Obesity Definition: Understanding BMI and Weight Ranges

Obesity is a chronic disease defined by excessive adiposity that impairs health. Body Mass Index (BMI) is the standard population-level screening tool.

BMI Classification (Adults)

Category	BMI (kg/m²)
Underweight	<18.5
Normal weight	18.5-24.9
Overweight	25.0-29.9
Obesity Class I	30.0-34.9
Obesity Class II	35.0-39.9
Obesity Class III (Severe)	≥40.0

BMI Calculation

BMI = weight (kg) / height (m)²
or (weight (lbs) × 703) / height (in)²

Limitations of BMI

• Doesn't distinguish fat vs. muscle mass
• Doesn't reflect fat distribution (visceral vs. subcutaneous)
• Less accurate in: athletes, elderly, certain ethnicities (Asian: lower cutoffs), pregnancy

Better Measures of Adiposity

• Waist circumference: >102 cm (40") men, >88 cm (35") women → high visceral fat risk
• Waist-to-height ratio: >0.5 increased risk
• Body fat %: DEXA, BIA, skinfolds
• Visceral fat imaging: CT/MRI (research)

Clinical Definition (Beyond BMI)

Obesity = abnormal/excessive fat accumulation presenting health risk. Diagnosis considers:

• BMI + waist circumference
• Comorbidities (T2DM, hypertension, dyslipidemia, OSA, NAFLD, osteoarthritis)
• Functional limitations
• Metabolic health markers

Pediatric

• BMI-for-age percentiles (CDC growth charts)
• Overweight: 85th-94th percentile
• Obesity: ≥95th percentile
• Severe obesity: ≥120% of 95th percentile or ≥35 kg/m²

Obesity is a complex, relapsing disease requiring comprehensive, individualized management—not just a number.