David S Freedman1, Gerald S Berenson2. 1. Division of Nutrition, Physical Activity and Obesity, Centers for Disease Control and Prevention, Atlanta, Georgia; and dxf1@cdc.gov. 2. Division of Cardiology, Louisiana State University Health Sciences New Orleans Medical Center, New Orleans, Louisiana.
Abstract
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.
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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