Background
Obesity is the most prevalent nutritional disorder among children and adolescents in the United States. Approximately 21-24% of American children and adolescents are overweight, and another 16-18% is obese; the prevalence of obesity is highest among specific ethnic groups.
Childhood obesity predisposes to insulin resistance and type 2 diabetes, hypertension, hyperlipidemia, liver and renal disease, and reproductive dysfunction. This condition also increases the risk of adult-onset obesity and cardiovascular disease.
Obesity in children is a complex disorder. Its prevalence has increased so significantly in recent years that many consider it a major health concern of the developed world. The National Health and Nutrition Examination Survey (NHANES) indicates that the prevalence of obesity is increasing in all pediatric age groups, in both sexes, and in various ethnic and racial groups. Many factors, including genetics, environment, metabolism, lifestyle, and eating habits, are believed to play a role in the development of obesity. However, more than 90% of cases are idiopathic; less than 10% are associated with hormonal or genetic causes.
Definitions
Operational definitions of obesity in adults are derived from statistical data that analyze the association between body mass and the risk of acute and long-term morbidity and mortality. Because acute medical complications of obesity are less common in children and adolescents than in adults, and because longitudinal data on the relation between childhood weight and adult morbidity and mortality are more difficult to interpret, no single definition of obesity in childhood and adolescence has gained universal approval.
Some investigators have used the terms overweight, obese, and morbidly obese to refer to children and adolescents whose weights exceed those expected for heights by 20%, 50%, and 80-100%, respectively. The body mass index (BMI) has not been consistently used or validated in children younger than 2 years. Because weight varies in a continuous rather than a stepwise fashion, the use of these arbitrary criteria is problematic and may be misleading. Nevertheless, children and adolescents defined as overweight or obese according to published criteria are highly likely to maintain this ponderal status as adults.
Body mass index
The BMI is a continuous, although imperfect, measure of body fatness. Calculated as weight (kg) divided by height (m2), BMI corrects for body size and can be readily and reliably quantified in clinical settings. The BMI correlates closely with total body fat (TBF), which is estimated using dual-energy x-ray absorptiometry (DEXA) scanning in children who are overweight and obese.
Normal values for BMI vary with age, sex, and pubertal status, and standard curves representing the 5th through the 95th percentiles for BMI in childhood and adolescence were generated using data from the 1988-1994 NHANES.
Consensus committees have recommended that children and adolescents be considered overweight or obese if the BMI exceeds the 85th or 95th percentiles, on curves generated from the 1963-1965 and 1966-1970 NHANES, or exceeds 30 kg/m2 at any age.
McGavock et al demonstrated that low cardiorespiratory fitness and reductions in fitness over time are significantly associated with weight gain and the risk of being overweight in children aged 6-15 years.
Analysis on a cohort of 902 schoolchildren showed higher waist circumference and disproportionate weight gain over a 12-month follow-up period in those children with low cardiorespiratory fitness. The 12-month risk of overweight classification was 3.5-fold higher in youth with low cardiorespiratory fitness, relative to fit peers.
Reductions in cardiorespiratory fitness were significantly and independently associated with increasing BMI. Low levels of cardiorespiratory fitness have also been associated with elevated depressive symptoms in obese adolescents.
One study suggests that a lack of adequate sleep time in young children is associated with increased BMI; this observation is independent of other confounding variables (eg, physical activity).
Furthermore, data indicate that over a 5-year period an increase in BMI among overweight children 6 to 11 years of age is associated with increases in both systolic and diastolic blood pressure, as well as with a decrease in sleep time.
A study by Mosli et al found that a birth of a sibling when the child is 24 to 54 months old is associated with a healthier body mass index z-score trajectory.