BMI is the most useful indicator of health risks associated with being overweight and underweight. This is what the High Authority for Health recalls in a recent argument (2) . This page dwells on the classification that is made of the BMI, and on the interpretation of the different “slices” in order to remove any ambiguity.
BMI: WHAT ARE THE SLOTS? (OR CLASSIFICATION)
Once the Body Mass Index has been calculated, the value obtained is expressed in kg/m 2 . However, this value should be interpreted correctly.
For this, the World Health Organization (WHO) provides an interpretation (classification) according to the value obtained from the BMI (1) . This interpretation is as follows:
| Classification according to WHO | BMI value (in kg/m 2 ) |
|---|---|
| Underweight | < 18.5 |
| Severe underweight | < 16.5 |
| Moderately underweight | 16.00 – 16.99 |
| Mild underweight | 17.00 – 18.49 |
| Normal build | 18.50 – 24.99 |
| Overweight | ≥ 25.00 |
| Pre-obesity | 25.00 – 29.99 |
| Obesity | ≥ 30.00 |
| Class I obesity | 30.00 – 34.99 |
| Class II obesity | 35.00 – 39.99 |
| Class III obesity | ≥ 40.00 |
These slices are the exact and official slices . Any other interpretation is wrong. There are many sites on the Internet that indicate that below 20, the situation is “ a situation of thinness ”. This is however false. Our dossier is based on information officially provided by the WHO (World Health Organization) as well as by the HAS (Haute Autorité de Santé). You will find at the very end of the file the sources and references that we used to constitute this page (in general this is true for each of our pages: you will find at the end of the article all the sources and references, for that you can consult totally transparent, verified and verifiable information ).
For children (up to 18 years old) refer to the specific curves defined in collaboration with INPES , this page only applies to adults. Teenagers should never interpret their Body Mass Index according to the classifications and ranges provided on this page. You have to go to the specific page for children.
SLOTS: WHAT EXACT MEANING?
This classification is subject to many questions from Internet users: what does class I obesity really mean, for example, compared to class II obesity?
It is precisely to answer these questions that this dossier has been compiled. To guide you, we rely on the recommendations made by the Haute Autorité de Santé (2) .
Below 20, the mortality rate increases
Even if underweight is only reached when the BMI is strictly less than 18.5 kg/m 2 , the fact remains that an increase in mortality is observed in all the different studies in people whose the BMI is less than 20. A BMI equal to 19 will therefore still be considered as a BMI in normal corpulence.
From 20 to 24.9 the mortality rate is the lowest
According to a study of 1.46 million white people aged 19 to 84, it is when the BMI is between 20 and 24.9 that the death rate is lowest.
Beyond 20, the risk of disease increases
When BMI increases from 20 to 30 kg/m 2 , there is a proportional relationship between weight and arterial hypertension, coronary heart disease and type 2 diabetes.
Clearly: a person has a BMI of 20kg/m2 ( you can calculate your BMI on this page ). This person has a priori less arterial hypertension (all other considerations being equal) than a person with a BMI of 26 kg/m 2 .
From 25, the mortality rate increases in young people (under 30)
For people under 30, as soon as the BMI exceeds 25 kg/m 2 , the mortality rate is on the rise . Clearly as soon as overweight is reached. However, when a person is overweight, his relative risk rate in the face of mortality is only increased by 13% compared to individuals who are in a situation of normal corpulence.
From 28, the mortality rate increases for everyone
In all cases, from a BMI at least equal to 28 kg/m 2 , the mortality rate increases (except for people aged over 85, for whom the BMI does not apply to all very few ways).
- For people with class I obesity (BMI between 30 and 34.99) the risk is 44% higher.
- For people with class II obesity (BMI between 35 and 39.99) the risk is 88% higher.
- For people with class III obesity (BMI over 40) the risk is 151% higher.
From 35, in some cases, bariatric surgery can be a solution
Individuals with a BMI greater than 35 and reporting comorbidity that can be alleviated after surgery (e.g. high blood pressure, etc.) may undergo bariatric surgery ( bariatric surgery ).
Patients with a BMI greater than 40, without necessarily gathering the condition of a comorbidity, can be the subject of a bariatric operation. However, this type of operation is subject to a collegial decision by the medical profession. Please note that other criteria must be met to benefit from such an operation (acceptable operational risk, etc.). In addition, bariatric operations are only considered if all prior treatments have failed.
Among the bariatric operations that are generally proposed there are:
- gastric bypass
- The gastric band
- Sleeve gastrectomy
SUBCATEGORY IN THE OBESITY RANGE (BMI OVER 30)
As for the overweight category, the HAS specifies certain sub-categories. Please note that these categories are specific to the Haute Autorité de Santé in France. They have no international value.
EXAMPLE OF INCORRECT INFORMATION
Too often a misinterpretation of BMI is made. For example on the official website of Weight Watchers, yet widely acclaimed, the following explanations are given (4) (see image below and comments below).
- The BMI is not intended to determine the ” status of a weight ” (it does not really make sense). BMI is simply an indicator of the state of corpulence. To learn more about this, you can consult our file on the definition of BMI .
- ” Experts believe that a BMI of less than 20 is a sign of too much thinness “: no, that’s not true . As we have seen in this dossier, it is below 18.5 that the World Health Organization and the High Authority for Health consider that there is underweight.
DID YOU KNOW ? OTHER INDICATORS EXIST
Indeed, the BMI (Body Mass Index) is not the only indicator relating to your height and your weight, there are others. Many “theoretical ideal weight calculators” are available on our site. Each of these calculators is presented with clear explanations, and always with the sources and references that go well. You can click on the image below to access the list of all our different tools.