Anthropometric assessment - body mass index (BMI)

Definition

BMI (weight in kg divided by height in metres squared) is the most widely used simple screening measure for weight status - classifying underweight, healthy weight, overweight, and obesity categories, though it is a population-level screening tool rather than a precise individual body composition measure
Universally used in clinical and public health practice given its simplicity, low cost, and reasonable correlation with adiposity-related health risk at a population level

BMI cannot distinguish fat mass from lean muscle mass, meaning it systematically misclassifies muscular individuals as overweight/obese and can under-classify adiposity in individuals with reduced muscle mass (e.g. some older adults, sarcopenic patients) despite meaningfully elevated fat mass
Muscular individuals misclassified as overweight/obese, ethnicity-specific threshold variation, sarcopenic obesity (normal/low BMI masking significant adiposity with low muscle mass), bariatric surgery eligibility criteria

Standard cut-offs (underweight <18.5, healthy 18.5-24.9, overweight 25-29.9, obese ≥30 kg/m²) apply reasonably well at a population level but should be interpreted alongside other measures (waist circumference/waist-to-hip ratio) when body composition is atypical
Ethnicity-specific BMI thresholds are recommended in some populations (e.g. lower cut-offs for some Asian populations) given differing body composition-risk relationships at a given BMI

BMI results feed into risk stratification for metabolic and cardiovascular disease, bariatric surgery eligibility assessment, and general clinical counselling, generally used alongside waist circumference/waist-to-hip ratio rather than in isolation
A single BMI measurement should be interpreted alongside trend over time and other anthropometric/clinical context, rather than as a standalone diagnostic threshold

BMI alone cannot distinguish fat mass from muscle mass, meaning a heavily muscled individual can be misclassified as overweight or obese by BMI despite low actual adiposity - this is a frequently tested limitation, and waist circumference/waist-to-hip ratio provides better discrimination of actual adiposity-related risk in these atypical body compositions

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