Body Mass Index – Don’t pay any attention to it

The Body Mass Index, or BMI,  is used to ascertain whether someone is overweight, obese or at the correct weight for their height.  It’s used by health professionals across this country as a guide for their patient’s health.  It’s wrong.

The reason for this blog is a conversation I had with a good friend of mine.  He also happens to own a gym and is a very talented and knowledgeable trainer/fitness coach.  We were discussing it in relation to his clients and my patients and how people are often times misled by the numbers they see when they use the BMI scale.

BMI was designed to be used as an easy tool for clinicians to assess their patients in terms of body weight relative to height. Before the BMI scale was invented it was hard to assess someone’s weight and say that it was appropriate because height is also an important factor in weight.  BMI combined those two.

How to calculate BMI:

The formula is simple.  You need your weight in pounds and your height in inches.  Take your weight and multiply it by 703.  Take your height and multiply it by itself (height squared).  Now divide the first number by the second number and you have your BMI.  Here is an example.  We’ll use my numbers.  I am 201 pounds and 71 inches tall.

201 lbs x 703 = 141,303

71 in. x 71 in. = 5,041

141,303/5,041 = 28.03

So my BMI is just over 28.  This puts me in the overweight category, actually moving close to obesity.  Wait…what?

If you look below you can see the classification system used for BMI.

You will see that anything above 30 is considered obese.  Technically anything above 29.9 is obese.  If we use this scale, I am only 13-14 pounds short of being considered obese.  People who know me will tell you that I do not look obese.  They will also tell you that I do not even look overweight.  So what’s the catch?

That is the problem with using BMI to assess health.  It doesn’t take into account many factors.

The problem for some people, like athletes, it does not take into account muscle mass.  A person that is heavily muscled will always be overweight according to the BMI.  As a matter of fact, I have been considered “overweight” since college despite always being is relatively good shape.  If we look at professional level athletes, most of them would be considered obese!

I understand that not everyone is an elite athlete.  What about the elderly?  BMI is not ideal for them either.  Many times an elderly person will fit nicely into the BMI by being considered “ideal weight” for their height.  This can be significantly misleading.  Why?  In the elderly muscle mass begins to drop.  It happens to all of us.  However, with this drop in muscle mass comes a drop in weight.  As weight is lost a person is likely to fall into the “ideal weight” category even though they should be concerned about muscle mass loss.  This loss in muscle mass causes a loss in strength and stability increasing the risk of falls and increasing the risk of osteoporosis.  Another problem with muscle loss is the change in your body composition.  As muscle mass is lost one’s body fat percentage increases.  Body fat percentage is a great indicator of health.  The lower it is (within reason) the healthier you are, generally speaking.

So does it work for anyone?  Yes, there are some people that it works for.  If a person is sedentary and eats a poor diet it may accurately depict your current weight status (ideal, overweight or obese).  There are, however, better ways to assess health.

Then how do I know if my weight is appropriate?

The best way to assess your weight status is to perform body composition.  This gives us a percentage number based on body fat.  For example, if you weigh 200 lbs and 50lbs of that is from fat, your body fat is 25%.  Below is the ideal body fat percentages for men and women.

The gold standard for measuring it is the caliper test or skin fold test.  A pinch of skin is precisely measured by calipers at several standardized points on the body to determine the subcutaneous fat layer thickness. These measurements are converted to an estimated body fat percentage by an equation.  It is most reliable when taken over time and it must be done by the same person to be accurate.  Techniques can vary from person to person and may change the results.

The other way to measure it by bioelectrical impedance.  You are hooked up to electrodes that are spaced far apart on your body; usually on each hand or on a hand and a foot.  An electrical signal is passed between the electrodes and the resistance to the current flow is measured.  This is a painless process.  Fat and muscle have different resistance rates so the machine can estimate the body fat percentage based on that.  It is affected by hydration levels so be sure to be hydrated when you take a test like this.  This is a very accurate method and does not depend on a person’s technique as the skin fold test does.

The best way to get your body fat to the desired level is a healthy diet and exercise.  You want to increase muscle mass and decrease fat.  This is done by weight training and short duration, intense circuit type workouts.  All of this can easily be done at any local gym.