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Updated: Mar 1, 2026
Assessment of Child Anthropometry in a Large Epidemiologic Study
Published on: February 2, 2017
BMI is a poor predictor of adiposity in young overweight and obese children
Cassandra Vanderwall1, R Randall Clark2, Jens Eickhoff3
1University of Wisconsin, Madison, WI, USA. CVanderwall@uwhealth.org.
Insights
Body Mass Index (BMI) z-score effectively predicts total fat mass in older children (9-18y). However, BMIz is a less reliable indicator of body fat percentage in younger children (<9y).
Area of Science:
- Pediatric Endocrinology
- Obesity Research
- Body Composition Analysis
Background:
- Body Mass Index (BMI) is a common screening tool for obesity.
- Accurate assessment of body composition, including total fat mass (TFM) and percent body fat (%FAT), is crucial in pediatric populations.
- The predictive accuracy of BMI for body composition in overweight and obese children requires further investigation.
Purpose of the Study:
- To evaluate the predictive ability of BMI z-score (BMIz) for TFM and %FAT in overweight and obese children.
- To determine if age influences the relationship between BMIz and body composition metrics.
- To clarify the utility and limitations of BMI as a screening tool for adiposity in pediatric populations.
Main Methods:
- An observational study involving 663 overweight and obese children.
- Body composition was assessed using dual-energy x-ray absorptiometry (DXA).
- Univariate and multivariate regression analyses were performed to assess BMIz as a predictor of TFM and %FAT, stratified by age groups.
Main Results:
- BMIz, sex, and age were significant predictors for both TFM and %FAT.
- In children aged 9-18 years, BMIz was a strong predictor of TFM (R² 0.57–0.73) but a moderate predictor of %FAT (R² 0.22–0.42).
- In children younger than 9 years, BMIz showed weak to moderate predictive power for both TFM (R² 0.03–0.26) and %FAT (R² 0.22–0.38).
Conclusions:
- BMIz is a robust predictor of TFM in older children (9-18 years).
- The predictive value of BMIz for %FAT is moderate across all age groups, particularly in younger children.
- Clinicians should exercise caution when using BMIz to estimate %FAT in children under 9 years old.
Background:
The body mass index (BMI) is a simple and widely utilized screening tool for obesity in children and adults. The purpose of this investigation was to evaluate if BMI could predict total fat mass (TFM) and percent body fat (%FAT) in a sample of overweight and obese children.
Methods:
In this observational study, body composition was measured by dual energy x-ray absorptiometry (DXA) in 663 male and female overweight and obese children at baseline within a multidisciplinary, pediatric fitness clinic at an academic medical center. Univariate and multivariate regression analyses were conducted to evaluate whether BMI z-score (BMIz) predicts TFM or %FAT.
Results:
The BMIz, sex and age of subjects were identified as significant predictors for both TFM and %FAT. In subjects younger than 9 years, the BMIz was a weak to moderate predictor for both TFM (R2 = 0.03 for males and 0.26 for females) and %FAT (R2 = 0.22 for males and 0.38 for females). For subjects between 9 and 18 years, the BMIz was a strong predictor for TFM (R2 between 0.57 and 0.73) while BMIz remained only moderately predictive for %FAT (R2 between 0.22 and 0.42).
Conclusions:
These findings advance the understanding of the utility and limitations of BMI in children and adolescents. In youth (9-18y), BMIz is a strong predictor for TFM, but a weaker predictor of relative body fat (%FAT). In children younger than 9y, BMIz is only a weak to moderate predictor for both TFM and %FAT. This study cautions the use of BMIz as a predictor of %FAT in children younger than 9 years.
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