Priority to Health

Sport: how do you know if you're doing the right weight for your health?

Every week, Dr. Jean-Marc Sène, sports physician, presents his sports column in Priorité Santé. Today, he talks to us about BMI or Body Mass Index.

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How to calculate your Body Mass Index (BMI)? © Getty Images - wragg

By: Caroline Paré Follow


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BMI is very well known for diagnosing overweight and obesity, but how do we know how to use it?

The BMI is a grader that allows us to estimate a person's corpulence. Invented in the mid-nineteenth century by Adolphe Quetelet, a Belgian mathematician and one of the founders of modern statistics, this index is also called the Quetelet index.

If you weigh yourself regularly and notice that you are gaining weight, talk to your GP. It will definitely calculate your BMI. Body mass index or BMI is calculated by dividing weight by height squared.

How to interpret BMI?

If the BMI is:

  • Between 25.0 and 29.9 kg/m²: there is an excess weight
  • Between 30.0 and 34.9 kg/m²: this is moderate obesity
  • Between 35.0 and 39.9 kg/m²: this is severe obesity
  • More than 40 kg/m²: we are talking about massive obesity

There is another measure, and that is waist circumference: another important indicator of overweight or obesity...

The waistline gives a simple picture of the excess fat accumulated in your abdomen. The measurement of the waist circumference is carried out under specific conditions:

  • Clothing-free, directly on the skin
  • Stand with your feet together, with your arms relaxed on either side of your body
  • At the end of a normal exhalation
  • By placing a tape measure horizontally, halfway between the lower part of the last rib and the highest part of the pelvic bone

The waist circumference is considered too high if it is greater than or equal to:

  • 80 cm for a woman
  • 94 cm for a man

It is then associated with the development of metabolic complications such as:

  • Diabetes
  • Increased cholesterol and/or triglycerides in the blood
  • Increased cardiovascular risk.

If this is the case, it is advisable to make an appointment with your doctor. He or she will carry out a check-up to determine any health problems associated with your overweight.

Since the isolated measurement of weight is not sufficient to identify overweight, can we trust scales that indicate fat mass?

No, measuring weight in isolation is not enough, as it does not allow you to determine the percentage of body fat, which is the real indicator to know. The scales that indicate fat mass correspond to a technique known as impedance metry: it uses variations in electrical conduction capacity according to the nature of the tissues crossed. Fat mass is not very conductive compared to other tissues rich in water and electrolytes that facilitate conduction. This method uses equations, some of which have not been validated. Results vary depending on age, gender, ethnicity, and hydration status.

The impedance method, usually offered in the form of a balance, should be excluded, due to unreliability. The results appear to be underestimated or not reproducible, with significant differences compared to the reference methods and the skinfold method.

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