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The BMI index is not suitable for everyone. Here’s why

The Body Mass Index system has been around for a while but many people don’t realise that it was never intended to measure obesity. It was a simplified system invented in about 1830 by a Belgian doctor and it was just designed to put various body types into categories. It wasn’t until the 1970s that it began to be adopted to measure height-to-fat ratios and to declare an individual as overweight or obese.

There are many categories of people for which BMI is not an accurate measure of obesity. In fact, if you are Asian, pregnant, elderly or an athlete, it may not be an accurate measurement for you. Here’s why.

Not good for people of Asian descent:

Recently, the World Health Organisation set some guidelines to indicate that a BMI of 25 is healthy, more than 25 is overweight and more than 30 would indicate obesity. However, one Taiwanese academic named Pan Wen-Harn recently spoke in Australia and said this criterion missed a large number of people in Asia. “Chinese, Japanese, Koreans and Indians experienced metabolic risks such as hypertension and diabetes at a much lower threshold. You don’t have to wait until you get a BMI of over 25 — even at 23 or 24 a substantial number of people have those diseases,” she said. (source)

Not accurate for children:

This is also not an ideal measurement for kids as their body fat ratios are different to those of adults. “Calculating a child’s BMI number starts out just like calculating an adult’s BMI. It’s based on height and weight. But for kids, height and weight alone aren’t as accurate as they are for adults.” This is due to the fact that as we age and mature, our body fat percentages age and mature as well. “Kids’ BMIs vary based on their age and gender. That’s why when health care professionals talk about a child’s BMI, you won’t usually hear a plain BMI number, like 25, but rather a BMI percentile, like 75th.” (source)

Not useful for athletes:

Again, if you are super-fit, your fat ratios compared to your weight may also be off, if you were to solely look at BMI as the only measurement. According to Wikipedia, ‘BMI is particularly inaccurate for people who are fit or athletic, as the higher muscle mass tends to put them in the overweight category by BMI, even though their body fat percentages frequently fall in the 10–15% category, which is below that of a more sedentary person of average build who has a normal BMI number’. (source).

One study done by the US National Library of Medicine says the same thing. “When using BMI to discriminate between athletes and non-athletes only 52.4% (22 of 42) were correctly classified. These results suggest that compared to BMI levels, subcutaneous fat patterns are a more accurate way of discriminating between athletes and non-athletes. In particular the neck and the trunk compartment in men and the upper back and arms compartment in women, were the best sites to discriminate between young athletes and non-athletes on the basis of their fat patterns”. (source).

Not useful for the elderly:

Again, I have heard this a few times, but I couldn’t find great sources for this online but I know they are out there. This has to do with the fact that muscle mass and bone density changes with age, so an elderly person may appear underweight on a BMI scale. This seems to be because of muscle wastage, primarily: “BMI calculations will underestimate the amount of body fat for: The elderly and people with a physical disability who are unable to walk and may have muscle wasting.” (source)

A final word on BMI

BMI is not without its failings as a method of managing your weight but I do still think that it’s worth making sure you fall into the correct category. You need to have a BMI that is >24.9. Please note, some sources say >25.

Here is how to calculate your BMI:

  • Take your height in meters squared. I am 5 foot 6 which is 168cms tall. That’s 1.68 meters. To square it, I need to find 1.68 x 1.68. This = 2.8224
  • Now, you take your weight in kilograms. I am about 135 pounds, this is 61.2 kilograms (on a good day, where I haven’t had nachos the night before).
  • Divide your weight in kilograms (61.2) by your height in meters squared (2.8224) this = 21.68 (rounded to 2 decimal places).

All that sounds too hard? Do you prefer imperial?

  • For an imperial BMI calculator, go here.
  • For a metric BMI calculator, go here.

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