It is a simple calculation, which only takes a few seconds. Divide your weight (in kg) by your height (in m) squared, and you will get your BMI. Example: a woman measuring 1.65 m and weighing 63 kg has a BMI of 63/1.65² = 23.16. “The figure obtained allows you to see if you have a normal weight, or, conversely, if you are overweight or obese” explains Pre Martine Duclos, endocrinologist. We owe the invention of this formula to the Belgian mathematician Adolphe Quetelet (1796-1874). This measure has an undeniable advantage.”It is very simple, and can be done by everyone. Whatever his age, whatever his sex, the BMI calculation is the same. This is what explains why the World Health Organization (WHO) has continued to recommend it all this time.
BMI: an excellent indicator of our state of health
The figure obtained makes it possible to make a link between our corpulence and the risks that we pose to our health. “The more the BMI increases, the more the health risks increase. More cancers, more type 2 diabetes, more cardiovascular diseases, more NASH, more sleep apnea, asthma, osteoarthritis. ..”,details Pre Duclos. But the WHO itself recognizes that this measure is not perfect, because it is incomplete. It only takes into account height and weight, leaving aside the distribution of fat, age, physical activity and gender. In other words, if, very often, the increase in BMI is proportional to the increase in body fat…, this is not always the case.
The WHO recommends this index for adults between the ages of 18 and 65. However, the BMI does not know everything, does not see everything. It does not distinguish between fatty mass (located under the skin and around organs) and lean mass (water, organs, muscles, etc.). Two people will thus be able to display the same figure, but not at all the same silhouette.
BMI: it does not take into account muscle mass or fat distribution
So the BMI does not take muscle mass into account. “The typical example is the 1.80m rugby player for 97 kilos, who will have a BMI of 30, but with a lot of muscle, and very little body fat”. This does not expose him to any risk to his health. While a woman with the same BMI, but sedentary and having accumulated a lot of fat, will be less fit. Similarly, a very muscular judoka would be considered obese, according to the BMI, which is obviously far from reality. “The clinical examination quickly makes the difference” reassures our expert.
We all need fat in our bodies to survive. But not too much is needed. The ideal ratio is between 20 and 30% of the total weight in women, between 18 and 25% in men. To measure it, there are impedance scales, which work with an electrical current – perfectly painless – which passes through the body. But this measurement is necessarily approximate. If used, care should be taken to always weigh yourself under the same conditions – in the morning, on an empty stomach. More precise, DEXA (Dual-energy X-ray absorptiometry) measures body composition (fat mass, lean mass and bone mass) by scanning the whole body with an X-ray beam. The radiation dose received by the patient is extremely low, the equivalent of that received during a 2 hour plane trip. This examination is performed in a radiology office. It is not covered by Social Security.
The waist circumference, an essential complement
“You also need to measure your waist circumference.” encourages Pre Duclos. Beyond a certain threshold, fat mass can harm our health. Even more depending on where it is located. Not everyone knows it, but abdominal obesity is more dangerous than fat lodged elsewhere, in the thighs or buttocks for example. A ticking time bomb for our health. “This visceral fat is a real risk factorin particular for type 2 diabetes and cardiovascular diseases”. However, some people may display a weight in the norm (BMI between 18 and 25), and have fat more particularly concentrated on the abdomen. “To take your waistline properly, you have to have your feet flat. With a tape measure, you take the measurement at the end of a normal breath. No forced inspiration or forced expiration.” From 80 cm in a woman, 94 cm in a man, the waist circumference presents health risks. Even more, of course, when combined with a BMI over 25.
What if BMI was the worst indicator, to the exclusion of all the others? Its incredible longevity is no coincidence. With the exception of measuring waist circumference, no other indicator is as simple, quick and inexpensive as the body mass index. Even if, of course, the doctor will go far beyond to have more complete information. But BMI is – definitely – a good start.
Our expert : Pre Martine Duclos, endocrinologist, head of the sports medicine department at Clermont-Ferrand University Hospital