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Tracking of BMI z Scores for Severe Obesity
David S Freedman1, Gerald S Berenson2
1Division of Nutrition, Physical Activity and Obesity, Centers for Disease Control and Prevention, Atlanta, Georgia; and dxf1@cdc.gov.
Insights
Body Mass Index (BMI) z scores are inaccurate for children with severe obesity. Alternative metrics like BMI percent of 95th percentile (%BMIp95) or adjusted BMI z score (BMIaz) better track BMI changes in this group.
Area of Science:
- Pediatrics
- Public Health
- Biostatistics
Background:
- Centers for Disease Control (CDC) growth charts are standard for childhood obesity studies.
- BMI z scores (BMIz) demonstrate inaccuracies for values exceeding the 97th percentile.
Purpose of the Study:
- To compare the tracking accuracy of three Body Mass Index (BMI) metrics in children with obesity.
- To identify the most reliable BMI metric for assessing severe obesity in children.
Main Methods:
- Utilized longitudinal data from 6994 children in the Bogalusa Heart Study.
- Compared tracking of BMI z score (BMIz), BMI as a percentage of the 95th percentile (%BMIp95), and adjusted BMI z score (BMIaz).
- Focused analysis on children with severe obesity (%BMIp95 ≥120%).
Main Results:
- BMIz tracking was substantially weaker (r=0.46) in children with severe obesity compared to %BMIp95 (r=0.61) and BMIaz (r=0.65).
- Weak BMIz tracking was particularly pronounced before age 10 (r=0.36 vs 0.57 for %BMIp95 and 0.60 for BMIaz).
- Some children with severe obesity showed decreasing BMIz despite increasing BMI values.
Conclusions:
- BMIz exhibits weak tracking in children with severe obesity due to CDC growth chart limitations.
- Expressing high BMIs relative to the 95th percentile (%BMIp95) or using adjusted BMI z scores (BMIaz) is preferable for this population.
Background:
Although the Centers for Disease Control (CDC) growth charts are widely used in studies of childhood obesity, BMI z scores are known to be inaccurate at values greater than the 97th percentile.
Methods:
We used longitudinal data from 6994 children in the Bogalusa Heart Study who were examined multiple times to compare tracking of 3 BMI metrics: BMI-for-sex/age z score (BMIz), BMI expressed as a percentage of the 95th percentile (%BMIp95), and levels of BMI z score that adjust for the compression of very high z scores (adjusted z score [BMIaz]). The later 2 metrics, unlike BMIz, do not have an upper limit. The mean interval between examinations was 2.8 years. We were particularly interested in these metrics among children with obesity or severe obesity (%BMIp95 ≥120%).
Results:
Although there was little difference in the tracking of the 3 metrics in the overall sample, among 247 children with severe obesity, the correlation of BMIz levels between examinations (r = 0.46) was substantially weaker than those for BMIaz and %BMIp95 (r = 0.65 and 0.61). Age-stratified analyses indicated that the weak tracking of BMIz was particularly evident before the age of 10 years (r = 0.36 vs 0.57 and 0.60). Several children with severe obesity showed BMIz decreases between examinations despite having BMI increases of over 5.
Conclusions:
Among children with severe obesity, the tracking of BMIz is weak. This is because of the constraints in converting very high BMIs into z scores based on the CDC growth charts. Rather than using BMIz, it would be preferable to express very high BMIs relative to the CDC 95th percentile or to use BMIaz.
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