The Body Mass Index Myth: What is BMI good for?

I’m overweight. According to the BMI, that is.

BMI stand for Body Mass Index. It takes two variables: your height and your weight and with these two metrics it guesses your health status. Which scares you, makes you happy, or pisses you off depending on the score.

I am 5 foot 11 inches tall.  I weigh 180 pounds (12 stone 12 pounds). If I put my height and weight into a BMI calculator like this one: I get a score of 25.1 (or you can try a different one here)

Anything between 25.0 and 29.9 is considered overweight. So I get a message like this:

Obesity Campaign PosterYour weight may be putting you at increased risk of health problems, such as heart disease and stroke. Talk to your doctor about healthy eating and exercise options. Even losing a few pounds can help to reduce your risk.”

Fortunately there was another message underneath:

“Although the BMI is useful for healthy adults (ages 20 to 65 years), it does not apply to infants, children, adolescents, pregnant or breastfeeding women, endurance athletes, highly muscular people and adults over 65 years of age.”

I am not an infant or child (but my girlfriend might argue that). Nor am I pregnant or breast feeding and if you met me I don’t think you would call me highly muscular. But I might be considered an ‘endurance athlete’ depending on how you define endurance athlete.

So here is the problem: If you don’t like the score you get you might read the second part and use one of the categories to dismiss the ranking. But perhaps you are not the best judge of yourself. Conversely, you might take the ranking  seriously and think that you’ve got to take action.

The BMI myth

The reality is BMI tells us nothing of a person’s level of fitness. Fitness is a better predictor of overall health status. You can be overweight and remarkably fit. You can be average weight and drastically unfit. Your medical doctor can help you figure out which one you are. But either way it will rarely hurt to try and get more fit.

I have a big problem with these ‘helpful’ parts of BMI calculator messages:

“Even losing a few pounds can help to reduce your risk.”

Putting the focus on changing your overall weight is not necessarily a good thing – you always need to think about exercise as a way to get fitter (and if your weight drops as a result – then hurray!) If you’ve read my previous posts you will have a good understanding about what it means to be fitter and how to improve your fitness.

Be fit, not thin

An improvement in fitness if far more meaningful than weight. One good way to measure if you’re getting aerobically fitter is a reduction in your resting heart rate.

To measure this, sit in a chair, completely calm and measure your pulse (count the number of beats over a minute). Each week measure it again and write it down. See if it drops as the weeks turn into months and the months into years.

Another measure of improved fitness is the ability to cover the same distance in less time on foot, or a bike, or in the pool. If it took you 1 hour of walking to cover 5km last month, but this month it took you 55 minutes then your pace has increased. If your pace increases then you’re probably getting more aerobically fit.

My Wii BMIBut don’t forget about your anaerobic fitness (see my post about your different types of muscle for more info). Always consult your medical doctor before beginning a new fitness routine, especially if you have been sedentary for a long period of time.

A wise nutritional biochemist once said “the best exercise is the one you’ll do”. Please find an activity (or two or three) that you enjoy and do them efficiently.

Never measure your success solely by a number on a scale.

Article by Matthew Linsdell

February 13, 2012

Matt is a certified personal trainer and has a degree in Environmental Science. He calls himself an evidence-based trainer, because training is a field which is littered with well-disguised pseudoscience – his emphasis is always on teaching the biology behind exercise. He lives at the edge of the beautiful and expansive Gatineau Park in Quebec and works across the water in Ottawa, Ontario. If he’s not out walking his two pit bulls, you’ll find him doing press ups with insanely large weights on his back. Follow Matt on Twitter at @smartfitmatt.

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4 thoughts on “The Body Mass Index Myth: What is BMI good for?”

  1. I sincerely hate the BMI, and have had a similar experience as yours. It was after the winter holiday, and I was about 2 points from “overweight” which is ridiculous – endurance athlete or not. I was just having a chat with your buddy Stu and asked if he has heard about this “normal weight obesity” thing. My head just about spun off my neck when I heard that! Now even normal-weight people are “fat”

  2. Suzanne, I have not heard about “normal weight obesity” before. I just googled it. It makes perfect sense to me and I think I mention it in the article. “You can be average weight and drastically unfit”. If you are a “Normal” weight and unfit and a good portion of your weight is from Fat then you might experience health issues that would normally been seen in a heavier, inactive, individual.

    Don’t take the term “fat” personally. Focus on fitness and forget about the labels. Like I say in the article, a number on a scale doesn’t tell us much about our health status.

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