There are few single measures in healthcare that seem to carry as much weight as body mass index, or BMI. We encounter it not just at doctors' offices, but with online calculators and smart scales, at gyms and even when determining eligibility for the Covid vaccine.
Its formula is simple: Take your weight (in kilograms), and divide by the square of your height (in metres).
The result, which slots you into one of four main categories, is meant to describe your body in a single word or two: underweight (BMI less than 18.5), normal weight (18.5-24.9), overweight (25.0-29.9) or obese (30 or greater).
After talking to an epidemiologist, two obesity medicine physicians, a health psychologist and a sociologist, none claimed that BMI was a very useful measure of a person’s health.
And, in fact, some said they would call it a scam.
How is BMI useful?
Introduced in the 1830s by a Belgian statistician who wanted to quantitatively describe the "average man", the calculation was named the body mass index and popularised in the 1970s by Minnesota physiologist Dr Ancel Keys. At the time, Keys was irritated that life insurance companies were estimating people's body fat – and hence, their risk of dying – by comparing their weights with the average weights of others of the same height, age and gender. In a 1972 study of more than 7,000 healthy, mostly middle-aged men, Keys and his colleagues showed that the body mass index was a more accurate – and far simpler – predictor of body fat than the methods used by the insurance industry.
"It is a very useful tool in epidemiologic research," says Dr JoAnn Manson, a professor of medicine at Harvard Medical School. Research has shown that across large groups of people, higher BMI is generally associated with greater risk of heart disease, type 2 diabetes and some types of cancer, she says. On average, people with a higher body mass index have more body fat, so it can be useful for tracking rates of obesity, which have nearly tripled globally over the last several decades. It also has a "J-shaped" relationship with mortality; very low and very high BMIs are associated with greater risk of dying sooner, whereas the "normal" to "overweight" range is associated with lower mortality risk.
As Keys found, BMI is also easy and inexpensive to measure, which is why it is still used in research studies and doctors’ offices today.
Does it accurately describe individual health?
For all its utility as a research tool, though, body mass index is "fairly useless when looking at the individual", says Dr Yoni Freedhoff, an associate professor of family medicine at the University of Ottawa.
BMI can’t tell, for instance, what percentage of a person’s weight is from their fat, muscle or bone. This explains why muscular athletes often have high BMIs despite having little body fat. And as people age, it’s common to lose muscle and bone mass but gain abdominal fat, a change in body composition that would be concerning for health but might go unnoticed if it didn’t change a person’s BMI, Manson says.
The measure also does a poor job of predicting a person's metabolic health. In a 2016 study of more than 40,000 adults in the United States, researchers compared people's BMIs with more specific measurements of their health, like their insulin resistance, markers of inflammation and blood pressure, triglyceride, cholesterol and glucose levels. Nearly half of those classified as overweight and about a quarter of those classified as obese were metabolically healthy by these measures. On the other hand, 31 per cent of those with a "normal" body mass index were metabolically unhealthy.
BMI can "label a huge swath of our population as somehow aberrant because of their weight, even if they're perfectly healthy", says A Janet Tomiyama, lead author of the study and an associate professor of health psychology at the University of California, Los Angeles.
Another problem with BMI is that it was developed and validated mostly in white men, says Sabrina Strings, an associate professor of sociology at the University of California, Irvine. But body composition and its relationship with health can vary depending on your gender, race and ethnicity. "Women and people of colour are largely not represented in many of these data," Strings says. "Nevertheless, they're being used to create a universal standard."
Can BMI be harmful?
It can be if it's used to set arbitrary goal posts for body size. Dr Rekha Kumar, an associate professor of clinical medicine at the Weill Cornell Medical College, says some patients who have lost weight and reached what she sees as their "healthy, happy weight" but who still have a high BMI, may feel they have to unnecessarily or unrealistically drop more pounds to be considered normal on the scale.
It can also be harmful if a doctor assumes that a person with a normal body mass index is healthy, and doesn’t probe them about potentially unhealthy habits they may have, like following a poor diet or not getting enough physical activity, Freedhoff says. And if doctors of patients with higher BMIs focus on weight alone as the cause of any health issues, they may miss more important diagnoses and risk stigmatising patients.
There is plenty of evidence that weight stigma is harmful, Tomiyama says. Research has shown that anti-fat bias is common among medical doctors, which can result in lower quality care and cause patients to avoid or delay medical attention. People who have felt discriminated against because of heavier weight are also about 2½ times more likely to have mood or anxiety disorders, and are more likely to gain weight and have a shorter life expectancy.
“You can see a situation where this hyper focus on body mass index, and that there are allowable and unallowable BMIs, could be highly stigmatising,” Tomiyama says. “And that stigma in turn could ironically be driving future increases in weight.”
If BMI isn’t important to monitor for health, what is?
If you’re concerned about your weight, a more direct and relevant way to gauge potentially unhealthy body fat is to measure your waist circumference, Manson says. This estimates abdominal fat, which lies deep within the abdomen and accumulates around vital organs. In excess, it can increase the risk of certain obesity-related conditions like type 2 diabetes, high blood pressure and coronary artery disease. It is also more harmful than subcutaneous fat – the soft fat you can pinch with your fingers that lies in a layer just beneath the skin.
But in Freedhoff’s weight management clinic, he and his team don’t set patient goals around BMI, weight or waist circumference at all.
“We discuss something we call ‘best weight’, which is whatever weight a person reaches when they’re living the healthiest life they can actually enjoy,” he says. If a patient’s weight is negatively affecting their health or quality of life, Freedhoff and his colleagues will explore weight-loss strategies including lifestyle changes, medication or, in some cases, surgery. But if it’s not, Freedhoff tells patients they may already be at their healthy weight, regardless of their BMI.
Rather than focusing on body size as a gauge of health, Tomiyama says blood glucose, triglyceride and blood pressure results can be better windows into wellbeing. How you feel in your body is important, too, she says. "Can you go up a flight of stairs and feel good about how you feel after that? How are you able to live your life in the body that you have?" she asks.
If you’re seeking better health, prioritise behaviours that are more within your control than your body mass index, Tomiyama says, like “better sleep, more exercise, getting a handle on stress and eating more fruits and vegetables”. – New York Times
