Assessing BMI Change in Research

What to know

Although extended body mass index (BMI) z-scores (BMIz) should work well in most analyses, the best metric for assessing BMI change is uncertain. The interpretation of these metrics may also differ by age.

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Overview

Although extended BMIz should work well in most analyses, the best metric for assessing BMI change is uncertain. Several studies123456 have shown that expressing BMI relative to the CDC 50th or 95th percentile is better than the LMS-based version of BMIz when assessing adiposity changes. This is because of the curvilinear relation of BMI to BMIz but would likely apply to both the original and extended7 versions of BMIz.

Further, a recent commentary8 suggested that until consensus is reached, “investigators can consider consistently reporting more metrics." Additional metrics could include:

  • Absolute and percentage change in BMI.
  • Change in percentage of the 95th BMI percentile.
  • Change in percentage of the median.

As of December 15, 2022, the SAS Program for CDC Growth Charts includes two additional variables for assessing BMI change:

  • bmip95: BMI expressed as a percentage of the 95th percentile of the CDC growth charts.
  • bmip50: BMI expressed as a percentage of the 50th percentile (median) of the CDC growth charts.

You can subtract 100 from bmip50 to get the %distance from the median.9

Be careful when using metrics to assess change. Their interpretations likely differ by age. The dispersion of BMI values is much greater at age 18 than at age 2 years. A bmip50 value of 150, for example, could be interpreted differently at these two ages. You can adjust these metrics for differences by sex and age by specifying the 'all=TRUE' option in the R program.

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Content Source:
National Center for Chronic Disease Prevention and Health Promotion (NCCDPHP)
  1. Cole TJ, Faith MS, Pietrobelli A, Heo M. What is the best measure of adiposity change in growing children: BMI, BMI%, BMI z-score or BMI centile? European Journal of Clinical Nutrition 2005;59:419–25.
  2. Barlow SE, Salahuddin M, Durand C, Pont SJ, Hoelscher DM, Butte NF. Evaluation of BMI Metrics to Assess Change in Adiposity in Children with Overweight and Moderate and Severe Obesity. Obesity 2020;28:1512–8.
  3. Berkey CS, Colditz GA. Adiposity in adolescents: change in actual BMI works better than change in BMI z score for longitudinal studies. Annals of Epidemiology 2007;17:44–50.
  4. Kakinami L, Henderson M, Chiolero A, Cole TJ, Paradis G. Identifying the best body mass index metric to assess adiposity change in children. Archives of Disease in Childhood 2014;99:1020–4.
  5. Freedman DS, Woo JG, Daniels SR. Longitudinal changes in various BMI metrics and adiposity in 3- to 7-year-olds. Pediatrics In Press 2022 Dec 1;150(6):e2022058302.
  6. Paluch RA, Epstein LH, Roemmich JN. Comparison of methods to evaluate changes in relative body mass index in pediatric weight control. American journal of human biology : the official Journal of the Human Biology Council 2007;19:487–94.
  7. Freedman DS, Goodwin Davies AJ, Phan T-LT, Cole FS, Pajor N, Rao S, Eneli I, Kompaniyets L, Lange SJ, Christakis DA, et al. Measuring BMI change among children and adolescents. Pediatr Obes 2022;e12889.
  8. Ryder JR, Kelly AS, Freedman DS. Metrics matter: Toward consensus reporting of BMI and weight-related outcomes in pediatric obesity clinical trials. Obesity (Silver Spring) 2022;30:571–2.
  9. Hales C, Freedman DS, Akinbami L, Wei R, Ogden CL. Using CDC growth charts to assess and monitor weight status in children and adolescents with extremely high BMI. National Center for Health Statistics Vital Health Stat 2 2022;197.