BMI calculator: A truly pertinent tool for athletic men and women?

The calculation of BMI is a recurring topic among women and men, whether they are athlete(s) or not. The Body Mass Index (BMI) is a tool primarily used by doctors. It is often summarized as a way to know if you are in good physical health or not. However, BMI is not quite that…

So what is BMI? Why calculate it? How to interpret its results? And why is it not always a very reliable tool, one that sometimes stirs debate? Elements of an answer.

The concept of BMI was invented in the mid-19th century by Belgian mathematician Adolphe Quetelet. It was initially developed to define a person's body size beyond just their weight data. 

Thus, BMI calculation does not solely take into account the number that appears on a scale. It relates it to height. A simple calculation that, besides characterizing body size, also aims to define an "ideal weight" or "healthy weight" and whether our weight is suitable for our height. In theory, that's what BMI is.

As we've just seen, the Body Mass Index is based on two main data points: a person's weight and height. 

For example, if you are 60 kilograms and 1.70 meter, your BMI will be 20.76.

But once the calculation is done and the result obtained, it can often leave you puzzled. To understand what it means and to interpret it correctly, you must know the reference thresholds to refer to.

In 1997, the World Health Organization (WHO) decided to make the Body Mass Index a standard to assess certain health risks such as underweight, overweight, obesity, etc. To do this, it established reference thresholds, themselves based on the statistically observed relationship between the BMI and the mortality rate. 

According to the WHO, a 'normal' BMI is therefore between 18.5 and 24.9. But does this apply to everyone (men, women, children, adults, etc.)?

It's rare enough to note that, for once, no distinction is made between men and women. In BMI calculation, sex and age are not taken into account. 

Whether you're a 40-year-old man or a 20-year-old woman, if you're 1.70 meters tall and weigh 60 kilograms, your BMI will be 20.76. That's all. Same data, same results. Similarly, for the question of 'normal' BMI: according to the WHO, as soon as you are an adult, regardless of your age and sex, it is between 18.5 and 24.9.

Another thing to know about BMI calculation: it was initially designed for adult men and women aged 18 to 65 years, not for children. But since its invention in the mid-19th century, some scientists have focused on the issue of calculating BMI for young people from 0 to 18 years old.

If the calculation method remains the same as for adults, several points differ in the interpretation of the Body Mass Index for children. 

Thus, age and sex are considered. Categories are also removed and replaced by growth charts (which you have probably seen in your health record) on which the calculation result should be placed. The goal remains the same: to be able to prevent potential health risks.

While BMI is a more interesting and relevant indicator than the number that appears on a scale, it has some imperfections. For example, it does not take into account certain aspects that are nevertheless determinant in the calculation result.

Considering weight relative to height is good. But what about the difference, in the weight number, between body fat, muscle mass, and bone mass? The distribution of mass in the body? Personal medical history? Metabolism? Sporting practice and eating habits? Pregnant women?

In short, there are many points that are not considered in BMI calculation and could yet explain – if not change – the result obtained. This is notably why the relevance of the Body Mass Index is sometimes questioned. 

But it's important to remember one thing: BMI is not a diagnosis or a standard to be followed. It's a statistical indicator. So you shouldn't take it literally. On the contrary, you should appreciate it differently according to your personal situation.

In addition, many nutritionists and dietitians say that the notion of "ideal weight" doesn't exist, while the healthy weight does. It's not about a number to be reached, but the weight you feel good with, in full possession of all your capabilities, both physically and mentally.

As you have understood from reading the previous lines, BMI is a tool with its limits and needs to be nuanced. This is particularly true for people with diseases, pregnant women, or even… athletes! 

For these profiles, the calculation of the Body Mass Index is indeed biased. Indeed, in the case of athletes, muscle weighs more than fat. And the more muscle there is in the body, the more you weigh, and therefore the higher the Body Mass Index is. 

If you are very muscular, it is therefore not impossible that the result of your BMI calculation places you in the "overweight" category even though you are in very good physical health. You may have already seen studies comparing Homer Simpson to San Goku, the hero of the manga Dragon Ball Z. Both characters have the same theoretical BMI. Yet, if you visualize them, you'll quickly realize that they have nothing to do with each other physically!

A contrario, some long-distance runners like Eliud Kipchoge, are very thin, with a Body Mass Index below the minimum recommended. They are not in poor health either! Your body adapts to your physical activity, especially when you practice it intensely. In this case, the BMI loses its relevance.

If the calculation of the Body Mass Index is not always relevant, what should you rely on instead? 

You may have heard some doctors talk about the method of bioelectrical impedance analysis. This is a device that accurately measures the body fat, bone, and water mass levels in the body. The results are thus more precise and detailed than those of BMI calculation. Waist measurement can also be a more relevant indicator.

In any case, except in certain circumstances, issues of BMI and, more broadly, weight should not be an obsession. The right weight is the one you feel good with, physically and mentally.

In conclusion, the Body Mass Index (BMI) is a measurement derived from your weight and height, offering a quick gauge of your body weight status for men and women. While a BMI table can help identify if your BMI is 25 (often considered the threshold for overweight) or if you fall into overweight or obese categories, it's crucial to understand its limitations. BMI doesn't differentiate between body fat and muscle mass, meaning a very muscular individual might appear overweight or obese despite being a healthy weight. For adults over 20 years, fixed thresholds apply, but for children and teens, interpretation relies on a growth chart using percentiles like the 5th percentile, 85th percentile, or 95th percentile. Ultimately, while BMI can indicate potential health risks such as heart disease, it's a statistical tool, not a definitive health diagnosis. Always consider individual context beyond just height and weight.
In short, the Body Mass Index is an indicator that, like many things, has its strengths and weaknesses. It is interesting because it helps prevent major risks related to underweight or overweight (anorexia, cardiovascular diseases, diabetes, stroke, hypertension, etc.). 

It can also play a role in screening certain diseases. But, in certain cases, it should be nuanced, especially for athletes. Don’t forget that good physical health is absolutely not just a matter of weight and height. Everyone is different.