BMI, bullshit or not?
To designate the corpulence of a person, the body mass index is based on two data only: the weight and the height.
Does body mass index remind you of physical education class? The painful memory that some keep of it — belonging to the category of overweight despite being in satisfactory physical shape —reflects a shift from the healthy body image that is valued today. Should we question the famous IMC?
First, let’s establish the basics. Invented in the 19th century, BMI is used to assess a person's corpulence (undernutrition, healthy weight, overweight or obesity) based on two data: weight and height.
The formula, used consistently for adult men and women over the age of 18, divides weight in kilograms by height in meters squared (i.e. height multiplied by height), explains Health Canada in the report Canadian guidelines for body weight classification in adults.
BMI Classification in kg/m2
Insufficient weight = less than 18.5
Normal weight = 18.5 to 24.9
Overweight = 25 to 29.9
Obese = 30 or more
Simple to calculate, the index makes it possible to estimate the risks of health pathologies associated with body weight such as diabetes, arterial hypertension, cardiovascular diseases or certain cancers, depending on whether the weight is considered too low or too high by compared to height, indicates the Government of Quebec.
Fats and muscles
However, what what about muscle mass?
Indeed, the BMI does not take into account major physiological components: fat, muscle, water mass and bone density.
It does not apply precisely to athletes or to very muscular people, no more than children, the elderly and pregnant women, reads Passeport Santé.
Nicolas Leduc-Savard, sports nutritionist, teacher at the University of Montreal and member of the Ordre des diététistes-nutritionnistes du Québec. Photo: Alison Slattery/Two Food Photographers
Stick to height and weight to determine “healthy weight” of a person is it reductive or even inadequate? This is the opinion of sports nutritionist and teacher at the University of Montreal Nicolas Leduc-Savard, who bluntly states, in an interview with Métro, that he does not use BMI in his practice< em>. Due to the limits of the tool, many health organizations no longer use it, points out the member of the Ordre des diététistes-nutritionnistes du Québec.
“ Instead, organizations like ÉquiLibre want to undo the link between weight and health, and educate people about having a better relationship with food and appreciating what their bodies allow them to accomplish,” he says. /p>
Paradoxes thus emanate from what the BMI designates as being a “healthy weight”. A person who is anorexic, but whose mental health is deteriorating and muscle mass is atrophying, may falsely belong to this “healthy” bracket, Nicolas cites as an example.
Conversely, a person with a certain body fat could be classified in the obese category, “but be in much better health metabolically,” he adds. Just as a very muscular person might be classified there, muscle weighs more than fat. “Therefore, health risk should not be estimated solely from an individual's body weight or waist circumference,” says Health Canada.Beyond the individual assessment of BMI, it can be used on a larger scale to “determine the distribution and evolution of body weight within a population”. and thus estimate “the prevalence of different levels of body weight” in said population, continues Health Canada.
This comparison within a population or between populations, this is the most appropriate use that Nicolas Leduc-Savard attributes to BMI , which states that “we could perhaps draw conclusions, but that these would remain based on not much”.
Body fat distribution is a much more reliable determinant of long-term health risks or mortality than BMI.
Nicolas Leduc-Savard, sports nutritionist
< p>In this regard, women enjoy a “metabolic advantage” on men, says the specialist, also co-creator of the Gourmand Gourmand blog, since on average, their body fat “is distributed much more in the hips than around the vital organs. On the other hand, that of men is distributed more around the vital organs: liver, pancreas, etc. ». This is called visceral (or intra-abdominal) fat, explains Health Canada. It's more harmful than subcutaneous fat (which you can pinch off), compares Community Health Center.
Genetics are involved
What about “big bones”, “slow metabolism” or the thyroid gland, factors often cited to justify that a more corpulent person can be in very good health?
“ There are so many things that can influence the weight and the bold. And a lot of that is determined by genetics,” emphasizes the nutritionist.
“Diet culture holds out the idea that we can build our bodies, lose and gain calories as we please, and control our weight and our cravings. In fact, our genetics influence these things a lot, and we can influence them up to a certain limit. »
The < em>shape of magazine, it is much more attainable by genetics than by will and effort. Showing what a star eats to get into a swimsuit is super guilt-inducing because the star's genetics differ from ours.
Nicolas Leduc-Savard, sports nutritionist
Rather than talking about “healthy weight”, which precisely connotes BMI, Nicolas favors the terminology “natural weight”, i.e. a stable weight that allows you to perform all the activities you want in being well in his body, he explains.
” It can therefore totally fall outside the weight range determined by the BMI “, continues Nicolas.
Natural weight falls under the principles of intuitive eating. “We are more attentive to our appetite, the foods and textures we want to eat to improve our relationship with food, rather than relying on the instructions of diet culture, such as “carbs make you fat” or “you shouldn't eat after such time”. »
Finally, does he believe that we are progressing socially with regard to the issues of acceptance of body diversity? If he answers in the affirmative, he nevertheless underlines that he is part of a ” bubble in Montreal and on Instagram “, where people advocate diversity in all its forms.
Mores may evolve, ” there will always be diets and harmful fashions — like the one valuing anorexic bodies — for mental health, or generations that have been less exposed to guilt-inducing messages that could still fall into something dangerous,” he recalls.
The Weight and Health podcast with researcher Benoît Arsenault, “which explains the role of the distribution of fat in the body as being more important than BMI as such,” says Nicolas.
The blog of Karine Gravel, doctor of nutrition, speaker and author of book From diet culture to intuitive eating (2021, KO Éditions)