, based on data from nearly 7,000 children in the Bogalusa Heart Study.
The current parameters used in the Centers for Disease Control and Prevention growth charts for children with high body mass index (BMI) “can result in estimates that differ substantially from those that are observed and constrains the maximum BMI z that is attainable at a given sex and age,” wrote David S. Freedman, PhD, of the Centers for Disease Control and Prevention in Atlanta, and Gerald S. Berenson, MD, of Louisiana State University Health Sciences Center, New Orleans, (Pediatrics. 2017;140:e20171072).
The BMI adjusted z score (BMIaz) or the BMI expressed as a percentage of the 95th percentile (%BMIp95) “will provide more accurate information on body size over time among children with very high BMIs,” they said.
In children with severe obesity, BMI z was a weaker measure (r = 0.46) than were measures of %BMIp95 (r = 0.61) or BMIaz scores with no upper boundary (r = 0.65).
BMI z scores were weakest when applied to children younger than 10 years, with correlations of r = 0.36 for BMI z vs. correlations of 0.60 and 0.57 for BMIaz and %BMIp95, respectively.
The results were limited by several factors including the age of the data (40 years ago, when the prevalence of severe obesity was lower, 2% compared with approximately 6% now) and long intervals between exams in some cases (5 years or more), the researchers noted. However, the results suggest that BMI z values “can differ substantially from empirical estimates, have an effective upper limit, and are strongly influenced by sex and age,” they said. As an alternative, the researchers recommended that “very high BMIs should be should expressed as z scores on the basis of linear extrapolations of a fixed SD or as percentage of the CDC 95th percentile,” or using multilevel models that adjust for age and sex.
The researchers had no financial conflicts to disclose. The National Institute on Aging, the National Heart, Lung, and Blood Institute, and the National Institutes of Health funded the study.