The World Health Organization (WHO) uses the following BMI (Body Mass Index) categories to define underweight, normal weight, overweight, and obesity: # WHO BMI Categories 1. *Underweight*: BMI < 18.5 2. *Normal weight*: BMI = 18.5-24.9 3. *Overweight*: BMI = 25-29.9 4. *Obesity*: BMI ≥ 30 # Obesity Subcategories 1. *Class 1 obesity*: BMI = 30-34.9 2. *Class 2 obesity*: BMI = 35-39.9 3. *Class 3 obesity*: BMI ≥ 40 # BMI Interpretation 1. *Underweight*: May indicate malnutrition, osteoporosis, or other health problems. 2. *Normal weight*: Generally indicates a healthy weight. 3. *Overweight*: May increase the risk of chronic diseases like diabetes, cardiovascular disease, and certain cancers. 4. *Obesity*: Significantly increases the risk of chronic diseases and may reduce life expectancy. # Limitations of BMI 1. *Does not measure body composition*: BMI does not distinguish between lean body mass and body fat. 2. *May not be accurate for athletes or muscular individuals*: BMI may classify athletes or muscular individuals as overweight or obese, even if they have a low body fat percentage. 3. *May not be accurate for older adults*: BMI may not accurately reflect the health risks associated with excess weight in older adults. # Calculating BMI BMI is calculated by dividing weight (in kilograms) by height (in meters) squared: BMI = weight (kg) / height (m)² For example, if a person weighs 70 kg and is 1.75 m tall, their BMI would be: BMI = 70 kg / (1.75 m)² = 22.9 This would fall into the normal weight category.