BMI Classification System
Body Mass Index (BMI) is classified into distinct categories: underweight (<18.5 kg/m²), normal weight (18.5-24.9 kg/m²), overweight (25.0-29.9 kg/m²), and obesity classes I (30.0-34.9 kg/m²), II (35.0-39.9 kg/m²), and III (≥40 kg/m²). 1, 2
Adult BMI Classification
The American Heart Association and World Health Organization have established the following BMI categories for adults:
- Underweight: BMI < 18.5 kg/m²
- Normal or acceptable weight: BMI 18.5-24.9 kg/m²
- Overweight: BMI 25.0-29.9 kg/m²
- Obesity:
Children's BMI Classification
For children and adolescents (ages 2-18 years), BMI is interpreted differently:
- Healthy weight: BMI between 5th and 85th percentile for age and sex
- Overweight: BMI between 85th and 95th percentile
- Obese: BMI at or above 95th percentile or BMI of 30 kg/m², whichever is lower
- Severe obesity: BMI at 99th percentile 1
Clinical Significance of BMI Categories
BMI classification has important clinical implications:
Underweight (BMI < 18.5): Associated with increased all-cause mortality risk, nutritional deficiencies, reduced immune function, hormonal imbalances, and bone density loss 2
Overweight and Obesity: BMI ≥ 30 kg/m² is associated with increased risk of mortality, coronary heart disease, cardiovascular disease, and to a lesser extent, cancer 3
Medication Considerations: For example, weight loss medications like phentermine are indicated for patients with BMI ≥ 30 kg/m² or ≥ 27 kg/m² with additional risk factors such as hypertension, diabetes, or hyperlipidemia 4
Limitations of BMI
It's important to recognize that BMI has several limitations:
- BMI is a surrogate measure of body fatness and doesn't directly measure body fat percentage 5, 6
- It doesn't account for differences in body composition (muscle vs. fat mass)
- It doesn't capture information on fat distribution in different body sites 6
- BMI interpretation may need adjustment for factors such as age, sex, race/ethnicity, and athletic status 5, 6
Additional Anthropometric Measurements
To complement BMI assessment, consider:
- Waist circumference: Provides information about abdominal fat distribution
- Waist-to-hip ratio: Helps assess body fat distribution patterns 7
These measurements can provide additional insight into health risks beyond what BMI alone can indicate.