Is BMI an Accurate Predictor of Health?

Body mass index (BMI) is a measure that calculates the relationship between a person's weight and the square of their height. BMI has been used for years in health and obesity assessments and has been accepted as a general guide. In recent years, however, there has been growing debate and research among health professionals about whether BMI is an accurate predictor of health. Because BMI only takes into account the ratio of weight to height, it may not reflect the complexity of body composition and metabolism.

What Is BMI?

“BMI” stands for “body mass index”. It was developed in 1832 by a Belgian mathematician named Lambert Adolphe Jacques Quetelet.

The BMI scale was developed to quickly estimate the degree of overweight and obesity in a given population, to help governments decide where to allocate health and financial resources.

How Is Body Mass Index Calculated?

The BMI scale is based on a mathematical formula that divides a person's weight in kilograms by the square of their height in metres to work out whether they are at a “healthy” weight:

BMI = weight (kg) / height (m²)

Alternatively, BMI can be calculated by dividing weight in pounds by height in inches squared and multiplying by 703:

BMI = (weight (lbs) / height (in²)) x 703

A similar online BMI calculator provided by the National Institutes of Health can also be used.

Is Body Mass Index a Good Indicator of Health?

Despite concerns that BMI does not accurately determine whether a person is healthy, most studies show that a person's risk of chronic disease and early death rises when their body mass index is under 18.5 (“underweight”) or 30.0 or higher (“obese”).

For example, a 2017 retrospective study of 103,218 deaths found that people with a BMI of 30.0 or more (“obese”) had a 1.5 to 2.7 times higher risk of death after 30 years of follow-up.

The researchers also found that people in the “underweight” category, and those in the “severely obese” or “extremely obese” categories, died on average 6.7 years and 3.7 years earlier respectively than those in the “normal” BMI category.

Because of the many studies showing that the risk of chronic disease is higher in people with obesity, many health professionals may use BMI as a general picture of a person's risk. However, it should not be the only diagnostic tool used.

What Are the Disadvantages of BMI?

There are many flaws in using body mass index. They include the following:

It Does Not Take Other Health Factors into Account

BMI gives a “yes” or “no” answer as to whether someone is at a “normal” weight, without taking into account a person's age, sex, genetics, lifestyle, medical history or other factors.

Relying on BMI alone can cause other important health measures, such as cholesterol, blood sugar, heart rate, blood pressure and inflammation levels, to be missed, and can lead to a person's true health being over- or underestimated.

It Assumes All Weight Is Equal

Although 1 pound or kilogram of muscle weighs the same as 1 pound or kilogram of fat, muscle is denser and takes up less space. As a result, a very lean person with a high muscle mass may weigh more on the scales.

If only BMI is considered, this can easily misclassify a person as “overweight” or “obese” despite a low fat mass. For this reason it is important to consider a person's muscle, fat and bone mass as well as their weight.

It Does Not Account for Fat Distribution

Although a higher body mass index is linked with poorer health outcomes, where fat is stored in the body can make an even bigger difference.

For example, in a review of 72 studies including data from more than 2.5 million people, researchers found that people with an apple-shaped fat distribution had a significantly higher risk of death from all causes, while those with a pear-shaped fat distribution had a lower risk.

It Can Lead to Weight Bias

A medical professional is expected to use their best judgement. That means taking the BMI result and seeing the patient as a unique individual. However, some health professionals use BMI alone to measure a person's health before giving medical advice. This can lead to weight bias and lower-quality healthcare.

It May Not Apply to All Populations

Although BMI is widely used among all adults, it may not accurately reflect the health of certain racial and ethnic populations.

For example, many studies have shown that people of Asian descent have an increased risk of chronic disease at lower BMI cut-off points than white people.

Better Alternatives

There are alternatives to body mass index that may be better indicators of a person's health. They include:

Waist Circumference

A larger waist circumference – more than 35 inches (88 cm) in women or 40 inches (102 cm) in men – points to more body fat around the abdomen, which is associated with a higher risk of chronic disease.

• Benefits

It is easy to measure and only needs a tape measure.

• Disadvantages

It does not take different body types into account.

Waist-to-Hip Ratio

A high ratio (greater than 0.80 in women or 0.95 in men) indicates higher fat stores around the stomach area, which is linked with a higher risk of heart and chronic disease.

• Benefits

It is easy to measure, as it only needs a tape measure and a calculator.

• Disadvantages

It does not take different body types (for example apple-shaped or pear-shaped) or builds (for example muscle and bone mass) into account.

Body Fat Percentage

Body fat percentage is the relative amount of body fat a person has.

• Benefits

It distinguishes between fat mass and lean mass and is a more accurate representation of health risk than BMI.

• Disadvantages

Convenient assessment tools (such as skinfold measurements, portable bioelectrical impedance analysis and home scales) carry a high risk of error.

More accurate tools (such as dual-energy X-ray absorptiometry, underwater weighing and the BodPod) are expensive and out of reach for most people.

Laboratory Tests

Laboratory tests are a variety of blood and vital-sign measurements that can indicate the risk of chronic disease.

• Benefits

These tests give a more detailed look at a person's metabolic health. They do not rely on body fat alone as a health measure.

• Disadvantages

Often a single laboratory value is not enough to diagnose or indicate risk.

Whichever assessment tool is used, it is important that health professionals do not rely on a single test. For example, a healthcare professional may measure a person's BMI and waist circumference and, if there is concern, carry out a blood test.

This article is for general information only; please consult your doctor and dietitian about your own situation.