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The Body Mass Index (BMI) is a simple but essential indicator used to quickly assess a person's corpulence. Created in the 19th century, this tool today allows the detection of situations from underweight to obesity, thus providing an important reference for health professionals. Although effective, the BMI has limitations that we will explore, while explaining how to calculate and interpret it correctly.
What is BMI and what is it used for?
BMI, or Body Mass Index, is a tool that allows you to quickly estimate an individual's corpulence based on their weight and height (1). It was invented in the 19th century by the Belgian mathematician Adolphe Quetelet, hence its other name of "Quetelet index" (2). But it was not until 1972 that it was used by scientists to assess corpulence and the risks associated with overweight and obesity (3). Since then, BMI has become the reference measurement used by the World Health Organization (WHO) and most health professionals to define overweight and obesity (1). BMI allows individuals to be classified into different categories.
This tool is therefore valuable for quickly detecting excess weight and the risks associated with it, such as type 2 diabetes, cardiovascular diseases or certain cancers (4).
How to calculate your BMI?
The formula for calculating BMI is very simple. You just have to divide your weight (in kilograms) by your height (in meters) squared (1): BMI = weight (kg) / height (m)² For example, for a person weighing 70 kg and measuring 1.75 m, the calculation will be:
BMI = 70 / (1.75)² = 70 / 3.0625 = 22.9 kg/m² This BMI of 22.9 kg/m² indicates a normal body size according to the WHO classification. No need to be a math whiz, many online tools allow you to calculate your BMI in one click by simply entering your weight and height. But once you have this figure, how do you interpret it?
How to interpret your BMI?
The interpretation of the BMI is based on the thresholds defined by the WHO (1):
| BMI in kg/m² | |
| Normal weight | 18.5-24.9 |
| Overweight | 25-29.9 |
| Obesity class I | 30-34.9 |
| Obesity class II (severe) | 35,39,9 |
| Class III (massive) obesity | ≥ 40 |
Thus, a BMI of less than 18.5 kg/m² indicates underweight, i.e. weight too low in relation to one's height. Conversely, a BMI of 25 kg/m² or more indicates overweight, which becomes obesity beyond 30 kg/m².
But be careful, these thresholds are general benchmarks that do not take into account individual characteristics such as age, gender, ethnic origin or muscle mass (5). For example, older people naturally have lower muscle mass and higher fat mass, which can overestimate their corpulence.
Conversely, very muscular athletes can have a high BMI without being overweight (6). It is therefore necessary to interpret one's BMI with caution and not to consider it as an absolute verdict on one's state of health. Especially since this index has other limits.
The Limitations of BMI
Although very useful and easy to calculate, BMI is not a perfect tool for assessing body size and associated risks. Here are its main limitations (5):
It does not take into account the distribution of fat: an accumulation of fat at the abdominal level (android obesity) is more harmful to health than a more uniform distribution.
It does not distinguish between fat mass and muscle mass: with equal BMI, a muscular person will be healthier than a person with high fat mass.
It does not apply to pregnant women, whose weight gain is normal and necessary.
It may overestimate excess weight in older people and certain ethnicities.
It may underestimate obesity in people with low muscle mass (sarcopenia).
To refine the assessment, the BMI must therefore be coupled with other measurements such as waist circumference, body fat percentage or impedancemetry (7). And above all, it must be interpreted by a health professional according to the overall context.
BMI is a quick and easy tool to assess body size based on weight and height.
It allows overweight and obesity to be detected according to the thresholds defined by the WHO.
But it has its limits and does not replace a complete assessment by a doctor, taking into account the specificities of each individual.
If you have any doubts about your weight, do not hesitate to talk to your doctor who will be able to calculate your BMI and advise you.
So, now you know everything about this index that everyone is talking about! But remember, beyond the numbers, the most important thing is to feel good in your body and adopt a balanced lifestyle. Your health is not just about your BMI!
Scientific references
(1) World Health Organization. (2021). Obesity and overweight.
(2) Eknoyan, G. (2008). Adolphe Quetelet (1796-1874)--the average man and indices of obesity. Nephrology, dialysis, transplantation.
(3) Keys, A., Fidanza, F., Karvonen, M.J., Kimura, N., & Taylor, H.L. (1972). Indices of relative weight and obesity. Journal of chronic diseases.
(4) Abdelaal, M., le Roux, CW, & Docherty, NG (2017). Morbidity and mortality associated with obesity. Annals of translational medicine.
(5) Nuttall FQ (2015). Body Mass Index: Obesity, BMI, and Health: A Critical Review. Nutrition today.
(6) Garrido-Chamorro, RP, Sirvent-Belando, JE, Gonzalez-Lorenzo, M., Martin-Carratala, ML, & Roche, E. (2009). Correlation between body mass index and body composition in elite athletes. The Journal of sports medicine and physical fitness.
(7) Andreoli, A., Garaci, F., Cafarelli, F.P., & Guglielmi, G. (2016). Body composition in clinical practice. European journal of radiology.
