Is BMI the Measure? Not the Last Word on Obesity

Feet on scale

Obesity is a problem throughout the world. Unhealthy diet, lack of exercise and quantity consumption are key contributors. Comfort foods and energy drinks add to the mix.

In 2022, 2.5 billion adults (18 years and older) were overweight.¹ Of these, 890 million were living with obesity, according to the World Health Organization. Over 390 million children and adolescents aged 5–19 years were overweight in 2022, including 160 million who were living with obesity.

Obesity has serious health implications. Excess body fat can trigger inflammation, high blood sugar and high blood pressure.² Levels of fat and cholesterol in the blood increase, raising the risk of heart disease and stroke. Fat can also get stored in places it normally wouldn’t be, such as the liver and kidneys. Obesity and some of its associated symptoms, such as high blood pressure and high blood sugar, contribute to a condition known as metabolic syndrome. People with metabolic syndrome have a higher risk of developing many chronic diseases, including type 2 diabetes, cardiovascular disease, and even certain cancers.

For some time, Body Mass Index – or BMI -- has been used as a clinical tool for patients with overweight and obesity. BMI is an approximate measure of a person’s best weight for health³ and is calculated by dividing weight in kilograms by height in meters squared (m2).3 BMI classifies people as underweight (under 18.5kg/m2), healthy weight (18.5 to 24.9kg/m2), overweight (25.0 to 29.9kg/m2) or obese (over 30kg/m2).

But medical professionals have long recognized BMI’s shortcomings and are seeking ways to improve how being overweight and obesity are identified and defined, and how treatment plans can be constructed accordingly.

Shortcomings of BMI

For example, in 2020 in the U.S., the National Committee for Quality Assurance (NCQA) retired the Adult BMI Assessment (ABA) as a performance measure. NCQA concluded that even though automatic calculation of BMI in electronic health records has become a common standard of practice during most outpatient visits, it fails to assess counseling or follow-up for patients either “at risk” or diagnosed as overweight or obese.⁴ “Documentation of BMI assessment alone sets a relatively low threshold for quality care to address the nation’s ongoing obesity epidemic.”

One of the primary shortcomings of BMI is that it cannot differentiate between body fat and muscle mass.⁵ Nor can it distinguish between subcutaneous fat (i.e., fat just below the skin) and visceral fat (i.e., fat found deep within the abdominal cavity). Quantifying the amount and location of body fat is critical when assessing health risk.

By itself, BMI leads to confusion and misinformation, wrote Nick Trefethen, professor of numerical analysis at Oxford University’s Mathematical Institute, in a letter to The Economist in 2022.⁶ The BMI weight/height2 formula divides weight by too much in short people and too little in tall individuals, he said. As a result, tall people believe they are fatter than they really are and short people think they are thinner. Trefethen believed a better calculation than weight/height2 for BMI would be weight/height2.5.

By itself, BMI leads to confusion and misinformation

Waste-to-Height Ratio

Many clinicians believe that waist circumference must be figured into the equation as well. The issue was addressed in the 2022 update to England’s National Institute for Health and Care Excellence (NICE) guidelines on “Obesity: Identification, Assessment and Management.”⁷

The guidelines advocate measurement of waist-to-height ratio. “Compared with BMI, visceral fat represents a useful indicator of cardiometabolic health,” wrote the authors. “Waist circumference correlates with visceral fat content, and the waist-to-height ratio takes account of variations in height within the population. Furthermore, a cutoff of 0.5 in the waist-to-height ratio to indicate increased health risk is simple and easy to remember.”

But the NICE guidelines don’t call for discarding BMI altogether.⁸ Instead, they suggest that waist-to-height ratio be measured in addition to BMI to get a practical estimate of central adiposity in adults with a BMI under 35kg/m2. This would in turn help professionals assess and predict health risks.

In the opinion of the authors of the NICE guidelines for obesity, adults should be encouraged to measure their own waist-to-height ratio for the assessment of abdominal fat, and to seek advice from a healthcare professional if this measurement indicates an increased health risk (defined as a waist to height ratio of ≥0.5).⁹

“Such an approach redirects a locus of control and responsibility from the healthcare professional toward the person living with obesity,” they write. “An emphasis on self-control will facilitate a supportive context for interactions between those living with obesity and healthcare professionals, in addition to encouraging improved self-efficacy and self-esteem. A further benefit of self-measurement of the waist-to-height ratio is that this will improve self-awareness and will also help to facilitate a more proactive approach to the self-identification of obesity.”

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