From the Guidelines
Normal body mass index (BMI) for children is between the 5th and 85th percentiles.
BMI Classification for Children
- Underweight: below the 5th percentile
- Normal weight: between the 5th and 85th percentiles
- Overweight: between the 85th and 95th percentiles
- Obese: at or above the 95th percentile This classification is based on age- and sex-specific BMI standards for the US population, as published by the Centers for Disease Control and Prevention 1. The use of BMI percentiles adjusted for age and sex is recommended because the absolute value of BMI is not used to classify weight status in children, as change in BMI is normal and expected as children grow and develop 1. The American Medical Association, the CDC, and the Maternal and Child Health Bureau expert committee also defined a BMI at the ≥95th percentile as obese and a BMI between the 85th and 94th percentiles as overweight 1.
Clinical Use of BMI in Children
The BMI is a reliable and valid measure of relative weight in children and is recommended for clinical use 1. Clinicians are encouraged to use the CDC growth charts to replace the older weight-for-height curves 1. For example, a child whose height is at the 50th percentile for age and sex should ideally have a weight around the 50th percentile for age and sex as well 1. Children and adolescents with a BMI greater than the 95th percentile should undergo additional evaluation for obesity-related complications, such as blood pressure measurement, lipoprotein analysis, and fasting insulin and glucose determination 1.
Growth Monitoring
Growth monitoring, including measurement of BMI, is an important aspect of clinical practice guidelines for the prevention of childhood obesity 1. The use of standardized growth charts, such as the CDC growth charts, is recommended for children aged 2 years or older 1. However, differences in percentile cutoffs for defining obesity and overweight were noted between clinical practice guidelines, highlighting the need for consistent and evidence-based guidelines 1.
From the Research
Definition and Calculation of BMI
- Body Mass Index (BMI) is a measure of weight adjusted for height, calculated as weight/height(2) 2
- BMI is a simple and noninvasive surrogate measure of body fat, but it has limitations in differentiating body fat from lean mass 2
BMI Categories for Children
- The Centers for Disease Control and Prevention (CDC) uses BMI-for-age categories to classify body fatness in children, including:
- Normal: <85th percentile
- Moderate: 85th-94th percentile
- Elevated: ≥95th percentile 3
- However, BMI-for-age categories may not accurately classify body fatness in all children, particularly those with high muscle mass 3
Reference Values for BMI in Children
- Reference values for BMI in children can be found in tables and figures developed from large epidemiologic studies, such as the NHANES I measures 4
- These reference values are age, race, and gender specific, and can be used to determine the relative ranking of BMI for patients compared with values derived from a large sample of healthy children and adolescents 4
Limitations of BMI in Children
- BMI has limitations in monitoring weight status in children and adolescents with extremely high BMI, as it requires extrapolation beyond the 97th percentile 5
- Alternative BMI metrics may be needed to monitor weight status in these children 5
- BMI is not diagnostic of level of adiposity, and should be used as a screening tool to identify children who should have further evaluation and follow-up 3