What is BMI and why you should care

April 25th, 2009 — Kid's Obesity

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The body mass index (BMI), or Quetelet index, is a statistical measurement which compares a person’s weight and height. Though it does not actually measure the percentage of body fat, it is a useful tool to estimate a healthy body weight based on how tall a person is. Due to its ease of measurement and calculation, it is the most widely used diagnostic tool to identify weight problem within a population including:underweight, overweight and obesity.

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It was invented between 1830 and 1850 by the Belgian polymath Adolphe Quetelet during the course of developing “social physics”.[1] Body mass index is defined as the individual’s body weight divided by the square of his height. The formulas universally used in medicine produce a unit of measure of kg/m2. BMI can also be determined using a BMI chart, which displays BMI as a function of weight (horizontal axis) and height (vertical axis) using contour lines for different values of BMI or colours for different BMI categories.

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As a measure, BMI became popular during the early 1950s and 60s[citation needed] as obesity started to become a discernible issue in prosperous Western societies. BMI provided a simple numeric measure of a person’s “fatness” or “thinness”, allowing health professionals to discuss over- and under-weight problems more objectively with their patients. However, BMI has become controversial because many people, including physicians, have come to rely on its apparent numerical authority for medical diagnosis, but that was never the BMI’s purpose.

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It is meant to be used as a simple means of classifying sedentary (physically inactive) individuals with an average body composition.[2] For these individuals, the current value settings are as follows: a BMI of 18.5 to 25 may indicate optimal weight; a BMI lower than 18.5 suggests the person is underweight while a number above 25 may indicate the person is overweight; a BMI below 17.5 may indicate the person has anorexia or a related disorder; a number above 30 suggests the person is obese (over 40, morbidly obese).

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For a given height, BMI is proportional to weight. However, for a given weight, BMI is inversely proportional to the square of the height. So, if all body dimensions double, and weight scales naturally with the cube of the height (as is the case with a spherical cow), then BMI doubles instead of remaining the same.

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This results in taller people having a reported BMI that is uncharacteristically high compared to their actual body fat levels. This anomaly is partially offset by the fact that many taller people are not just “scaled up” short people, but tend to have narrower frames in proportion to their height. It has been suggested that instead of squaring the body height (as the BMI does) or cubing the body height (as seems natural), it would be more appropriate to use an exponent of between 2.3 to 2.7.[3]