At some point or another in your life you have probably come face to face with your body mass index (BMI). Maybe you found it out using an online calculator, or your treatment center used it to track your progress, or pre DSM-5 it was used as a way to diagnose you…or not diagnose you. But what is the scientific background of this formula that is so commonly used in healthcare? I did some research on the subject and decided to share my results with you dear reader.
I hope by seeing it’s inaccuracies you will be more skeptical of the BMI and not let your BMI validate or invalidate you.
1. It wasn’t even designed for its current use
Flashback to the mid 19th century. Belgian mathematician, astronomer, and statistician Adolphe Quetelet derives the Quetelet Index. He had no interest in obesity when he came up with the index. He was a pioneer in the field of ‘social physics’ and wanted to define the characteristics of the ‘normal man’ using mathematical analysis. He used data from the heights and weights of the French and Scottish armies to show that most cases fell within the range defined as a person’s weight in kilograms divided by the square of the person’s height in meters. His work got published in 1835 in the book entitled A Treatise of Man and the development of his aptitudes. The index sat unused for nearly 140 years until Ancel Keys came along in 1972 and popularized it when he published a paper in the Journal of Chronic Diseases that coined the term body mass index. Interestingly enough Ancel Keys explicitly said that the BMI is appropriate for population studies, and inappropriate for individual diagnosis.
2. One’s health should not be defined by a decimal point.
The BMI uses sharp boundaries that hinge on a decimal point to determine if someone is underweight, ideal, overweight, and obese. A focus on only a number takes away from other important indicators of health such as diet, exercise, and lifestyle.
BMI category ranges
- Below 18.5 = Underweight
- 18.5 to 24.9 = Ideal
- 25.0 to 29.9 = Overweight
- 30.0 and above = Obese
3. It is physiologically incorrect
The BMI formula does not take into account the relative proportions of bone, muscle, and fat in the body nor does it distinguish between weight due to muscle or weight due to fat. Muscle has a higher density, more mass per unit volume, than fat. This is why many athletes have high BMIs and are classified as overweight or obese.
4. It’s a one size fits all approach
BMI does not account for differences in race, gender and age.
5. Not all fat is created equal
BMI does not measure the percentage of body fat, nor where it is distributed on the body. Belly fat (fat around the abdominal organs) puts people at risk of more life-threatening diseases such as diabetes and heart disease, whereas peripheral fat (fat beneath the skin elsewhere in the body) is associated with less health risks. Even relatively thin people can have a high level of belly fat, which means they might be healthy by BMI standards, but internally they might be at higher risk of developing health problems.
The BMI is not perfect, it comes with it’s fair share of inaccuracies.
There’s no single number that can represent a healthy weight so why are we being held accountable to comply to this metric?
The only answer I could find is that it is convenient, inexpensive, and insurance companies like it. I believe that it’s time to get over the convenience factor and get innovative. With all the technological advances in medicine that have been made this last century it baffles me that we are still using this heuristic statistical measure. I conclude this post with the words of the inventor of the BMI himself.
The more advanced the sciences have become, the more they have tended to enter the domain of mathematics, which is a sort of centre towards which they converge. We can judge of the perfection to which a science has come by the facility, more or less great, with which it may be approached by calculation.Adolphe Quetelet