Customize your JAMA Network experience by selecting one or more topics from the list below.
Body mass index (BMI; calculated as weight in kilograms divided by height in meters squared) has become the standard metric for assessing excess weight in clinical, public health, and research contexts because of its high levels of accessibility, measurement reliability and validity, clinical validity, and sensitivity to change over time.1,2 In children and adolescents, overweight and obesity are defined as BMI at or above the 85th and 95th percentiles, respectively, for age and sex on the US Centers for Disease Control and Prevention (CDC) BMI growth charts.1,3 However, BMI units and their corresponding percentiles and z scores may not be easy to understand for many patients, families, and clinicians themselves, potentially making these measures of excess weight difficult to interpret, communicate, compare over time, and act on. Therefore, this analysis investigates new age-adjusted and sex-adjusted metrics, kilograms (or pounds) overweight and kilograms (or pounds) obese, arithmetically transformed from BMI data and the CDC growth references.
Participants were children aged 7 to 11 years at study entry, had BMIs at or above the 85th percentile for age and sex, were recruited from low-income neighborhoods in the San Francisco Bay Area, California, to participate in a community-based trial, and were measured annually for 3 years, from September 2012 to December 2016.4 Parents and children provided written consent and assent for participation, and the protocol was approved by the Stanford University institutional review board.
Kilograms (or pounds) overweight and obese are defined as the number of kilograms (or pounds) above the age-adjusted and sex-adjusted BMI thresholds for overweight and obesity, respectively. These are calculated from measures of height, weight, and the CDC BMI growth charts or online calculators, using the following formulas:
Kilograms Overweight = [BMI – BMIow] × Height (in Meters)2
Pounds Overweight = [BMI – BMIow] × Height (in Meters)2 × 2.205
Kilograms Obese = [BMI – BMIob] × Height (in Meters)2
Pounds Obese = [BMI – BMIob] × Height (in Meters)2 × 2.205
where BMIow indicates the 85th percentile BMI for age and sex and BMIob indicates the 95th percentile BMI for age and sex. For ease of manual calculations, multiplying by height2 is equivalent to multiplying by height twice (ie, height × height).
To judge measurement validity and clinical validity for use in clinical care, research and surveillance, we first calculated Pearson correlation coefficients between kilograms (or pounds) overweight and kilograms (or pounds) obese and other accepted measures of obesity and weight-associated physiological risk factors at 2 different times, at study entry and approximately 3 years later (Table 1). We then conducted a similar analysis comparing changes in each measure over approximately 1 and 3 years (Table 2). The threshold of statistical significance was set at a 2-tailed P < .05. Analysis was conducted with SAS version 9.4 (SAS Institute) between February 2019 and November 2019.
Of the 268 children participating, 147 (54.9%) were girls and 263 (98.1%) were Latino. A total of 202 (75.4%) had BMIs at or above the 95th percentile, 149 (55.6%) at or above the 97th percentile, and 87 (32.4%) at or above 120% of the 95th percentile.
Kilograms (or pounds) overweight and kilograms (or pounds) obese were highly correlated with other measures of obesity (baseline r range, 0.92 to 0.98; year 3 r range, 0.97 to 0.99) and comparably with those measures with physiological risk factors (Table 1). All correlations generally increased as children aged 7 to 11 to 10 to 14 years. One-year and 3-year changes in kilograms (or pounds) overweight and kilograms (or pounds) obese correlated highly with concurrent changes in other obesity measures (1-year changes: r range, 0.92 to 0.97; 3-year changes: r range, 0.86 to 0.95) and comparably with those measures with concurrent changes in physiological risk factors (Table 2).
Kilograms (or pounds) overweight and kilograms (or pounds) obese are simple and valid metrics to express excess weight and weight changes among children with overweight and obesity. This could mean, for example, describing a person as having a weight that is 4.55 kg (or 10 lb) overweight or 4.55 kg (or 10 lb) obese, rather than (or in addition to) having a weight that is 3 BMI units greater than the cutoff for overweight or obesity, being at 180% of the median BMI for age and sex, or being 20% above the 95th percentile BMI for age and sex. This approach may be more meaningful for patients, families, and clinicians who are used to thinking about weight, weight gain, and weight loss in terms of kilograms (or pounds).
Accepted for Publication: June 16, 2020.
Published Online: December 21, 2020. doi:10.1001/jamapediatrics.2020.5196
Corresponding Author: Thomas N. Robinson, MD, MPH, Stanford Solutions Science Lab, Division of General Pediatrics, Department of Pediatrics, Stanford Prevention Research Center, 1265 Welch Rd, MSOB X129, Stanford, CA 94305 (email@example.com).
Author Contributions: Dr Robinson had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Conflict of Interest Disclosures: Dr Robinson reported grants from National Institutes of Health and other financial support from Stanford Maternal and Child Health Institute and Department of Pediatrics at Stanford University during the conduct of the study and grants from the US Centers for Disease Control and Prevention and personal fees from WW International Inc outside the submitted work.
Funding/Support: The data used in this study were collected with support by the National Heart, Lung, and Blood Institute of the National Institutes of Health under award U01HL103629. This study was supported in part by the Stanford Maternal and Child Health Research Institute and the Department of Pediatrics at Stanford University.
Role of the Funder/Sponsor: The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.
Disclaimer: The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
Additional Contributions: I thank K. Farish Haydel, BA, Stanford Solutions Science Lab, Department of Pediatrics, Stanford University, for assistance with database management and analysis and the participants and staff of the Stanford GOALS trial, without whom this study would not be possible. Ms Haydel was compensated for her contributions.
Robinson TN. Using Metrics of Kilograms (or Pounds) Overweight or Kilograms (or Pounds) Obese to Help Interpret and Communicate Magnitudes of Excess Body Mass Index. JAMA Pediatr. 2021;175(4):410–412. doi:10.1001/jamapediatrics.2020.5196
Artificial Intelligence Resource Center