Over the last decade, the term BMI has been tossed around a lot, but I feel it is very important to know what BMI is and what BMI is not. It stands for Body Mass Index, which is one way researchers have used to measure body composition, that is to know the approximate lean body mass (Muscles, bones, organs, etc.) compared to the non-lean mass (adipose tissue). They calculate this by taking your height and weight and putting it through a formula, and this gives us a number that is the BMI. A number under 18.5 is underweight, 18.5 to 24.9 is normal, 25-29.9 is underweight, and anything over 30 is obese. This is nice and simple, but it also is a gross over simplification that leads to very rough data, which for the individual is not useful.

A BMI calculation does not look at what your weight is actually composed of, it only compares it to what an average person’s height and weight should be. If you have a very large frame, so larger bones and organs, compared to most people your height, BMI rankings will probably show you overweight or even obese, even though the extra weight may not be coming from fat stores. If you are a well muscled athlete, this extra muscle is just extra weight as far as the BMI formula is concerned, and you will be marked as obese! While it is certainly a far end of the spectrum, if you look at Arnold Schwarzenegger, who is absolutely nothing but muscle, the BMI says he is obese!

So why is it that researchers use this obviously inaccurate formula? Well the thing is that for most people in any given population it is set up for, it will be correct for the population, even if it is not accurate for the individual. The BMI was designed to be used by epidemiologists, who are researchers who study entire populations for the purposes of health so that they can see how certain elements that affect health are interacting with a population, so that they can advise policy makers and the health industry on certain related elements of health. If a population’s BMI goes up or down, it is likely indicative of gain or loss in fatty weight.

So what I’m trying to get at, is that if you calculate your BMI, and it says you are obese, you might be, but you also might have a big frame or have lots of muscle. You will most likely know which you are, obese or muscular, but obviously if you both have lots of muscle but are also overweight, the BMI number will not help tell you how overweight you are. It is simply not a precise tool. It is good for tracking populations, but is essentially useless for use with an individual. What is better, is to use any one of the three (or all) of the methods we use during assessments: circumference, caliper, or bio-impedence measurements that will much more accurately tell us your lean and non-lean mass. These look at actual measurements in adipose tissue compared to your weight to let us know your real measurements.

The BMI is useless for any practical application for an individual, and remains in service of of health scientists and the misuse of insurance companies.

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