What is BMI? And is it outdated?

Photo:  Adrian Peterson from Minnesota Vikings.   His BMI Classification may surprise you.

First of all what is BMI, and how is it calculated?

Body Mass Index (BMI) is a relatively straightforward equation that measures a person’s body fat by comparing their weight to their height.  It is the “go to” formula for establishing a healthy weight.

The formula is weight (lb) / [height (in)]2 x 703

Calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703.

Example: Weight = 140 lbs, Height = 5’6″ (66″)
Calculation: [140 ÷ (66)2] x 703 = 22.5                           

Or you can use a simple conversion calculator by googling it on the internet.

So my BMI is 22.5.  What do I do with that?

I go to the BMI table and it says the following:

Weight Status
Below 18.5 Underweight
18.5 – 24.9 Normal
25.0 – 29.9 Overweight
30.0 and Above Obese

Seems simple enough. This table is basically used to show whether I am at risk for heart disease or how healthy I am based on my weight.   However, there are many underlying problems with this because it really doesn’t take into account your sex, body composition (whether or not excess weight is fat or muscle), or bone density  – which is why fit people often find themselves in the fat category of the BMI rating system.

BMI was also never meant to be used for individuals, rather the collective weight of an entire population.  Although the BMI equation was created in the 1850’s, it was coined “BMI” and popularized by  Ansel Keys in the 1970’s.  However, it was never intended for physicians or insurance companies to use this equation — but BMI was just too perfect. Because it is simply a math equation, it is quicker, easier, and cheaper to use than more direct and accurate measures; such as the underwater weighing test, which measures how much you weigh by how much water you displace.

The BMI equation also cannot take into account how the fat is distributed around the body as many the real problems occur when fat accumulates in the central abdominal region.  Sometimes health issues are not about how much you weigh, but where is the fat distributed.

For a specific example of how the BMI can miscalculate whether or not someone is obese, look at an athlete like Adrian Peterson for the Minnesota Vikings.  He is 6’1″ and 217lbs.  How would you describe him?  Athletic?  Fit?  Lean? In amazing shape? Ripped?


Probably so, but guess what?  His BMI classification puts him at overweight.

Now this designation doesn’t really do much harm, but it is inaccurate.

Let’s look at an example where the inaccuracy can cause real harm.  In our society, weight has become crucial to many young girls self-esteem.  This BMI index can do some serious damage.

lily grasso

Take into account Lily Grasso (pictured above)

She is an 11 year old girl from Florida who is 5’3″ and weighs 117lbs.  She is the star volleyball player at her school and trains 6 days a week.  She is a healthy active teen, yet received a letter from a mandatory health screening last year which labeled her as “at risk” and over-weight.  Apparently this letter has been nicknamed the “fat letter”.  The Collier County Health Department says BMI is a part of their screening, and based on your BMI index, they urge parents to take their kids to their doctors and follow up.  The Health Department does note the body mass index could be off for kids that are athletes, but still I don’t believe these letters should be sent home.  The person doing the screening should look at the child in front of them and have some rapport  not just put some quick calculations on paper for an assessment.

BMI is NOT bad to use as a general baseline, but understand it isn’t truly accurate, and look beyond the numbers.  Look at yourself in the mirror.  You know if you are overweight.  You know your body. If you think you are overweight, see a doctor and have proper testing done which includes looking at your family history.  Then come up with a plan to make yourself healthier.  Exercise and a good diet have proven to prevent heart disease and help us live longer lives.


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