BMI: an indicator of my weight problem?

BMI or Body Mass Index is a measure that compares an individual’s weight to their height. It is mainly used today in the medical field, to define healthy weight limits, and in the insurance field in North America, to define health risks and adapt prices accordingly. However, more and more studies are questioning the relevance and validity of this tool. Is it really a good measure of weight problems? This is what we will see.

1. Where does BMI come from?

Contrary to what one might think, the BMI was not developed by a doctor or a health or nutrition professional, but by a Belgian astronomer, mathematician, sociologist and statistician, Adolphe Quetelet, between the years 1830 and 1850. The index was then developed for sociological use. It was later developed in the 1970s by Ancel Keys, who mentioned that this tool is useful for studying masses, a set of individuals, but that it is not a good tool for individual use. In short, its simplicity is relevant for statistical use but does not allow for a good analysis of each individual.

2. What does BMI measure?

The body mass index measures the mass per square metre. It is measured with the following formula: mass/height².

The mass is in kilograms and the height in metres.

Depending on this score, several categories exist in relation to a weight standard.

Below 18.5 kg/m², we speak of thinness, between 18.5 and 25 kg/m² is the range considered normal, between 25.1 and 30, we speak of overweight and above 30 of obesity (with sub-levels of moderate, severe and morbid obesity). These thresholds are defined by the WHO, the World Health Organisation.

Today, BMI is primarily used for medical purposes. It is used to identify people whose weight may pose a health risk, whether they are undernourished (BMI below 16.5) or obese (BMI above 30). This is the real value of it. It gives, for each category of population, the future health risks. It is therefore a measure of prevention and not a measure of health status at the present time.

In France and other European countries, for example, models with a BMI of less than 18.5 are not allowed to work in order to combat anorexia in the fashion industry.

3. Is the BMI reliable?

The short answer to this question is no. The long answer is a bit more nuanced.

In reality, it is a good tool for studying populations, but it is often ill-suited to its most common use.

In statistical studies, the large number of subjects studied makes it possible to smooth out variations between individuals and the margin of error is much smaller.

In individual use, the BMI does not take into account different body shapes, gender, ethnicity or body fat.

It is much more of an indicator of health risks than BMI in general.

For example, some sportsmen and women have a BMI of overweight or even obesity, even though they are in excellent physical shape.

It should never be used as a diagnostic or assessment tool alone. It is primarily a dialogue tool, which should be complemented by other measures to assess a patient’s health status.

Studies show that people with an overweight or moderately obese BMI with physical activity have as low a mortality risk as people with a normal BMI with physical activity, and even a lower mortality risk than people with a normal or lean BMI without physical activity.

It is generally more reliable for measuring leanness and undernutrition, as there is less fat, but it also does not take into account different body types.

In general, BMI alone is not an indicator of a weight problem or health problem. If you are worried about your health, make an appointment with a doctor specialising in dietetics or nutrition. He or she will be able to tell you what the real risks are and suggest ways to reorganise your diet and physical activity to prevent the risks and increase your life expectancy.

BMI should not be used as a weight loss measure for cosmetic purposes. Like most weight loss figures, BMI can lead to a form of mental rigidity around this value, which can lead to obsessions and eating disorders such as anorexia or bulimia. Again, BMI is not a reliable measure of body fat or body shape. 

If food is a concern, please see our test What is your relationship with food?