Obesity Classification Based on BMI
In adults, obesity is defined as a BMI ≥30 kg/m², with overweight defined as BMI 25-29.9 kg/m², and obesity is further subdivided into three classes: Class 1 (BMI 30-34.9 kg/m²), Class 2 (BMI 35-39.9 kg/m²), and Class 3 (BMI ≥40 kg/m²). 1, 2
Adult BMI Classification
The standard classification system, adopted by the World Health Organization and the National Heart, Lung, and Blood Institute, defines the following categories 1, 2:
- Underweight: BMI <18.5 kg/m² 1
- Normal weight: BMI 18.5-24.9 kg/m² 1, 2
- Overweight: BMI 25.0-29.9 kg/m² 1
- Obesity: BMI ≥30.0 kg/m² 1
Obesity Subclassification
Obesity is graded into three classes based on increasing health risk 1, 2:
- Class 1 (Mild): BMI 30.0-34.9 kg/m² 1
- Class 2 (Moderate): BMI 35.0-39.9 kg/m² 1
- Class 3 (Severe/Morbid): BMI ≥40.0 kg/m² 1
BMI Calculation
BMI is calculated as weight in kilograms divided by height in meters squared (kg/m²) 1, 2. For measurements in pounds and inches, use the formula: [weight (pounds) ÷ height (inches)²] × 703 1.
Ethnic-Specific Adjustments
For Asian populations, lower BMI thresholds should be used because they have greater adiposity and higher comorbidity risks at lower BMI levels 1, 2, 3:
- Overweight in Asians: BMI ≥23 kg/m² 2, 3
- Obesity in Asians: Lower than standard 30 kg/m² threshold 1, 3
The standard WHO classifications are not appropriate for Indian and other South/Southeast Asian populations 3. Studies demonstrate that BMI cut points between 23-27 kg/m² more accurately identify obesity in these populations 3.
Pediatric BMI Classification
In children and adolescents (ages 2-18 years), BMI percentiles adjusted for age and sex are used instead of absolute BMI values 1:
- Healthy weight: BMI 5th to <85th percentile 1
- Overweight: BMI 85th to <95th percentile 1
- Obese: BMI ≥95th percentile or BMI ≥30 kg/m² (whichever is lower) 1
- Severe obesity: BMI ≥99th percentile 1
The rapid changes in body size and composition during growth make assessment more complicated in children than adults, requiring age- and sex-specific target values 1.
Additional Measurements Beyond BMI
Waist circumference should be measured in addition to BMI to assess central adiposity and cardiovascular risk 2, 4:
- Men: >102 cm (>40 inches) indicates increased risk 2
- Women: >88 cm (>35 inches) indicates increased risk 2, 4
- Asian women: ≥80 cm indicates abdominal obesity 3
- Asian men: ≥90 cm indicates abdominal obesity 3
Waist circumference is particularly important for risk stratification in patients with BMI 25-34.9 kg/m² 4. Central (abdominal) adiposity increases cardiovascular disease risk independent of overall obesity 2.
Clinical Application for Treatment Decisions
Weight loss treatment is recommended based on the following algorithm 2:
- BMI 18.5-24.9 without risk factors: Maintain current weight, avoid weight gain 2
- BMI 25.0-29.9 without cardiovascular risk factors: Avoid further weight gain through regular weight monitoring and physical activity 2
- BMI ≥25.0 with cardiovascular risk factors OR BMI ≥30.0: Weight loss treatment recommended with initial goal of 10% body weight reduction 2
For Asian populations with BMI ≥23 kg/m², perform comprehensive clinical assessment to identify obesity-related complications and potential barriers to treatment 3.
Important Limitations of BMI
BMI has good specificity (90%) but low sensitivity (50%) for diagnosing excess adiposity 2. It does not distinguish between lean and fat mass, potentially misclassifying individuals with high muscle mass as overweight or obese 2. BMI may not correlate as well with body fat in certain age, sex, and ethnic groups 2.
Common Pitfalls
Failing to consider ethnic-specific body composition differences can result in delayed intervention for obesity-related complications in Asian populations 3. Always assess waist circumference alongside BMI, as central adiposity confers additional cardiovascular risk even at lower BMI levels 2, 4. In children, never use absolute BMI cutoffs—always use age- and sex-adjusted percentiles 1.