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BMI z scores fall short for tracking severe obesity

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Abandon BMI z scores for tracking severe obesity

The use of BMI z scores to assess and track severe obesity in children should be abandoned.

In the study by Freedman et al., BMI z scores, which are extrapolations of BMI measurements, did not correlate well with other measures of adiposity. Their use to assess severe obesity is problematic because large changes in weight and BMI are linked to small changes in BMI z or BMI percentiles.

Dr. William H. Dietz of Sumner M. Redstone Global Center for Prevention and Wellness, Milken Institute School of Public Health at George Washington University in Washington Courtesy Dr. William Dietz

Dr. William H. Dietz

Satisfactory alternatives to BMI z to measure severe obesity include absolute BMI, BMIp95, BMI minus BMI at the 95th percentile, or BMI as a percentage of the median BMI for age and sex. These measures give quantifiable increases and decreases in BMI for a single patient, and allow comparison of changes with other children or adolescents over time.

William H. Dietz, MD, PhD, is at the Sumner M. Redstone Global Center for Prevention and Wellness, Milken Institute School of Public Health at George Washington University in Washington. He had no relevant financial disclosures, but disclosed that he serves on the scientific advisory board for Weight Watchers and is on the board of directors for the Partnership for a Healthier America. He discussed the article by Freedman et al. in an editorial (Pediatrics. 2017;140:e20172148).



Body mass index z scores fall short for tracking children with severe obesity, based on data from nearly 7,000 children in the Bogalusa Heart Study.

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.

An overweight child eats a hamburger. iStockphoto
To compare different BMI metrics, the researchers reviewed data from 6,977 children aged 2-17 years participating in the longitudinal Bogalusa Heart Study who were examined in two or more cross-sectional studies. Among the 247 who met criteria for severe obesity, the average BMI was 32 kg/m2, the average BMI z score was 2.4 standard deviation (SD), and the average BMI adjusted z score (BMIaz) was +3.0 SD. Correlations for BMI z scores were “consistently weaker than those for BMI, BMIaz, and %BMIp95,” for children with obesity or severe obesity over an average of 2.8 years. Obesity was defined as BMI in the 95th percentile or higher.

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.

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