What is Body Composition and Why Should You Care (Why I Hate BMI)


May 18, 2012 by Darlene McC

I hate BMI

There.  I said it.  I feel a little better.

Every time I do a consultation on a woman who really, truly wants to lose weight she will either (a) tell me what her BMI is or (b) ask me what her BMI is.  I secretly want to tell them a) “I don’t care” or b) “I don’t know off the top of my head and I don’t care” and c) Neither should you.

A Brief History of BMI

BMI, which stands for Body Mass Index, is a height and weight chart.  It runs under the assumption that everyone of a given height should be the same general weight to be healthy.  It was designed almost 200 years ago (yes, really.  In the 1830s) as a general mathematical equation of the body.  In the 1970’s BMI was popularized as a quick and easy way for medical professionals to determine if someone could stand to lose a few pounds…because doctors are trained to identify and deal with problems, not on preventing them.

Our Current BMI Obsession

Over the last 30 years, as the “obesity epidemic” had escalated in the United Stated, people have become more and more focused on BMI.  Docs could use it for an easy and quickly reference with a little graphic where they think you maybe-should be without them having to take the time to determine the composition of your weight.  The more they talked about it the more their patients talked about it.  The more the general population talked about it the more trainers and other wellness pros cared…and so forth…

But what does BMI actually mean?  Well, consider these two examples:

Example 1: You have before you two men.  One of them never works out, eats like crap, and has no lean muscle mass; but he’s from a relatively scrawny family and, because he only eats twice a day, doesn’t get more than 2,500 calories a day.  He’s the metaphorical 140 lb weakling.  The other guy is a vegan weight lifter.  He thinks about everything he eats, balances his meals, could bench press your mom without breaking a sweat, and can sprint down your dog for kicks.  He’s about 210… and according to BMI is unhealthy because he’s “overweight”.

Example 2: Now lets look at two women.  Woman one is a marathon runner from a broad-shouldered family (what my dear friend Ann would call “peasant stock”).  She’s tough and loves to work out, meaning she has a bunch of lean muscle tissue.  Woman two is what I like to call a “cardio queen”.  She rocks that elliptical for 60 minutes every other day, drinks like a fish, and her weight hasn’t changed more than 2 lbs in five years (neither has her lean tissue).  She wouldn’t know a muscle if it punched her in the nose.  BMI claims woman 2 is the health nut.

You with me?

If I’m Supposed to Hate the Scale, and I’m Supposed to Hate BMI, What Do I Use?

Here’s where a new vocab phrase comes in: Body Composition.

Ask yourself: Do you actually care what the number on the scale is?  For reals; if we decided to measure weight tomorrow in gum drops and set a standard and you were able to determine how many gum drops of weight you were and how many Jessica Biel is would you be willing to use that?

So another system of measure is an option?  What if that system is more accurate and drives you less insane when it wont. fucking. move?  Does that sound better?

What you want to know then is Body Composition… basically: “what is your body made of?”  How much is muscle and organs and bones (the good stuff) and how much is fat and water retention and other junk (the less-than-good-stuff).

There are quite a few options out there, but really 2 are practical for us normal folks:

Girth Measurements

One of the best ways to gauge your progress is how big you are when you start and how big you are now.  It’s simple: Measure the following places on the regular while you’re trying to change the composition of your body:

  • Upper arm (around the biscept)
  • Upper theigh (thickest part)
  • Neck
  • Under bust (where your bra sits)
  • Waist
  • Hips (the fullest/widest part… not your butt, just a little higher)

For myself, I also like to measure my bust because my boob size will fluctuate with fat loss and muscle mass.  There are a few decent calculators on-line for using those measurements to gauge body composition.


If you have a gym membership or a friend who is a trainer you may have access to another method: body fat calipers.  A trained individual will pinch you in 3 to 7 points on the body and use that information to determine (using a regulated formula) how much of your body is fat and how much is lean.

Why do I care?

Armed with body composition information you can make more reasonable goals and set realistic expectations for your weight loss.  (Remember ladies: less than 12% body fat messes with your liver function, ovulation, and brain… so pushing it too low can really fuck you up.)

Here’s where a little basic math comes in (if you’re working with a trainer they’ll probably do it for you):

Take your weight and multiply it by your body composition (as a decimal).  That would mean if someone was 180 lbs and had 20% body fat they would multiply 180 x .20 = 36.  That person has 36 lbs of fat and 144 lbs of everything else.  It’s true that women’s bodies need 12%; but that’s a very hard number to shoot for.  Even very athletic women then to be around 18% (elite athletes get down into the teens), so I suggest us normal ladies to shoot for somewhere around 20%.  This person is right on target.

An example of someone who is less on target: 190 lbs and 28% body fat.  Here’s the math: 190 x .28 =53.2 | 190 – 53.2 = 136.8 lbs of lean tissue.  Here’s how we calculate target weight: take the lean tissue and multiply by the desired percentage.  As women we want to be around 20%:  136.8 x .20 = 27.36 – so this person should be carrying around 28 lbs of fat.  Their goal weight should be in the low 160’s

Two Quick Tips

A quick detail you’ll want: as you age more and more of your fat moves toward your organs to cushion them.  Because of that, people in the later decades of their lives will have a hard time getting down into the lower percentage ranges.  Don’t set yourself up for failure – if you’re in your 50s, don’t expect a 20% body composition.  Use girth measurements and how you feel to gauge progress.

Give yourself a range: No one likes that feeling of seeing the scale rise and fall over the month and worrying that they’ve started to put a little weight on.  Don’t stress over 5lbs… give yourself a healthy range and expect your weight to live in it.

6 thoughts on “What is Body Composition and Why Should You Care (Why I Hate BMI)

  1. Ann says:

    How nice to get a shout out!

    I also wanted to point out that the use of BMI to indicate health was actually a result of the health insurance companies trying to categorize their clients in a simple and systematic way. They’ve also encouraged the continuing decrease of “acceptable” BMIs (don’t get me started on the corruption in privatized insurance). This isn’t the article I had in mind, but it’s what I found after a cursory search: http://www.playnormous.com/blog/?p=825

  2. UrbanAmazon says:

    Just wanted to share a tweet by @PersonalGenius today:
    “BMI is also ethnically biased. ¦ RT @darlenemcc What is Body Composition and Why Should You Care (Why I Hate BMI)”

    It never occurred to me before, but it makes so much sense that a mathematical ratio made in Europe 200 years ago would be bias to the pale folks.

    Anyone have any other thoughts?

  3. […] looking at yourself in the mirror is hard.  This might be one of them.  If you’ve read Why I Hate BMI then you know that I feel strongly that pounds and BMI are not a good measure of fitness, […]

  4. […] about those who just want to get stronger.   For those of you who are generally happy with your body composition but would like to work toward throwing a car over your head, this is for […]

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