been proved among children. Densitometry through underwater weighing, determining the displacement of air by the body (BodPod), determination of skinfold thickness, and bioelectric impedance analysis indicate amounts of body fat and body water.

Body fat percentages change normally with age. Infants have a relatively high BMI followed by a decrease at 5 to 6 years of age, the adiposity rebound. An early adiposity rebound is predictive of persistent overweight status with an odds ratio of 6. In childhood the values approximate 12% for boys and 15% for girls, with a temporary increase among boys until values at puberty return to childhood levels. Among girls the amount of body fat increases to 22 to 24% in late puberty. When BMI in children exceeds the 95th percentile, body fat percentage by DEXA or bioelectric impedance exceeds 40%.

Between the second National Health Examination Survey (NHANES II) in 1984 and NHANES III in 1993, the prevalence of BMI greater than the 95th percentile (defined as obesity by some and overweight by others) in the 6- to 11-year-old population increased from 6.5% to 11.4% for boys and 5.5 to 9.9% for girls. Among 12- to 17-year-olds it increased from 4.7% to 11.4% among boys and from 4.9% to 9.9% among girls. The prevalence of BMI in the 85th to 95th percentile (classified as at risk of obesity) increased to 22% during the same period. Hispanic and African-American children have a higher BMI, are more prone to obesity, and are at higher risk of type 2 diabetes mellitus than are white and Asian Americans at most ages.