Classification of Weight Status During Childhood
As in adults,
BMI is the recommended screening assessment of weight status in children aged 2 years and older.
2,
3 Although not a direct measure of body fat,
BMI generally correlates with more direct and robust methods for measuring body fat in children and correlates with both concurrent and future health risks, including morbidity and mortality. Consequently, the calculation, documentation, and interpretation of
BMI is recommended at least annually as part of routine pediatric health care.
However, the clinical interpretation of
BMI in children is more complex than in adults, as weight
and height are dynamic and anticipated to change as part of a child’s normal growth and development. Moreover, unlike other growth parameters,
BMI does not simply increase from birth to adulthood. In fact,
BMI is typically expected to decrease beginning in the second year of life, reaching a nadir at the age of 5 to 6 years, and then increase steadily throughout later childhood and adolescence. The vertex of this v- or u-shaped pattern in
BMI trajectory is termed “adiposity rebound,” and children who experience adiposity rebound prior to 4 years of age are at increased risk of obesity later in life.
4
In light of these normal developmental patterns, an absolute
BMI value is typically not sufficient for characterizing weight and adiposity status in children and younger adolescents, and
BMI percentiles specific for age and sex should be determined for children ≥2 years of age in the United States according to the revised 2000 Centers for Disease Control and Prevention (
CDC)
BMI data (https://www.cdc.gov/growthcharts/cdc_charts.htm). Age- and sex-specific
BMI percentiles may be calculated by plotting the child’s calculated
BMI on the appropriate
CDC BMI chart (
Figure 11.1). Alternatively, numerous online pediatric
BMI percentile calculators
are publicly available (https://www.cdc.gov/healthy-weight/bmi/calculator.html); many electronic medical records also contain automated functions for calculating
BMI percentiles for pediatric patients.
The body weight status of a child or adolescent ≥2 years of age can then be classified according to the determined age- and sex-specific
BMI percentiles (
Table 11.1). Specifically, a
BMI that is ≥85th percentile but <95th percentile is consistent with overweight, and a
BMI ≥95th percentile (
or an absolute
BMI ≥30 kg/m
2, whichever is lower) is consistent with obesity. Astute HCPs (and parents) may question how more than 5% of children can have obesity in light of the proposed definition (i.e.,
BMI ≥95th percentile). To avoid an upward shift in the weight- and BMI-for-age growth curves as a result of the secular increases in body weight that occurred during the 1980s and 1990s, the weight data for children aged 6 years and older used in the development of the 2000
CDC curves exclude data collected after 1980.
More recently, the classification of obesity in youth has been revised to include a definition for severe obesity (
BMI ≥120% of the age- and sex-appropriate 95th percentile
or an absolute
BMI ≥35 kg/m
2) in order to underscore the increased health risks of severe weight gain among children and adolescents.
5 This category of severe obesity in youth has been further refined to include proposed definitions for Class 2 (
BMI ≥120%
of the age- and sex-appropriate 95th percentile
or an absolute
BMI ≥35 kg/m
2) and Class 3 (
BMI ≥140% of the 95th percentile
or an absolute
BMI ≥40 kg/m
2) to coincide with adult definitions, as well as criteria for
MBS in adolescents.
1,
2,
6 Figure 11.2 contains specialized growth charts that can be used to determine pediatric obesity severity and class.
5
Although increasing data support that rapid weight gain in early life and infancy may portend an increased risk of obesity in later childhood, the importance of an elevated
BMI in children <2 years of age has not been definitively established.
3 Moreover, both the
CDC and the American Academy of Pediatrics (
AAP) recommend using 2006 World Health Organization (
WHO) growth standards (https://www.cdc.gov/growthcharts/who_charts.htm), as opposed to the 2000
CDC growth references, for assessing growth patterns for infants <2 years of age given the
WHO growth standards’ more robust longitudinal data and inclusion of predominantly breastfeed infants.
7 Even though the 2006
WHO growth standards include a
BMI reference chart beginning at birth, the CDC-AAP expert panel does not recommend its clinical use before 2 years of age.
7 Alternatively, guidelines have proposed that in children <2 years of age, a weight-for-recumbent length ratio ≥97.7th percentile according
WHO growth standards may represent an appropriate definition of obesity
2; however, the clinical utility of this definition remains unclear.
An additional approach that addresses the dynamic characteristics of
BMI among youth is the use of
BMI z-scores (or standard scores); a
BMI z-score corresponds to the number of standard deviations that a given child’s
BMI is away from the corresponding age- and sex-specific 50th percentile. However, even though automated pediatric
BMI percentile calculators may also generate z-scores and
BMI z-scores are frequently reported in the research literature, the clinical utility of a
BMI z-score in direct patient care is limited.
Prevalence of Overweight and Obesity in Youth
Rates of overweight and obesity among US youth have increased substantially over the past 40 to 50 years across all age and racial/ethnic groups.
1,
2 According to the 2015-2016 National Health and Nutrition Examination Survey (
NHANES) data, 18.5% of children in the United States aged 2 to 19 years have obesity and 35.1% have overweight.
8 Prevalence rates of obesity among children and adolescents rise with increasing age and are slightly higher in boys compared with girls across age groups (
Figure 11.3).
8 Obesity also disproportionately affects African American and Hispanic youth; however, no racial/ethnic group is immune from the epidemic of pediatric obesity.
8 Even though reported rates are lowest in Asian American children, there is concern that the proposed
BMI percentile definitions of pediatric obesity may underestimate the health risks associated with increasing adiposity in this population.
2
Although overall rates of obesity may be plateauing in some pediatric age groups,
8 the prevalence of severe obesity continues to rise at alarming rates.
1 Based on 2015-2016
NHANES data, 6.0% of US children aged 2 to 19 years have severe (i.e., Class 2 or higher) obesity. Prevalence estimates of severe obesity climb with age, with an estimated 9.5% and 4.5% of US youth aged 16 to 19 years meeting proposed definitions of Class 2 and Class 3 obesity, respectively.
1
Health Impact of Overweight and Obesity During Childhood
Children with overweight and obesity are likely to have persistent obesity as adults. Based on the current prevalence of excess weight in youth, predictive modeling suggests that ˜50% to 60% of the current generation of US children will have obesity when they are 35 years old.
9 Childhood obesity is associated with significant life-long morbidity and mortality, including an increased risk of
T2D, cardiovascular disease, and premature death in adulthood.
3,
5,
10 The risk for the development of many weight-related conditions in childhood (
Figure 11.4) increases with obesity severity. Furthermore, pediatric obesity is associated with significant reductions in quality of life.
11 As a result of obesity and the weight-comorbidities, a decline in life expectancy is projected, with today’s youth living shorter lives than their parents.
12 Thus, the development of skills required for the compassionate and comprehensive care of youth with overweight and obesity, including an understanding of the complexities of the disease and evidenced-based treatment recommendations, is critical for all HCPs who care for children.