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.

The explanation for this dramatic increase in the incidence of obesity is multifactorial. Energy intake comes from food, and energy expenditure occurs as the result of a combination of basal metabolism, activity, and growth. If these factors are equal, weight stays stable along a growth or BMI percentile. When energy intake exceeds energy expenditure in a positive balance, weight gain with fat deposition occurs. Genetic factors modify the efficiency of conversion of energy intake to tissue as well as the energy expended for a given amount of activity in each person. Specific alleles of the mitochondrial uncoupling protein-2 are associated with obesity among children. Obese children and adults may be more efficient in their use of consumed calories. Thus with the same caloric intake and energy expenditure, one child may gain excess weight while another does not. Because the genetic factors relating to energy use have not changed, environmental influences that affect energy intake and expenditure are primarily responsible for the increased trend toward obesity among children. In most cases, obesity is caused by a subtle positive increase in caloric balance in a genetically susceptible person over a prolonged time. There is a 50% likelihood that obesity will persist into adulthood if the BMI in adolescence is greater than the 95th percentile. Parental fatness, low socioeconomic status, high birth weight, earlier and rapid secondary sexual maturation, and inactivity are predictive of persistence of obesity into adulthood.

The risk of morbidity from being overweight is unclear for children. Adults have an increased rate of complications as BMI increases to more than 25, which is approximately the 90th percentile BMI for the adult population. A BMI of 30 is approximately the 99th percentile. The BMI linked to complications in childhood has not been established, but a BMI above the 85th percentile is considered worrisome. Counseling is needed to prevent further weight gain. If risk factors such as a personal or family history of hypertension or dyslipidemia are present, weight loss is recommended. Hypertension and dyslipidemia also are more common when BMI is greater than the 85th percentile. Therefore it is reasonable to consider obese any child or adolescent with a BMI above the 95th percentile. The goal of prevention and management of obesity is improvement of metabolic function, not emulation of thin body types.