3 Reasons Why the BMI is Obsolete

Posted on 18. Mar, 2009 by in Motivation

Yesterday was a big day for my husband. He’s in the Air Force and it was time for his annual physical training (PT) test. He was really nervous that he would score poorly on this test but it’s not because he is out of shape or overweight. The real reason for his worries was the BMI.

Photo by Life Design Strategies

Photo by Life Design Strategies

So, what is the BMI?

Here is the definition by the National Institutes of Health’s (NIH):

Body Mass Index (BMI) is a measure of body fat based on height and weight that applies to both adult men and women.

The military still uses the BMI as one of their standards for measuring body fat which becomes a problem for people who are heavier relative to their height because of their muscle mass (as you’ve probably heard that muscle weighs more than fat). In the case of my husband, he can’t be more than 200 pounds for his PT test because with a height of 6 feet, 3 inches, this would put him at a BMI of 25.1 which would mean that he’s overweight relative to this “standard.” This totally doesn’t make sense because if you see him in person, the word “overweight” would not even describe him one bit. In fact, he is tall and lean and I’m not even exagerrating here.

The BMI Table

Underweight 18.5
Normal Weight 18.5 to 24.9
Overweight 25 to 29.9
Obesity 30 or greater

So, now that I’ve made that point, let me tell you about why it’s not a good measure of physical fitness or body composition.

  • It is based on your height and weight which means that these numbers do not even have anything to do with body fat percentage which is a big factor when assessing health risks.
  • It may overestimate body fat in athletes and others who have a muscular build.
  • It may underestimate body fat in older persons and others who have lost muscle mass.

Take note that the last two reasons are actually limitations that I copied from the NIH’s web site. I am sure that there are some old research studies that support the BMI and which have made it into what it is today but I won’t even bother looking for them because my main point is this: measure your body fat percentage instead. I actually wrote a post about this in another post entitled, “Why Weight is Just Another Number,” and you can view that one here. Body fat percentages actually measure your body fat in a few different sites of your body where you would most likely have body fat such as the lower abs, the back of the triceps, the front of your biceps, etc. So, even if the measurements may be inaccurate, it will give you a better picture of your body composition.

I know that sometimes it can be difficult to get your body fat measured because you either have to get a personal trainer to do it or have someone wiling enough to learn how to perform the measurements on you. If this is the case, I read about another type of measurement which takes into account your waist to height ratio. Brad Pilon, author of Eat Stop Eat, talks about this in his post and you can see it here and scroll down to question #4. I asked him if it applies to women as well but he hasn’t gotten back to me yet.

Another option is to buy a bathroom scale that measures body fat but I find that they also have their own limitations as they can be inaccurate. But if you were to buy a scale with this feature, use it as a measure of progress. In other words, even if the number on it is not truly reprsentative of your actual body fat percentage, use the number as your baseline which means that if the measurements go up or down, take the difference between your baseline and the new number. If the number goes down, then you’re making progress and vice versa.

The only other time that I would ever use my weight in any measurement of fitness is when I want figure out my lean mass once I know my body fat percentage – that’s it. As for the military, my husband says that they may me changing the BMI into the waist to height ratio as described by Brad Pilon but it might take awhile for this change to happen (as with any goverment changes but that’s a whole different story).

The lesson I learned is that there are a lot more measurements out there that will give us a better idea of where we are in terms of health risks and physical fitness besides the more general, government issued standards. Sometimes the best thing to do is to not focus on the numbers too much but pay more attention to more important things like eating healthier and living a more active lifestyle instead.

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  • Archaic and beyond outdated (as if BMI should have ever been implemented as a tool). How ’bout this for a working example? My friend Patrick is 6′, weighs 175lbs, has a BMI of 23.8 and a bodyfat percentage of 23.3%. According to those with degrees, Pat is “healthy” even ignoring the 23% bodyfat. I on the other hand am 6’1.5″, weigh 213lbs, have a bodyfat percentage of 13.8 but also have a BMI of 27.8.

    Pat carries approximately 41lbs of fat, while I carry 29lbs, yet I am the unhealthy one?

    Another great post!

  • Hi Anna, just found your blog from SmoketheBlowfish, Mike Z. What a great post about BMI. My husband is 5’8″ and weighs 185, but he’s around 8% bodyfat and is a total mesomorph. Didn’t think much about BMI until we were changing insurance policies and they ask for height and weight, and whether or not you’re a smoker. They don’t seem to care about bodyfat, exercise, diet or anything. Amazing!

  • admin

    Mike, that is a nice example! Archaic really is a great way to describe the BMI and you made my point even clearerer. Thanks for visiting.

    Hi Kelley, thanks for stopping by and I really appreciate you leaving a comment about the BMI. I visited your blog and saw pictures of your husband. He looks great especially knowing that he’s 59 years old. Amazing! My husband is a mesomorph as well but he hasn’t quite realized his full potential yet 🙂

  • Pingback: 3 Ways to Break the Fat Loss Plateau | My Fat Loss Strategies()

  • Archaic and beyond outdated (as if BMI should have ever been implemented as a tool). How 'bout this for a working example? My friend Patrick is 6', weighs 175lbs, has a BMI of 23.8 and a bodyfat percentage of 23.3%. According to those with degrees, Pat is “healthy” even ignoring the 23% bodyfat. I on the other hand am 6'1.5″, weigh 213lbs, have a bodyfat percentage of 13.8 but also have a BMI of 27.8.

    Pat carries approximately 41lbs of fat, while I carry 29lbs, yet I am the unhealthy one?

    Another great post!

  • I think a BMI between 22 and 27 would be ideal, provided the person is between 8 and 14% body fat.

  • Leonard Hilley

    1 lb of muscle weighs the same as 1 lb of fat. The issue is that muscle is more compact and dense, so a person with a lot of muscle looks smaller than an obese person at the same weight. BMI is so outdated, and in some cases, actually unhealthy for athletic people to follow (bodybuilders, fitness trainers, etc.). I do believe a new system should be set up, based on an individual’s physical activities and diet.