Having obesity increases your chances for health problems later in life. The BMI, or body mass index, is the most commonly used measure to determine if a person has obesity. It’s used in fitness settings, wellness programs and healthcare. BMI is a measure of weight (in kilograms) divided by height (in metres) squared. The diagnosis for obesity is a BMI of ≥30 kg/m2. In addition, a BMI of 25-29.9 kg/m2 is used to identify people as overweight, while a BMI of 18.5-24.9 kg/m2 is considered to be the ideal range.
How BMI Became a Health Measure
BMI was first described in 1832 by Adolphe Quetelet, a Belgian mathematician. His interest in the Quetelet Index (as it was known then) was purely statistical. It was a way for him to describe how weight changed in relation to height as people age. It was Ancel Keys who in 1972 gave it the name of body mass index and suggested it as a way to assess body fat.
From there, BMI categories of overweight and obesity were based on population-wide studies, which showed that as BMI increased above 25 kg/m2, the chances for getting heart disease and dying early also increased. Higher BMI values are also associated with being more likely to get a number of other chronic diseases and mental illnesses. Plus, people with obesity face a great amount of social stigma.
Worldwide, approximately 13% of adults have obesity. This is triple what it was in 1975. But rates vary by country. In Saudi Arabia 24.7% of adults have obesity, while that number is 41.9% in the United States.
The Problem with BMI
However, there are many problems with using BMI to determine health status. First and foremost, BMI is a measure of weight. And your weight is a combination of body fat, bone and lean mass (predominantly muscle). But excess weight is not really the concern, it’s excess body fat.
On a population level out of 1000 people with a BMI ≥30 kg/m2 most are likely to have excess body fat. But BMI falls short at an individual level. For example, Dwayne Johnson has a BMI around 34 kg/m2, yet not many people would say he has obesity. It’s possible to have two people with a BMI of 31 kg/m2, yet one with 50% body fat and the other less than 30%. And it’s still common to hear stories of people who are told by their doctors they’re obese even when they don’t have much fat.
BMI also doesn’t apply equally across populations. People of Asian background tend to have more body fat at a given BMI than those of European background. This has led some to suggest having ethnic-specific categories. Similarly, women generally also have more body fat than men at the same BMI. And BMI fails to be associated with early death in older adults, except at much higher levels (≥35 kg/m2). Other studies have also challenged whether the BMI categories of overweight and obesity are appropriate at any adult age.
So why do doctors still use BMI?
First off, it’s cheap, requiring very little equipment. And it’s easy to do. In some clinics, physician assistants and clerks measure a patient’s height and weight. It’s also ingrained into many aspects of the medical system. Doctors learn to use BMI in medical school. BMI is also included in many electronic medical records as a vital sign similar to heart rate and blood pressure. It’s used to determine eligibility for medical procedures such as infertility treatment and heart transplantation for both donor and recipient. Life insurance companies also use BMI to help determine how much your insurance costs.
People also understand weight. Many people have a scale in their homes for self-monitoring. And once a person reaches adult age, their height isn’t going to change, so changes in BMI are likely to reflect changes in weight.
Is there a better alternative?
Two common alternatives are waist circumference and the waist-to-hip ratio. (Click here for how to measure waist circumference and waist-to-hip ratio.) These are simple measures that only require a tape measure. They’re based on the fact that excess fat around the waist is associated with greater chances for type 2 diabetes, heart disease, certain cancers and early death, than fat elsewhere on the body.
The BMI is unable to distinguish between fat around the waist and fat elsewhere. And unlike BMI, a change in waist circumference is almost always due to a change in fat. It’s indeed possible for someone to have an ideal BMI but have an elevated waist circumference or elevated waist-to-hip ratio. And these people have greater chances for getting disease and early death compared to someone with an elevated BMI but a lower waist circumference.
However, the waist circumference or waist-to hip ratio aren’t the solutions for diagnosing obesity. Some people find it uncomfortable to have their waist and hips measured (or measure someone else’s) as the ideal method is measuring against the skin. And while waist circumference is important for assessing metabolic risk, it’s not a measure of overall body fat. Excess fat anywhere in the body can still increase your chances for joint pain and mobility issues, and your chances of facing social stigma. It’s also hard to measure one’s waist or hips by yourself, making self-monitoring a challenge.
The Bottom Line
There are other ways to assess body fat. These include the use of dual energy x-ray absorptiometry (DEXA), CT scan and MRI scan. While more accurate, these are far more costly. And it’s not clear whether they add to the assessment of your health/disease risk by a doctor. Therefore, these methods are felt to be unnecessary.
Instead, recent guidelines suggest doctors move away from using BMI to diagnose obesity and instead use it along with waist circumference or waist-to-hip ratio as part of a person’s global health assessment. This can include measures such as blood pressure, metabolic markers from blood tests and psychological screening. These guidelines define obesity as a disease at which a person having excess body fat impairs their health. In doing so, it reduces the focus of weight as a factor of obesity and shifts it to body fat and its effects on health.
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