Using BMI to measure your health is nonsense. here’s why

We are a society obsessed with numbers, and no more than with the management of our health.

We use smartwatches to count steps and track our daily activity, create fitness scores, and monitor our heart rate and sleep quality to measure our health and well-being.

Doctors can be just as obsessed with numbers, relying on measurements and equations to create scores for our health, one of the most popular of which is body mass index (BMI).

But BMI – a measure of the relationship between your weight and your height – is coming under increasing scrutiny. More and more experts are questioning its accuracy and the fixation of health practitioners on its use as a single indicator of health and healthy weight.

Here’s everything you need to know about BMI — and why using it as your sole measure of your health is nonsense, starting with a quick history lesson.

Where does BMI come from and why is it associated with health?

The concept of BMI was developed in 1832 (yes, almost 200 years ago!) by Belgian statistician Lambert Adolphe Quetelet, who was called upon to create a description of the “average man” to help the government estimate the number of obesity in the general population.

Fast forward 100 years to the United States, where life insurance companies had begun comparing people’s weight to an average population weight for similar individuals in order to calculate insurance premiums based on a predicted risk of death.

Annoyed by this somewhat unscientific approach, American physiologist Ancel Keys conducted research on 7,000 healthy men using the Quetelet measure, finding this method to be a more accurate and simple predictor of health that was also cheap.

Quetelet’s calculation was later renamed BMI and adopted as the primary indicator of health, with later studies confirming increased risks of heart disease, liver disease, arthritis, certain cancers, diabetes, and heart apnea. sleep with increased BMI.

Read more: Explained: Overweight, obesity, BMI – what does it all mean?

Its use quickly spread and today BMI can be found everywhere, from the doctor’s office to the gym.

How is BMI measured and what do the scores mean?

The BMI formula is simple and easy to calculate with the many free BMI calculators available online.

To calculate BMI:

  • take your weight in kilograms

  • to get your index, divide your weight by the square of your height in meters.

  • Your result puts you in one of four categories describing your body weight in one word:

    • underweight – a BMI of less than 18.5

    • normal – a BMI between 18.5 and 24.9

    • overweight – a BMI between 25.0 and 29.9

    • obese – a BMI of 30 or more.

    Read more: Numbers don’t have it: Why measuring won’t lead to better health

    So is BMI an accurate measure of health?

    In short: no.

    Although BMI is an accessible and affordable way to screen a person’s health, it should not be relied upon as a sole measure of health.

    Here’s why.

    1. BMI misses a more important measure – body fat percentage

    BMI is based on body weight, but a person’s risk of disease is related to body fat, not weight.

    Although body weight can be a proxy for body fat, there’s one important reason it doesn’t always tell an accurate story: muscle is much denser than fat.

    Because BMI calculators cannot differentiate fat from muscle, people can easily be misclassified. At the extreme, the BMI has classified athletes in peak physical condition, such as sprinter Usain Bolt, as nearly overweight, and American football player Tom Brady as obese.

    2. BMI does not measure body fat distribution

    Numerous studies have shown that people with the same BMI can have very different disease risk profiles, mainly depending on where fat is distributed in their body. This is because not all fats are created equal.

    If you have fat stored around your stomach, your risk of chronic disease is much higher than people who have fat stored around their hips because it’s an indicator of how much visceral fat you have. – the type of fat deep in your belly that increases your risk of stroke, type 2 diabetes and heart disease.

    In white populations, a waist circumference of more than 80 cm for women and more than 94 cm for men is associated with an increased risk of chronic disease, and for Asian populations it is greater than 80 cm for men. women or 90 cm for men.

    3. BMI does not take demographic differences into account

    BMI is something none of us like – racist and sexist.

    When Quetelet created and Keys validated the BMI, they studied middle-aged, largely male, Anglo-Saxon populations. Their method prevails, even though BMI calculations and classifications are universally used today.

    Our bodies, by nature, have distinct characteristics related to our sex, including the fact that women generally have less muscle mass and more fat mass than men. We also know that muscle mass decreases and shifts around the body as we age.

    Read more: Too fat, too thin? How do you calculate your ideal weight?

    Research has also confirmed significant differences in body weight, composition, and disease risk based on ethnicity. This includes findings from the early 2000s that found on measures of optimal health, people of Asian descent should have lower BMIs, and people of Polynesian descent could be healthier with higher BMIs. .

    This problem has led to the proposal of redefined BMI thresholds for people of Asian origin (where a healthy BMI is less than 23) and Polynesians (where a healthy BMI is less than 26).

    So what should we use instead?

    To be clear: weight and health are linked, with countless studies showing that people who are obese or overweight have an increased risk of disease.

    But while BMI can be used as a screening tool, it shouldn’t be the only tool relied on to assess a person’s health and healthy weight.

    Instead, we need to focus on metrics that tell us more about fat in the body and where it’s distributed, measuring weight circumference, waist-to-hip ratios, and body fat to better understand health. and risks.

    We also need to consider the many other ways to measure your health and the likelihood of disease, including levels of triglycerides (a type of fat found in your blood), blood pressure, blood glucose (sugar), heart rate, the presence of inflammation and stress levels.

    As a single measure, BMI is not a good measure of health – it lacks precision and clarity, and in its current form it does not measure the many important factors that influence your risk of disease.

    Although BMI can be a useful starting point for understanding your health, it should never be the only measurement you use.

    Author: Nicholas Fuller – Charles Perkins Center Research Program Manager, University of Sydney The conversation

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