This subject came to me recently as I got an email from school (my kids are 5 & 8) saying that due to the health concerns for some students, the school will measure the BMI of pupils to make parents aware of those kids at risk of obesity. I decided this would be good subject to write about, as I believe GP doctors still use these measurements to determine one’s health.
But is it reliable? Is it necessary?
Considering the relative oddness of what BMI measures are (weight in kilograms divided by the square of the height in meters), I wonder how this managed to become such an important indicator of good health. It is a basic calculation to determine if you’re at the right weight for your height. It’s generally a dependable measure of your risk of developing chronic diseases related to obesity. Even though BMI may be useful for helping to identify long-term trends for the average person, it is quite a poor metric for assessing the health of individuals. Norms don’t apply to everyone, and some may fall well outside the standard deviation.
It is easy to see that body weight itself can often be very misleading – take a bodybuilder as an example: the BMI could put him/her in the overweight category. The issue with the body mass index for bodybuilders is that it doesn’t take body composition into account. Because they have more muscle mass than the average person, their BMI will probably fall into the overweight or even obese range and the truth is that you just have a high amount of muscle mass, which makes you weigh more. This BMI error doesn’t occur only in professional bodybuilders. Well-muscled people are often given higher BMIs and the subsequent “medical” diagnosis of being overweight or obese.
There are individuals who are overweight but otherwise healthy, and others, who have a “normal” BMI, but who are at high risk for cardio-metabolic disease (thin outside, fat inside). And in no way does BMI calculate a person’s body fat directly.
In conclusion:
Instead of worrying about weight or body mass index, we should be trying to maximize and maintain muscle mass.
Building muscle mass is important in decreasing metabolic health risks. Fact is: building muscle and increasing your BMI (by increasing your overall body weight) could actually help to improve your health and decrease your risk of premature death (explained in detail here).
Going back to the email I received, are parents really that blind that they can’t notice if their child is overweight or obese? Furthermore, can’t an adult himself not notice they have the same problem? …
Inspired by Miranda 
A more accurate measure is probably waist girth circumference (for adults, not sure how it applies to children though).
BMI is not very reliable and I have had similar discussions with General Practitioners/Family Doctors as to why they insist on using it still. Simple answer… it’s easy.
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