What is BMI and how to interpret it correctly?

Body Mass Index (BMI) is a numerical measure that relates a person's body weight to their height. It was developed in the 19th century by Belgian mathematician Adolphe Quetelet and adopted by the World Health Organization as the standard indicator for classifying the weight status of the adult population.

How is BMI calculated?

The formula is straightforward: BMI = weight (kg) ÷ height² (m). For example, a person weighing 75 kg and 1.75 m tall has a BMI of 75 ÷ (1.75 × 1.75) = 24.5. This value falls in the normal weight range according to the WHO.

WHO categories

• •Underweight: BMI < 18.5 — may indicate malnutrition or underlying health problems.
• •Normal weight: 18.5 – 24.9 — associated with the lowest risk of weight-related disease.
• •Overweight: 25 – 29.9 — increases the risk of hypertension, type 2 diabetes and cardiovascular disease.
• •Obesity class I: 30 – 34.9 — moderate-to-high risk.
• •Obesity class II: 35 – 39.9 — high risk.
• •Obesity class III (morbid): ≥ 40 — very high risk; may require specialist medical intervention.

Limitations of BMI

BMI does not distinguish between muscle mass and body fat. An athlete with a lot of muscle may have a BMI of 27 (overweight according to the table) yet enjoy excellent metabolic health. Equally, a person with a normal BMI but low muscle mass and high visceral fat may have an elevated cardiovascular risk. BMI also fails to account for age, sex or fat distribution — abdominal fat is more dangerous than peripheral fat.

What other indicators complement BMI?

• •Waist circumference: elevated risk above 88 cm in women or 102 cm in men.
• •Waist-to-hip ratio (WHR): values > 0.85 in women and > 0.90 in men indicate cardiovascular risk.
• •Body fat percentage: measured by bioelectrical impedance or skinfold calipers.
• •Blood tests: blood glucose, cholesterol, triglycerides.

When should I see a doctor?

If your BMI is below 18.5 or above 30, it is advisable to speak with your doctor for a full assessment. A BMI in the normal range does not guarantee good health if other risk factors are present. Remember that BMI is a population screening tool, not an individual diagnosis.