Using Metrics of Kilograms (or Pounds) Overweight or Kilograms (or Pounds) Obese to Help Interpret and Communicate Magnitudes of Excess Body Mass Index

Abstract

This study uses weight data from a cohort of children aged 7 to 11 years to assess a new method of communicating about body mass index, overweight, and obesity with patients and their families.

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.

Methods

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 – BMI_ow] × Height (in Meters)²

Pounds Overweight = [BMI – BMI_ow] × Height (in Meters)² × 2.205

Kilograms Obese = [BMI – BMI_ob] × Height (in Meters)²

Pounds Obese = [BMI – BMI_ob] × Height (in Meters)² × 2.205

where BMI_ow indicates the 85th percentile BMI for age and sex and BMI_ob 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.

Table 1. Correlations Between Measures of Obesity and With Obesity-Associated Physiological Risk Factors in 2 Age Ranges^a.

Point	kg or lb Overweight	kg or lb Obese	BMI	% Of median BMI for age and sex	% Of 95th percentile BMI for age and sex	Waist circumference, cm	Triceps skinfold thickness, mm
Baselineb
kg or lb Overweight	1.00	0.98	0.97	0.97	0.92	0.89	0.80
kg or lb Obese	0.98	1.00	0.93	0.98	0.96	0.81	0.77
BMI	0.97	0.93	1.00	0.96	0.88	0.93	0.82
% Of median BMI for age and sex	0.97	0.98	0.96	1.00	0.98	0.84	0.79
% Of 95th percentile BMI for age and sex	0.92	0.96	0.88	0.98	1.00	0.73	0.73
Waist circumference	0.89	0.81	0.93	0.84	0.73	1.00	0.78
Triceps skinfold thickness	0.80	0.77	0.82	0.79	0.73	0.78	1.00
Fasting serum levels
Total cholesterol	NA	NA	NA	NA	NA	NA	NA
LDL cholesterol	NA	NA	NA	NA	NA	NA	NA
HDL cholesterol	−0.27	−0.27	−0.28	−0.29	−0.27	−0.28	−0.22
Insulin	0.47	0.44	0.51	0.47	0.41	0.49	0.44
Blood pressure
Systolic	0.27	0.23	0.27	0.19	0.14	0.29	0.21
Diastolic	0.13	NA	0.13	NA	NA	0.14	NA
Year 3c
kg or lb Overweight	NA	0.99	0.98	0.98	0.97	0.94	0.71
kg or lb Obese	0.99	NA	0.96	0.99	0.99	0.92	0.71
BMI	0.98	0.96	NA	0.96	0.94	0.95	0.73
% Of median BMI for age and sex	0.98	0.99	0.96	NA	1.00	0.90	0.74
% Of 95th percentile BMI for age and sex	0.97	0.99	0.94	1.00	NA	0.89	0.72
Waist circumference	0.94	0.92	0.95	0.90	0.89	NA	0.68
Triceps skinfold thickness	0.71	0.71	0.73	0.74	0.72	0.68	NA
Fasting serum levels
Total cholesterol	NA	NA	NA	NA	NA	NA	NA
LDL cholesterol	NA	NA	NA	NA	NA	NA	NA
HDL cholesterol	−0.38	−0.39	−0.38	−0.39	−0.40	−0.35	−0.32
Insulin	0.57	0.56	0.57	0.56	0.55	0.58	0.44
Blood pressure
Systolic	0.34	0.31	0.33	0.26	0.27	0.37	0.15
Diastolic	0.25	0.23	0.25	0.22	0.23	0.25	0.16

Abbreviations: BMI, body mass index (calculated as weight in kilograms divided by height in meters squared); HDL, high-density lipoprotein; LDL, low-density lipoprotein; NA, not applicable.

^aPearson correlation coefficients. Results were similar when using Spearman correlation coefficients. Nonsignificant correlation coefficients (2-sided P > .05) are not shown.

^bThe number of participants was 268 for all measures. Age, mean (SD): 9.53 (1.46) years (range, 7.00-11.98 years).

^cThe number of participants was 251 for kilograms (or pounds) greater than the 85th-percentile level for age and sex, kilograms (or pounds) greater than the 95th-percentile level for age and sex, BMI, percentage of median BMI for age and sex, and percentage of the 95th-percentile BMI level for age and sex; 249 for waist circumference and triceps skinfold thickness; 237 for fasting total cholesterol level, LDL cholesterol level, HDL cholesterol level, and insulin level; and 242 for systolic and diastolic blood pressures. Age, mean (SD), 12.68 (1.46) years (range, 10.03-15.49 years).

Table 2. Correlations Between Changes in Measures of Obesity and With Changes in Obesity-Associated Physiological Risk Factors Over 1 Year and Over 3 Years^a.

Outcome	kg or lb Overweight	kg or lb Obese	BMI	% Of median BMI for age and sex	% Of 95th-percentile BMI for age and sex	Waist circumference, cm	Triceps skinfold thickness, mm
1-y Changesb
kg or lb Overweight	1.00	1.00	0.96	0.94	0.92	0.77	0.59
kg or lb Obese	1.00	1.00	0.97	0.94	0.92	0.76	0.59
BMI	0.96	0.97	1.00	0.99	0.99	0.78	0.64
% Of median BMI for age and sex	0.94	0.94	0.99	1.00	0.99	0.78	0.64
% Of 95th-percentile BMI for age and sex	0.92	0.92	0.99	0.99	1.00	0.77	0.64
Waist circumference	0.77	0.76	0.78	0.78	0.77	1.00	0.56
Triceps skinfold thickness	0.59	0.59	0.64	0.64	0.64	0.56	1.00
Fasting serum levels
Total cholesterol	0.20	0.21	0.23	0.25	0.24	0.19	0.16
LDL cholesterol	0.22	0.22	0.24	0.25	0.24	0.21	0.15
HDL cholesterol	−0.16	−0.16	−0.15	−0.16	−0.16	−0.17	NA
Insulin	0.18	0.17	0.16	0.16	0.15	NA	0.19
Blood pressure
Systolic	0.16	0.16	0.17	0.17	0.15	0.13	0.17
Diastolic	0.20	0.20	0.21	0.22	0.21	0.18	0.15
3 y Changesc
kg or lb Overweight	NA	1.00	0.95	0.89	0.86	0.85	0.46
kg or lb Obese	1.00	NA	0.95	0.89	0.87	0.84	0.46
BMI	0.95	0.95	NA	0.97	0.96	0.87	0.57
% Of median BMI for age and sex	0.89	0.89	0.97	NA	0.97	0.89	0.63
% Of 95th-percentile BMI for age and sex	0.86	0.87	0.96	0.97	NA	0.82	0.59
Waist circumference	0.85	0.84	0.87	0.89	0.82	NA	0.55
Triceps skinfold thickness	0.46	0.46	0.57	0.63	0.59	0.55	NA
Fasting serum levels
Total cholesterol	0.21	0.22	0.26	0.27	0.27	0.20	0.14
LDL cholesterol	0.21	0.22	0.25	0.23	0.25	0.15	NA
HDL cholesterol	−0.32	−0.32	−0.33	−0.35	−0.35	−0.39	−0.29
Insulin	0.43	0.42	0.40	0.38	0.37	0.42	0.23
Blood pressure
Systolic	0.14	0.13	NA	NA	NA	0.15	NA
Diastolic	0.21	0.20	0.20	0.20	0.18	0.22	NA

Abbreviations: BMI, body mass index (calculated as weight in kilograms divided by height in meters squared); HDL, high-density lipoprotein; LDL, low-density lipoprotein; NA, not applicable.

^aPearson correlation coefficients. Results were similar when using Spearman correlation coefficients. Nonsignificant correlation coefficients (2-sided P > .05) are not shown.

^bThe number of participants was 265 for kilograms (or pounds) greater than the 85th-percentile level for age and sex, kilograms (or pounds) greater than the 95th-percentile level for age and sex, BMI, percentage of the median BMI for age and sex, and percentage of the 95th-percentile BMI level for age and sex; 264 for waist circumference, triceps skinfold thickness, and systolic and diastolic blood pressures; and 257 for fasting total cholesterol, LDL cholesterol, HDL cholesterol, and insulin levels.

^cThe number of participants was 251 for kilograms (or pounds) greater than the 85th-percentile level for age and sex, kilograms (or pounds) greater than the 95th-percentile level for age and sex, BMI, percentage of the median BMI for age and sex, and percentage of the 95th-percentile BMI for age and sex; 249 for waist circumference and triceps skinfold thickness; 237 for fasting total cholesterol, LDL cholesterol, HDL cholesterol, and insulin levels; and 242 for systolic and diastolic blood pressures.

Results

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).

Discussion

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).