What is Body Mass Index and Is It Useful?
Posted on 05. May, 2009 by Rich in Fitness
What is Body Mass Index?
Lots of doctors, health professionals, and trainers use BMI, or Body Mass Index when assessing a person’s weight and health, but what is body mass index really? The technical definition is that BMI is a statistical measurement that compares a person’s body weight to their height to determine if a person is underweight or overweight. The resulting number is used to lump you into one weight category or another. Typically anything under 18.5 is underweight, 18.5 to 24.9 is acceptable weight, 25 to 29.9 is overweight, and BMI’s of 30 and above are considered obese.
BMI is calculated by taking your weight in pounds, multiplying it by the number 703, and then dividing it by your height in inches squared. Confused yet? If you remember algebra here’s the equation for you:
As you can see, BMI was originally invented in Europe and was intended to be calculated using kilograms and meters. It makes the calculation much simpler.
Is Body Mass Index a Valuable Number?
So now that you know how to calculate your BMI, how valuable is the number really? Let’s take a look at a few examples and see what we can come up with.
For starters if you’re a star athlete or body builder the number is practically useless. An average NFL linebacker stands at 6′4″ and weighs 250 pounds. Using our handy calculation we come up with a BMI of 30.4 meaning that the average linebacker is technically “obese”. Considering that most NFL linebackers have very little body fat, our BMI calculation isn’t accurate or helpful in this instance.
A recent study in the International Journal of Obesity using a cross section of 13,601 subjects (49% men) found that BMI-defined obesity was present in 19% of the men and 25% of the women, but when using actual body fat percentage it was found that 44% of the men and 52% of the women qualified as obese. That’s a huge discrepancy! Again it seems that BMI isn’t an accurate tool.
In an attempt to illustrate the problem with BMI in greater detail I developed this example based on body fat and muscle percentages that are commonly found in society:
Sally is 5′4″ tall and weighs 125 pounds. Sally works out regularly and is on the local softball team. Her body fat and muscle percentages are on the fit side at 20% and 44% respectively.
Sally’s Body Fat = 125lbs x 20% = 25lbs of fat
Sally’s Muscle = 125lbs x 44% = 55lbs of muscle
Jane is 5′4″ tall and also weighs 125 pounds. Jane works a desk job and hangs out on the couch when she gets home. Her body fat percentage is high at 34% and due to her sedentary lifestyle her muscle percentage is low at 30%.
Jane’s Body Fat = 125lbs x 34% = 42.5lbs of fat
Jane’s Muscle = 125lbs x 30% = 37.5lbs of muscle
So what can we deduce from this? With identical BMI’s Jane has 17.5 more pounds of fat than Sally!!! I think this example illustrates the problem with relying on BMI as an accurate measure of weight and health.
What should we use instead of BMI?
There are many ways to measure body composition, but the top 3 recommended ways are:
- Body Volume Index, or BVI has been proposed as a more useful alternative to BMI. It was devised in 2000 and uses a 3D body scanner to measure for actual obesity. At this time BVI is still in clinical trials, but watch for it to replace BMI in the near future.
- Dual energy X-ray absorptiometry, or DXA is another method for calculating body fat percentage. DXA is currently considered the best way to get an accurate measurement of body composition as it takes 2 different X-rays of the body – one that detects all the mass, and one that detects all the mass except for fat. By comparing the 2 X-rays you can get a clear picture of fat content.
- Prior to DXA the most commonly used method for breaking down body composition was a body density test. By testing your average body density doctors could deduce what percentage of your body was fat.
I know that there are do-it-yourself kits that claim to give accurate results, but be aware that these 3 methods are the best. Whichever method you decide to use, I think it’s clear that BMI is insufficient as a weight and health measurement as it completely neglects body fat percentage. After all, weight loss is really about fat loss, so if BMI doesn’t measure body composition how can we rely on it at all?
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