BMI: Bad Measurement Index

by Charlotte on February 12, 2008 · 9 comments

The BMI has been villified almost since the day it appeared on the scene as being a terribly inaccurate measure at worst and a reasonable guess at best. It is famous for overclassifying people with higher than average amounts of muscle, like weight lifters, as “overweight” and underclassifying people with less than average muscle mass, such as the elderly, as being “healthy.” And yet everyone from President Bush to personal trainers still use it. The argument is that for average people, the BMI is pretty accurate. But is that true?

The Study
Researchers took 63 volunteers (23 men & 40 women) and then measured their BMI (weight compared to height), body fat percentage, evidence of metabolic syndrome, waist circumference, and body fat mass index (like the BMI but comparing your body fat to your height). They then compared the BMI to the other measures to see if it was a relatively accurate gauge of health.

The Results
Surprising no one, the BMI performed miserably. But the interesting part of this research is just how miserably. The basic gist is that the BMI significantly missed people on both ends of the spectrum (those who are underweight or overweight) that would be referred for treatment based on the other criteria.

Based on the BMI, only 11% of the participants would be strongly encouraged by a doctor to lose weight. Based on waist circumference 25% would. Body fat percentage indicated 28% would. And evidence of metabolic syndrome would indicate 34% needed to lose weight.

Caveats
63 is a ridiculously small sample size (unless you’re in Hollywood and then it’s an oxymoron). However, I found this study that tested 14,924 people in a similar manner and came to the same conclusions.

If Not BMI, Then What?
I think it is safe to say that if you are truly interested in measuring your health, the BMI is not a good shortcut. You would do better to factor in your waist circumference, blood pressure, resting heart rate, cholesterol & body fat percentage. But if you need just one number (checking all those factors every week or month is silly), experts are saying that waist circumference is the best indicator of health & best predictor of heart disease and diabetes. (Notice that I did not say anything about your thighs – according to scientists, fat stored in your hips and thighs, while irritating to most of us, does not adversely affect your health and in fact, helps women have smarter, healthier babies. Yet another reason looking like a catwalk model is a bad idea.)

It also happens to be a lot easier to calculate than BMI, for those of us who are math-challenged. Just take a measuring tape, wrap it around your waist (the smallest part of your torso – approx. where your bellybutton is) and measure! If you are a woman, you want to be under 34.5 inches. For a man: under 40 inches. Ideal waist size for a woman is 32.5 for women and 35 for men.

PS> In case you are curious, the average Victoria’s Secret model has a waist between 23 and 25 inches – the same size as your average 12-year-old girl.

{ 8 comments… read them below or add one }

Lucas February 13, 2008 at 3:18 pm

Count me in on the anti-BMI bandwagon! It’s a bit ridiculous that people think of it as the ultimate indicator of health. We already know that weight isn’t a reliable indicator of health and fitness (we do all know that, right?). So if weight doesn’t necessarily indicate health, then why would height/weight ratio be more reliable?

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Gena February 13, 2008 at 3:37 pm

That’s really interesting! I hate the BMI with a passion, but I always thought that the BMI tends to shift people to the “fat and unhealthy” category when they are actually neither. And yet these results suggest that even though BMI is skewed, its a bad measurement for a whole different set of reasons.

Thanks for sharing the VS model tidbit. Apparently they are all about the size of my thigh. Which is disturbing for several reasons.

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Charlotte February 13, 2008 at 11:00 pm

Holy crap Gena – you’re RIGHT!! I never thought of it that way but yeah, VS models ARE the size of my thigh. Eeek.

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Stephanie Quilao February 14, 2008 at 12:35 am

“Bad Measurement Index”…love that…Have you seen Kate Harding’s BMI project slideshow. It’s interesting because she shows pics of people and shares their BMIs, and visually you can see just how off the BMI scale looks. Truly fascinating. Check it out: http://kateharding.net/bmi-illustrated/

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Anonymous October 2, 2008 at 2:50 pm

I do know that whenever I’m in the healthy range of my BMI, I feel great! I do think it’s a good guidleine, but we’ve made the mistake of putting too much emphasis and value in BMI. When it comes to weight problems and health in general, it’s realistically just not that simple to streamline it to one calculation. There is just way too many other factors involved. People that are showing obese according to their BMI, but are not actually obese because they have muscle mass. Well, your heart still has to work harder to supply oxygen to those body builder/football player muscles than it would if you had the lean muscle look like Ryan Reynolds did in the movie Blade 3, who happens to be in the healthiest range of his BMI.

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Anonymous March 12, 2009 at 12:32 am

Wow, thanks for informing me of this. It made my day. Last time I went to the doctor she told me my BMI was higher than it should be and I should loose weight. But I have a 25 inch waist! Does this mean I should tell the doctor where to stick her opinion and apply for a job at Victoria’s Secret?

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Ivory May 28, 2011 at 10:53 pm

I love this article :)
The only thing I disagree with is using waist circumference as the main indication of health. I have a naturally small build. (I’m quite short) My mom has the same biuld, and currently has a waist of 32 inches and is overweight. My waist is just under 24 inches, and told kinesiologist told me that I’m at a perfectly healthy weight. Are you sure they didn’t mean to use wait-to-hip ratio as an indication of health? waist divided by hip? And that females should aim to be around 0.7 and males around 0.85?
I personnaly use body fat percentage to ensure that I’m healthy. It takes into account my muscle mass and frame size. It’s not perfectly accurate, but its close enough.

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