The Validity of BMI as an Indicator of Body Fatness and Risk Among Children

Purpose of Review

• Obesity prevalence in children has tripled over the last three decades.
• BMI is a measure of excess weight but not exclusively excess body fat.
• This review examines BMI's correlation with body fatness and related health risks, particularly in identifying children with excessive body fatness based on BMI-for-age percentiles.
• Analysis includes consideration of race/ethnicity and the usefulness of skinfold/circumference measurements.

Results

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• The accuracy of BMI as a measure of body fatness varies:
  • For fat children, BMI effectively indicates excess adiposity.
  • For thinner children, BMI variations may be influenced by fat-free mass.
• High BMI-for-age (95th percentile) has:
  • Sensitivity: 70%-80%
  • Positive predictive value: Moderate
  • High specificity: 95%
• High BMI correlates with poor risk factor levels and increased likelihood of adult obesity.
• Alternative measurements (skinfold, waist circumference) can help identify children at risk who fall into moderately elevated BMI categories (85th-94th percentiles).

Conclusion

• A BMI-for-age at or above the 95th percentile is reasonably sensitive and specific for identifying children with excess body fat.

Understanding BMI as a Measure of Body Fatness

• BMI (kg/m²) is widely used but measures weight relative to height, not solely body fat.
• It does not distinguish between fat mass, muscle mass, and bone mass.
• Growth in children leads to substantial increases in BMI, complicating its interpretation.
  • Example: BMI increases during adolescence often result from rises in fat-free mass rather than fat.
• Despite its limitations, a high BMI for a child's age is generally associated with excess body fatness.

Obesity Terminology and Definitions

• “Obesity” is often used synonymously with high BMI; adult cutoff is BMI ≥ 30 kg/m².
• For children, high BMI cutoffs are defined relative to peers by age and gender.
• Excess body fatness is assessed through specific BMI cut points, including those defined in CDC growth charts (children in the 85th and 95th percentiles considered overweight and obese, respectively).

Challenges in Measuring Body Fatness

• Ideal body fatness measures should be accurate, precise, and accessible, yet no method fulfills all qualifications.
• The four-compartment model, relating mineral, water, density, and weight, is accurate but costly and impractical for widespread use.
• DEXA (dual-energy x-ray absorptiometry) is correlated with the four-compartment model and frequently used in studies, but it can yield biased results due to hydration and equipment variability.

Anthropometric Assessment

• Anthropometry is a low-cost method to assess body fat (using weight/height^p), with BMI as a power index (p=2).
• Skinfold and waist circumference measures have higher error rates compared to weight and height.
• The correlation of childhood BMI with body fatness significantly decreases with a child's thinness and can be influenced by race/ethnicity differences along with growth dynamics.

Classification Systems for Pediatric BMI

• Two primary systems for classifying childhood BMI:
  • CDC growth charts based on four surveys of US children from 1963-1980.
  • IOTF (International Obesity Taskforce) cut points based on linking adult BMI levels to younger populations, potentially leading to differing obesity prevalence due to varying methodologies.

Identification of Excess Body Fatness

• BMI is a useful screening tool for obesity risk but often assessed in conjunction with body fat measurements for more precise evaluations.
• Sensitivity and specificity of BMI depend on chosen cut points and prevalence among examined populations.
• Studies show a high predictive value of BMI for assessing body fat in children, with caveats regarding its efficacy among varying populations.

Racial and Ethnic Differences in Body Fatness

• Research suggests varying body fat compositions across races even at similar BMI levels, necessitating careful consideration in assessments.
• Black, white, and Asian children show differing body fat percentages even at equivalent BMIs, complicating health risk evaluations across diverse groups.

Health Risks Associated with Childhood BMI

• High childhood BMI correlates with cardiovascular disease risk factors and is predictive of adult obesity.
• Tracking childhood BMI into adulthood shows strong correlation with adult obesity rates; puberty and early childhood metrics significantly influence health outcomes.

Additional Body Fat Measurement Tools

• Skinfold thickness measures provide more accurate body fat estimates compared to BMI but have measurement error concerns.
• Waist-to-height ratio (WHtR) is a promising indicator of fat distribution and associated risks, correlating well with BMI but simpler to communicate.

Conclusion Recap

• BMI is a reasonable screening tool for excess body fat in children, especially when accompanied by complementary measurements.
• While shortcomings exist due to variable body compositions, appropriate cut points could make BMI a reliable indicator of children at risk for health issues linked to excess body fat.