BMI Categories Explained: Underweight to Obese

The World Health Organization (WHO) classifies adult Body Mass Index (BMI) into four categories based on health risk associations observed in population studies.

Underweight: BMI below 18.5. Associated with nutritional deficiencies, weakened immune function, osteoporosis risk, and fertility issues. May indicate inadequate caloric intake, malabsorption disorders, or underlying medical conditions. Weight gain through increased caloric intake and strength training is typically recommended.

Normal weight: BMI 18.5–24.9. Associated with the lowest overall health risk in population studies. This range is the target for most adults. However, "normal" BMI does not guarantee good health — body composition, diet quality, physical activity, and other health markers matter independently.

Overweight: BMI 25.0–29.9. Associated with increased risk of cardiovascular disease, type 2 diabetes, hypertension, and joint problems. The risk increase is modest at the lower end of this range and becomes more significant as BMI approaches 30.

Obese: BMI 30.0 and above. Associated with substantially elevated risk for cardiovascular disease, type 2 diabetes, certain cancers, sleep apnea, and osteoarthritis. This category is further divided into Class I (30.0–34.9), Class II (35.0–39.9), and Class III (40.0+), with health risks increasing at each level.

These categories are statistical generalizations. Individual health assessment should consider body composition, waist circumference, blood markers, family history, and lifestyle factors alongside BMI.