The Theory of BMI

A critical reading of the body-mass index — and what it cannot tell you

No. 01

What is BMI?

Body Mass Index is a simple mathematical calculation:

Weight in kilograms divided by height in metres squared.

BMI = Weight in kilogram / (Height in meter)²

For adults, the standard categories are:

▪ Below 18.5 - Underweight

▪ 18.5 to 24.9 - Healthy weight

▪ 25 to 29.9 - Overweight

▪ 30 and above Obesity

On paper, it sounds very simple.

But the human body is not simple enough to be completely understood through one number.

No. 02

BMI measures weight - not body composition.

This is perhaps the biggest limitation of BMI.

BMI does not actually measure body fat.

It does not distinguish between:

▪ muscle and fat,

▪ bone and fat,

▪ visceral fat and subcutaneous fat,

▪ or different patterns of fat distribution.

Two people can have exactly the same height and weight, and therefore exactly the same BMI, while having completely different body compositions.

One could have a high percentage of muscle and very little body fat.

The other could have significantly more body fat and much less muscle.

“

The BMI would be identical. Their bodies would not.

— On the theory of BMI

No. 03

The history of BMI is also important.

The concept goes back to the 19th century and Belgian mathematician, astronomer and statistician Adolphe Quetelet. It was originally known as the Quetelet Index and was developed as a statistical relationship between height and weight.

It wasn't originally created as a modern diagnostic tool for obesity.

The term Body Mass Index was later popularised by physiologist Ancel Keys in 1972, when he examined different methods of measuring relative weight and obesity.

Over time, BMI became extremely popular because it was simple, inexpensive and easy to apply to large populations.

And that simplicity is both its greatest strength and its greatest limitation.

No. 04

Does that mean BMI is useless?

No.

BMI still has value.

It is useful as a screening tool, particularly when studying large populations. It is quick, inexpensive and easy to calculate.

The problem begins when a screening tool is treated as a complete assessment of an individual person's health.

BMI can indicate that a person's weight relative to their height falls into a particular category.

But it cannot, by itself, establish that person's overall health.

That distinction is extremely important.

No. 05

Athletes are a classic example.

Consider a highly muscular athlete.

.

Muscle contributes significantly to body weight.

A person with a large amount of lean muscle can therefore have a BMI that falls into the overweight or even obesity category, despite having relatively low body fat.

Arnold Schwarzenegger is a classic example of the limitation.

At around 6 feet 2 inches, or 188 centimetres, and weighing roughly 237 pounds, or about 108 kilograms, his BMI would be approximately 30.5.

According to the conventional BMI classification, a BMI above 30 falls into the obesity category.

But Arnold Schwarzenegger was a professional bodybuilder with an exceptionally high amount of lean muscle mass.

So would it make sense to look at that number alone and conclude that he had the body composition we normally associate with obesity?

“

Clearly, BMI by itself cannot answer that question.

— On the theory of BMI

No. 06

BMI also ignores where the fat is stored.

This is another major limitation.

Two people can have the same BMI but completely different fat distributions.

For example, one person may carry more fat around the abdomen, while another may carry it differently.

Measurements such as waist circumference can therefore provide additional information that BMI cannot.

The location and distribution of body fat can matter for health not simply the total number on the weighing scale.

No. 07

BMI also doesn't capture many other aspects of health.

BMI doesn't directly tell us about:

▪ muscle mass,

▪ body-fat percentage,

▪ cardiovascular fitness,

▪ strength,

▪ blood pressure,

▪ blood glucose,

▪ cholesterol,

▪ diet,

▪ sleep,

▪ physical activity,

▪ or overall metabolic health.

A person can have a “normal” BMI and still have unhealthy lifestyle habits or poor fitness.

At the same time, a person with a higher BMI may be physically active, muscular and metabolically healthy.

So reducing the entire concept of health to BMI is simply too narrow.

No. 08

This is where people can become misled.

For years, many people have been taught to think:

Lower weight equals better health.

And that isn't necessarily true.

A person can lose weight by losing muscle.

The weighing scale goes down.

The BMI goes down.

But that doesn't automatically mean the person's health has improved.

Similarly, someone can gain muscle through strength training.

Their weight may increase.

Their BMI may increase.

But their body composition, strength and physical fitness may improve.

So the question shouldn't always be:

“How much do I weigh?”

It should also be:

“What is that weight made of?”

No. 09

The psychological impact of BMI is another issue.

A number can very easily become a label.

FIG. A number can become a label. Health is more complicated than that.

Someone is told that they are “overweight”, and they may start associating that word with being unhealthy.

Someone loses a few kilograms and moves into another BMI category, and suddenly that number becomes a definition of success.

But health is much more complicated than that.

“

A BMI number should be a piece of information not an identity.

— On the theory of BMI

No. 10

So what should be considered?

BMI can be one part of the assessment.

But it should be considered alongside:

▪ waist circumference,

▪ body composition,

▪ muscle mass,

▪ physical activity,

▪ cardiovascular fitness,

▪ blood pressure,

▪ blood glucose,

▪ cholesterol,

▪ dietary habits,

▪ sleep,

▪ medical history,

▪ and overall physical function.

That gives a much more complete picture of health.

No. 11

So is BMI irrelevant today?

I wouldn't say that BMI is completely irrelevant.

I would say that using BMI as a standalone measurement of individual health is outdated and inadequate.

Its simplicity makes it useful.

But its simplicity also means that it leaves out a lot of information.

The modern understanding of health is moving beyond simply asking:

“How much does this person weigh?”

and towards asking:

“What is this person's body composition? Where is their body fat? How metabolically healthy are they? How fit and strong are they? And how well does their body function?”

No. 12

The real objective shouldn't simply be weight loss.

This is perhaps the most important point.

Weight loss and fat loss are not always the same thing.

If the objective is better health and fitness, the focus should be on reducing excessive body fat while preserving or building muscle, improving strength, maintaining cardiovascular health and developing sustainable lifestyle habits.

The weighing scale can be one measurement.

BMI can be another.

But neither should be treated as the entire story.

Because ultimately, the goal shouldn't simply be to become lighter.

The goal should be to become healthier, stronger and more capable.

“

And that is why BMI should be used as a tool, not as the final definition of health.

— On the theory of BMI

About the Author

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