What is body mass index (BMI)?

• A. A diagnostic tool for obesity.
• B. A screening tool used to identify children over 2 years of age and adolescents who are outside of their healthy weight ranges.
• C. A measure of waist circumference relative to height.
• D. A measure of body fat percentage.

Answer: (D)

BMI is a simple index that relates a person’s weight to their height to screen for weight-related health risk. It is not a direct measure of body fat; rather, it estimates whether weight relative to height falls into ranges associated with health concerns. It’s calculated by dividing weight in kilograms by height in meters squared (kg/m^2).

As a screening tool, BMI helps identify individuals who may be underweight, normal weight, overweight, or obese and signals when further assessment is needed. In adults, specific cutoffs define these categories, while in children and teens, BMI-for-age and sex percentiles are used to account for growth.

However, BMI has limitations. It does not distinguish between fat and lean mass, so highly muscular individuals may have a high BMI without excess fat, and some people with normal BMI may have higher body fat. It also doesn’t capture where fat is distributed. For precise body fat measurement, other methods such as bioelectrical impedance, DEXA, or skinfolds are used.

So, BMI describes weight relative to height as a screening measure, not as a direct measure of body fat percentage.