4 min read

The Problem with BMI

In this series of articles, medical students from across the country will discuss a range of topics from medical ethics to the NHS to public health to medical conditions to clinical governance.

What is BMI?

BMI or Body Mass Index has been used to measure body mass with respect to height in clinical settings. It is calculated by dividing body mass/weight (kg) by height squared (m²). BMI has been used to determine whether a patient is overweight, obese, underweight or normal. However, right from its inception, there have been multiple problems associated with the calculation of BMI and its applications, making it an inaccurate method to determine if a person is healthy or not.

A BMI of 18.5-25 is classified as ‘normal’
25-30 is considered ‘overweight’
Anything above 30 is classified as ‘obese’

BMI is currently used as a measure of body fat which is strongly correlated with weight-related medical conditions such as heart disease, stroke, type 2 diabetes, sleep apnea, breathing problems and multiple forms of cancer.

The History of BMI:

BMI was invented nearly 200 years ago by Adolphe Quetelet - a mathematician not a physician. That’s our first problem. Quetelet believed that the mathematical mean of a population was its ideal, and his desire to prove it resulted in the invention of the BMI.

Initially called Quetelet’s Index, Quetelet derived the formula based solely on the size and measurements of French and Scottish participants. That is, the Index was devised exclusively by and for white, male Western Europeans. By the turn of the next century, Quetelet's BMI calculation was used to determine whether or not someone was healthy - according to exclusively western and male standards. Ethnic groups such as South East Asians have 3-5% additional body fat than white Europeans - not because of their eating or activity habits, but rather because of their genes. At the same time, women tend to have larger hips than men and this can result in a higher BMI calculation. Since BMI was not made for people of colour or women, the parameters used to check whether their BMI is ‘normal’ do not apply to them. This makes traditional BMI ranges invalid for people of colour or the female population.

The Many Flaws of BMI:

The problem with BMI, while it does adjust for height, is that it does not adjust for what makes up the weight. This means that somebody who's extremely muscular could have a high BMI and yet have a very low per cent body fat and somebody who has very little musculature could actually register as lean on the BMI scale, and still have excess body fat. Many professional athletes are considered clinically obese based on the BMI scale because it cannot distinguish the weight of muscle from the weight of fat. People who have excess body fat will definitely have a high BMI, but it doesn’t work the other way around i.e. someone with a high BMI may not necessarily have excess body fat and will therefore not be prone to diseases that come with obesity.

Factors such as age, sex, ethnicity, and muscle mass can influence the relationship between BMI and body fat. Also, BMI does not distinguish between excess fat, muscle, or bone mass, nor does it provide any indication of the distribution of fat among individuals. (CDC)

For its inventor, the BMI was a way of measuring the population as a whole, not individuals — and it was designed for the purposes of statistics, not individual health. As a global health statistics tool, BMI is of great use - governments and health organizations can recognize trends in obesity and weight gain and can accordingly make laws to bring the country’s population on a more healthy path. However, each individual’s health is determined by a number of factors such as body fat percentage, ethnicity, gender, subcutaneous fat, body water, cholesterol levels, iron levels - not just their weight and height.

Why do we still use BMI?

Despite its flaws, BMI is an inexpensive and easy screening method to check body fat. Its accuracy ranges from around 50-80%. While other methods have been devised to check body fat percentage, they are far more expensive, inconvenient and subjective. Methods such as MRI scans and checking one’s underwater weight (to check body density) are more expensive and not easily accessible. Checking waist circumference, which is much more strongly correlated with weight-related diseases such as Type 2 diabetes, is much harder to standardize. As for differences in ethnicities and body fat percentages, it is nearly impossible to come up with different systems for every single ethnic group to exist. So what’s the solution?

BMI simply does not provide enough context about a person’s lifestyle and health. It is firstly important that healthcare professionals recognize and acknowledge the flaws of BMI. With this knowledge, it is important that they incorporate other tools while determining the health of their patient, along with BMI. For example, along with checking one’s BMI, doctors can check the patient’s cholesterol level - which is also a predictor of cardiovascular diseases. This will result in a more individualized course of action for patients - which fits perfectly into the ‘patient centred’ healthcare model that most health institutions are moving towards.

Summary:

Body Mass Index or BMI was created more than 200 years ago by a mathematician - Adolphe Quetelet. Many healthcare professionals argue that it is not an accurate tool for assessing body fat.
BMI is currently used as a measure of body fat which is strongly correlated with weight-related medical conditions such as heart disease, stroke, type 2 diabetes, sleep apnea, breathing problems and multiple forms of cancer.
When BMI was being invented, Quetelet primarily involved white western Europeans. This makes the system less applicable to people of colour or women. BMI also can’t distinguish between weight due to muscle/bone and weight due to fat, often resulting in wrong diagnoses.
BMI is a better statistical tool for the whole population rather than individual patients’ health. Therefore, BMI should be used with a number of other tools to determine the health of a patient.