What is BMI, and is it important?

The body mass index (BMI) is a tool that healthcare professionals and scientists can use to check whether your weight falls in a moderate, low, or high category. It’s calculated by dividing your weight by your height squared. 

Weight is a sensitive topic. And losing weight is more complex than simply eating less. If you’re concerned about your weight or BMI, work with a healthcare professional to figure out the best weight range for your body and health.  

In this article, we’ll take you on a whistle-stop tour of how BMI was developed, its uses and flaws, and why your health and weight aren’t defined by one simple number. 

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What is BMI?

BMI is a number that’s calculated by dividing a person’s weight by their height squared. Scientists and healthcare professionals can use it to estimate whether a person’s weight is within a moderate — or healthy — range, or if it falls below or above this. 

For example, if you weigh 145 pounds and you’re 5 feet 4 inches tall, your BMI is 24.9. 

The BMI weight categories as determined by the World Health Organization (WHO) are:

• below 18.5: underweight

• between 18.5 and 24.9: healthy or moderate weight

• between 25.0 and 29.9: overweight

• above 30.0: obesity

BMI correlates moderately with body fat on average, though this isn’t true for everyone. More on that later.

According to the WHO, “Overweight and obesity are defined as abnormal or excessive fat accumulation that presents a risk to health.” 

Carrying excess fat, particularly belly fat, can on average increase the risk of chronic health conditions, including heart disease, type 2 diabetes, osteoarthritis, and some cancers. 

However, it’s difficult to measure body fat. So, healthcare professionals use BMI instead. 

But a healthcare professional can’t use BMI alone to make a diagnosis about someone’s health or the amount of fat their body carries.

If they think a person’s BMI indicates a greater risk of chronic health conditions, they’ll perform additional tests to assess this risk. 

In other words, your BMI is a rough estimate of whether your weight falls within the moderate range, based on your height. 

BMI through time

The history of this measurement traces back to the 19th century astronomer and statistician Dr. Lambert Adolphe Jacques Quetelet. He studied human characteristics in European males and developed the Quetelet Index, which later became the BMI.

Around the turn of the 20th century, scientists started to draw connections between body fat and life expectancy. 

In the late 1950s, the Metropolitan Life Insurance Company published tables of average weight and height for men and women at different ages. They used this to analyze connections between different health conditions, weight, and height. 

The company used a different formula in their calculations — weight divided by height — which had some rather complex mathematical problems and didn’t work as well for taller people as it did for shorter people. 

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In 1972, Prof. Ancel Keys, from the University of Minnesota, highlighted these mathematical flaws in a seminal paper. He championed the use of the Quetelet Index as a better alternative and coined the term BMI. 

Yet the Metropolitan Life Insurance Company’s tables prevailed in the United States as the most common measure to define obesity — until the WHO published their weight categories, based on the BMI, in 1995. 

Health departments across the world now use the BMI’s weight categories, but these are increasingly coming under scrutiny. 

Flaws aplenty

The average BMI in most Western populations is 24, just below the top cut-off for a moderate weight. This puts nearly half of the population in the overweight or obesity categories. 

But does a higher BMI really mean that you and almost 50% of other people have a greater risk of chronic health conditions? 

We’ve already mentioned that BMI correlates moderately with body fat. And scientists can use it to study large populations. But BMI doesn’t necessarily work at the level of the individual.  

And here’s where it gets personal.

A higher weight, and by extension a higher BMI, can result from a number of factors. 

BMI doesn’t differentiate whether your body has more lean muscle than fat mass, or vice versa.

Two people’s body compositions — their lean muscle mass, bones, and body fat — can be vastly different, even if they weigh exactly the same.

Also, much of the research that went into establishing the BMI categories harks back to “the average man,” and more specifically the average European white man.

BMI and metabolic health

Being in a bigger body doesn’t automatically mean that a person has a higher risk of chronic health conditions or is metabolically unhealthy. 

A large study of over 40,000 adults in the U.S. found that nearly half of the participants with a BMI in the overweight category and 16% of those in the obesity category were, in fact, metabolically healthy when they took part in the research. 

And at the same time, a BMI in the moderate weight category doesn’t automatically mean that a person has good metabolic health. 

By focusing solely on BMI, it’s easy to overlook other measures, like waist circumference, and many of the important behaviors that contribute to good health. 

Overall, losing weight is very challenging for the vast majority of people.

Getting enough sleep, focusing on mental health, having a quality diet, and building exercise into your routine are all important in supporting metabolic health, no matter the number on the scale. 

BMI in research

There’s one place where BMI is useful, and that’s in research. When scientists study large groups of people and their health outcomes, they like to group the participants into different categories.

At the population level, BMI is just one of many measures that allow researchers to study health interventions and health behaviors. 

There are others, including sex, age, smoking status, exercise, and diet, for example. 

While a large amount of research shows that being in a bigger body is linked to a higher risk of chronic conditions, none of these measures in isolation can define any participant’s health status. 

What should you do?

BMI is a useful tool that scientists can use in research, so it’s here to stay. 

However, it’s an imperfect indicator of your personal health and doesn’t need to define how you feel about yourself. 

Whether you’re looking to gain or lose weight, it’s best to work with a healthcare professional to establish what range is best for your unique body and your overall health. 

Being in a bigger body indicates that, on average, you may have a higher risk of metabolic diseases. But your doctor needs to perform other tests to assess this with you. 

And when doing this, they should take into account your personal circumstances.

If you’re trying to lose weight and having a hard time with it, you can still improve your metabolic health.

Other behaviors that could make a big difference include having a healthy, varied diet that's rich in plant foods, exercising, supporting your mental health, and getting enough quality sleep.