Is BMI just a bunch of BS?

English: 8 women with the same Body Mass Index...
All these ladies have the same BMI. All this photo is missing is a lady with some muscles. (Photo credit: Wikipedia)

I first learned about the Body Mass Index (BMI) calculation my junior year of college while munching a turkey and swiss wrap in the back row during my Health Psychology class. I must have heard of it before then, but I’d never taken the time to calculate my own. During class, we used a simple height/weight calculation and I remember falling within the “normal” range and thinking “well, crap, I should probably lose some weight.” Of course, that was about the time I had just begun down the whole dieting path, caloric obsessions and exercise addictions, and now I see this number as an arbitrary figure that doesn’t consider density of muscle versus fat.

But in discounting BMI, am I just rationalizing that it’s okay to have a higher BMI despite being a long-distance runner? Or are my suspicions right: is this BMI thing really just a bunch of BS?

BMI is a simple and universal measure of body fat that can determine whether or not one falls within the healthy range for their height. BMI is found by simply dividing your weight by your height and squaring the resulting number, or (w÷h)² if you like math formulas. It is used to calculate the BMI of men, women, and kids of all ages. According to the measure:

  • under 18.5 is considered underweight
  • between18.5 and 24.9 is “normal”
  • between 25 and 29.9 is overweight
  • over 30.0 is obese

I don’t like numbers. I don’t like labels. So I might be biased against BMI to begin with, but below are some common blanket statements regarding BMI that should be taken with a grain of salt, so to speak.

English: Jadever Body Fat Scale. Old version o...
High tech scales provide body fat readings and BMI for you. (Photo credit: Wikipedia)

#1: BMI reveals accurate body composition readings
False. Body Mass Index readings are arbitrary in that they do not consider fat versus muscle. BMI is a numerical relation between one’s weight and one’s height. That’s it.

#2: A low BMI means you are healthy
False. You may mathematically fit the part; however, you could have very low physical activity and a crappy diet.  You may be able to subside on Big Macs and candy and never gain weight. That’s genetics. Diet, work conditions, genetic factors: these all contribute to health and eventually such a lifestyle will result in poor health, despite an arbitrary number.

#3: A high BMI means you are unhealthy
False. A very muscular person (hello long-distance runners!) may have the same or similar BMI to that of an overweight person despite having extremely different levels of muscle and fat. Here density comes into play. Many of us think muscle weighs more than fat, but actually muscle is merely much more dense than fat. Ultimately, this means when looking  at five pounds of muscle and five pounds of fat side by side, the fat takes up more volume, or space, than the muscle. One can be extremely fit and quite dense – resulting in a similar BMI to someone obese with a “lighter” volume of fat. Does that make sense?

What I’m trying to say here is: don’t stress about a number. If you follow basic guidelines of physical activity and diet recommendations, chances are you are doing just fine. I remember trying out this app during the Summer Olympics, which calculated your BMI and gave you your counterpart Olympian. According to this, I’d make a great competitive 20k walker. I wouldn’t take it too seriously, but it’s fun, nonetheless. Another tidbit of random trivia for your next cocktail party: according to the standards of BMI, Lebron James is just short of being considered obese with a 29.6 BMI (he is 6’5″ and weighs in at 250 pounds).

How do you feel about BMI? Arbitrary? A good starting point for measure of health? For the birds?

I'm a new momma, full-time non-profiter, and coffee lover. I write about healthy body image, half marathon training, and recovery from eating disorders. I'm currently training to maintain fitness throughout the winter and break 1:27:00 in my next half marathon.