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The Body Mass Index (BMI) Explained

Apr 30 2010 by pskaf

What is BMI?

BMI is an often used clinical measurement tool that assesses an individual’s body weight in relation to their height. Essentially, clinicians, researchers, and trainers use this tool to determine a person’s level of obesity (or lack thereof). The higher a BMI value is, the more fat mass is assumed to exist. The BMI value is then categorized, placing the person in one of 4 main categories: Underweight, Normal weight, Overweight, and Obese (see the chart below).

How is BMI Calculated?

BMI = weight (kg) / height (m) ²

For example, let’s assume Dave is a 20 year-old male who weighs 90 kg, with a height of 1.8 m. The calculation would be as follows:

90kg / (1.8m ²) = 27.8 kg/m²

Interpreting BMI

27.8 would place the individual in the BMI category of “overweight”. However, this value is left to some interpretation, and this is where the shortcoming of the BMI systems arises. Even though Dave is classified as overweight, we can’t accept this as an accurate and telling figure. Dave’s total bodyweight is used in the equation, that means we can’t differentiate between how much of the 90kg is fat, and how much is lean mass. If BMI is the only measure we have, it is impossible to determine if Dave has excess fat, or excess muscle! Dave could be a muscular sprinter, or conversely a chubby guy with underdeveloped musculature in comparison to a sprinter. In order for the BMI to have any meaningfulness, we need additional information such as body fat percentage, and waist circumference.
Look for upcoming articles further exploring methods of body fat analysis.

- Peter Skaf

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