Home Training Why is the BMI calculation unreliable?

Why is the BMI calculation unreliable?

The body mass index or BMI is a quantity that makes it possible to estimate the build of a person. It corresponds to the weight divided by the square of the height and offers four possible results: Lean (BMI <18.5), normal weight (18.5 <BMI <24.9), overweight (25 <BMI <29.9) , obesity (BMI> 30).

Some of my clients are really fixated on this figure, which can be demoralizing. Yet BMI is not a good representation of body weight or ideal health in general, and I’ll tell you why.

BMI does not take age or gender into consideration 

BMI is based on weight and height only. It does not take into account logical criteria such as sex and age differences between individuals. So if a man and a woman have the same height and the same weight, they will necessarily have the same BMI. Yet women naturally have more body fat than men.

It’s physiologically wrong

BMI does not take into account the density of bones and muscles, which is higher than that of fat. He rates them all equally. Every time your weight increases, whether through gains in muscle or body fat, your BMI increases.
Thus, athletes with strong bones, good muscle tone and low fat content tend to end up classified as overweight or even obese.

BMI does not indicate the location of fat

BMI also does not take into account the distribution of body fat, a critical factor that can be an indicator of future weight-related health problems according to many studies. Indeed, if your fat is more localized in the abdomen (abdominal or visceral fat) rather than around the hips and thighs, the risks to your health are greater (in particular diabetes and cardiovascular disease).

Why do healthcare professionals use it?

Although the BMI is not necessarily representative of our health, this indicator is validated by the WHO. Its calculation is quick and simple, unlike measuring the percentage of body fat. Many professionals therefore continue to use it, but increasingly associate it with other indicators.

Other more precise methods exist such as computed tomography and MRI which give indications as to the distribution of fat, but these tests are expensive and require a lot of skilled labor.

Personally, I recommend that my clients take their measurements, weigh themselves and rely on their sense of well-being to assess their progress.

In summary 

No longer let your BMI fool you one way or another. Moreover, your weight varies during the same day, according to the meals consumed, your digestion, your level of hydration and other factors. So no need to worry about your BMI! You have to rely on your own physical feelings, not just a number.

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