Is the Body Mass Index (BMI) accurate?

In the 1800s, Adolphe Quetelet, a noted statistician, mathematician, and astronomer, created a measure of body fat by a simple calculation involving your height and weight. The calculation is as follows:

Your weight in kilograms (kg) / height² (squared) in meters

Today, doctors, nutritionists, and many other healthcare professionals may use BMI as a screening measurement.

However, initial BMI calculations were meant for populations rather than individuals, and this could be one of its contributing flaws.

Is BMI accurate as a healthy weight indicator?

There are several factors that the BMI does not consider, making it difficult to say that it is accurate for everyone. It may indicate to healthcare professionals whether you are at risk of developing other health conditions, but it does not automatically mean that you are unhealthy.

Here are some of the reasons why.

Body fat vs. muscle

BMI cannot distinguish between body fat and lean muscle weight, which may be problematic if you are an athlete or someone who regularly works out.

You may have higher muscle mass and lower body fat but register still as “overweight” or “obese,” according to BMI calculators.

A healthcare professional should recognize the difference and perform other measurements that may be more appropriate for different body types.

Weight distribution

According to the Centers for Disease Control and Prevention (CDC), higher waist measurements may indicate a higher risk of specific health conditions, including type 2 diabetes, heart disease, and high blood pressure (hypertension).

BMI alone does not account for this, and two people may weigh the same but carry the weight in different places.

Bone density

While BMI does not consider how dense your bones are, there may be a link between weight and bone mineral density (BMD).

According to a 2020 commentary, a higher BMI can mean a higher bone density, which may be an important factor for people with osteoporosis — a condition that affects bone structure and strength.

However, the same commentary notes that other complications, like type 2 diabetes, can appear with a higher BMI, so finding an optimal body weight is crucial for people with osteoporosis.

Ethnicity and race

The American College of Cardiology (ACC) notes that BMI data largely comes from a white population, and how body fat distributes itself differs in Black, Hispanic, East, and South Asian populations.

Healthcare professionals may refer to a “cut-off” for BMI. Above this point, your risk of developing obesity-related complications increases. A 2021 study that followed 1.4 million people in England found that the cut-off point for type 2 diabetes risk differs among populations as follows:

This data demonstrates the considerations that healthcare professionals should make when looking at BMI information in the wider population.

Are there more accurate options?

There are other ways to measure body mass, such as skinfold calipers, body circumference measurements, and dual-energy x‐ray absorptiometry (DEXA) scans. 

Healthcare professionals can use a DEXA scan to tell you a lot about your body, including the amount of fat tissue and lean muscle you have, and how dense your bones are.

Additionally, a waist-to-hip ratio can tell you about how you carry your weight, which can also affect health risks.

Latest news on BMI

In June 2023, the American Medical Association (AMA) held its annual meeting with its House of Delegates. Here, they accepted the policy to clarify how the BMI fits into today’s medicine.

Under the new policy, the AMA recognized challenges with BMI because it is based on data collected from earlier generations of non-Hispanic white populations. They also recognized the harm caused by BMI, due to its use for racist exclusion.

It suggests that, ultimately, doctors and healthcare professionals should use the BMI alongside other valid risk criteria, including visceral fat, adiposity index, body composition information, relative fat mass, genetic or metabolic factors, and waist circumference.

Summary

The body mass index (BMI) has some flaws when it comes to measuring obesity.

Despite healthcare professionals using it for over 100 years, it has inaccuracies since the original data came from non-Hispanic white populations. Different races and ethnicities have differing body compositions, so using the BMI for everyone will likely be inaccurate.

BMI does not consider a number of factors, including lean muscle mass, bone density, and weight distribution.

The AMA suggests that doctors and healthcare professionals use a combination of risk measures to establish your health status, like genetic and metabolic factors, waist circumference, and relative fat mass.

If you are concerned about your weight, consider discussing it with a healthcare professional, as they will be able to offer the best advice for your circumstances.

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