This is why using BMI as an indicator of health can be dangerous

Being overweight can cause health problems. Being underweight can cause health problems. Sometimes you can be overweight or underweight and have no health problems, and never get any, just like smoking can vastly diminish your life expectancy but equally some smokers will live until they’re 100.

Because being over the ideal weight for your body, or under it, does mean you are at risk of a whole host of illnesses such as heart disease, cancer, osteoporosis, and mental health problems, doctors use a means of gauging an ‘ideal’ weight range, the body mass index (BMI).

The BMI is said to have been invented nearly 200 years ago by mathematician Adolphe Quetelet,¹ and was then published, as we know it today, in a paper by Ancel Keys in the Journal of Chronic Diseases.² This scale was developed purely to measure the weights of large populations, and Keys allegedly said himself that it shouldn’t be used to look at the health of individuals.

Your body mass index (BMI) is a number taken from your weight divided by your body height squared. The range of what constitutes ‘healthy’ is pretty big, to cater for different body shapes, and how much muscle mass someone may have, for example.

Muscle weighs a lot more than fat, so someone who is fit and healthy but who does a lot of sport could end up showing a BMI a lot higher than someone who doesn’t do much sport but is actually a little bit on the overweight side, so the BMI it has its flaws. Indeed, research has shown that some people with BMIs in the normal range are actually in worse health than their “overweight” peers.³ But can those flaws ever be dangerous? Does judging someone’s health on their weight mean doctors might overlook other problems?

This scale was developed purely to measure the weights of large population, and wasn’t intended to be used to look at the health of individuals.

According to Dr. Jane Woyka, a GP at the Harrow Health Care Centre, the BMI is currently the most useful tool doctors have of assessing someone’s general, physical health, alongside blood pressure and cholesterol levels. “Having a BMI outside the ‘normal’ range is one of the risk factors for almost every huge, important disease you can possibly think of,” she continues. “You can work out your QRISK (risk of certain conditions) if you know the parameters, and you get your risk of having a heart attack or stroke in the next ten years. Your BMI is part of this assessment.“

Almost all studies used to look at various health risks include BMI, Dr. Woyka adds, stating that it’s as important as blood pressure, smoking status, and cholesterol levels when it comes to assessing health. Of course, doctors are aware that there can be issues when it comes to deciding whether or not a specific individual is fat. Dr. Woyka uses the example of the Irish rugby squad, of which 40% of players showed up as obese when their BMI was measured, highlighting the need to take muscle mass into account.

Despite the BMI’s uses, there have been instances when BMI has proved dangerous, that much is certain. One woman, for example, wrote to her MP pleading with her to bring about changes in the system.⁴ “I never got diagnosed with anorexia, despite going from a size 16 to a size eight in less than a year,” she wrote. “I went through the monthly humiliation of being dragged onto scales and told I wasn’t thin enough to be helped yet.”

“Having a BMI outside the ‘normal’ range is one of the risk factors for almost every huge, important disease you can possibly think of.”

Although the situation is, in part, due to lack of NHS funding, it does beg the question: Why such an arbitrary cut off point? Why can’t we take other factors into account? Anorexia and other eating disorders have the highest mortality rates among psychiatric conditions, and early intervention has been proven to drastically improve your chances of recovery.⁵

It can work at the other end of the spectrum too, as people who are classified as ‘overweight’ or ‘obese’ have been talking about trips to the doctor for something such as, for example, sinus pain, and found themselves being lectured on their weight. Indeed, our dependence on BMI is arguably more dangerous for overweight people: due to societal stigma, people immediately jump to the conclusion that an overweight person’s health problems must be related to their larger size.

Chloe, 21, has plenty of examples where doctors’ obsession with her weight has led to other aspects of her health being overlooked, including her mental health. In fact, it was a situation like that described above that led to her developing an eating disorder.

Having gone to the doctors aged around eight, she found herself being signed up for a genetic study “to find out why I was fat”. As an active and healthy child, she had always been overweight, and given her lifestyle, doctors were baffled. What was overlooked, however, was the psychological implications of carrying out such a study on a child of that age.

“The whole time I was healthy and normal, but they were telling my mum things like ‘she isn’t going to have normal brain function’.”

“They were looking specifically at the hunger gene, which is where children produce extra hormones that tell them constantly that they’re hungry,” Chloe says. “I was put through tests and it was found that I had neither this gene, nor a food addiction.” Despite finding nothing, they carried on testing her and then labeled her as having ‘wonky genes’ – genetic mutations.

But during this testing process she was put on starvation diets, tested for numerous other conditions, and taken out of school for a long time. “The whole time I was healthy and normal, but they were telling my mum things like ‘she isn’t going to have normal brain function’,” says Chloe. She developed an eating disorder after being continually told her body was wrong, even though she was very healthy.

And that’s not the only time she has been judged for her weight before she has even had a chance to discuss the problem she’d gone to the doctor to solve. Chloe has severe OCD, and at one point she went to discuss this with her GP. “She said, ‘Do you mind if we weigh you?’, but I didn’t know why it was relevant,” says Chloe. “I told her my body has always been like this, so she said ‘OK, let’s focus on getting your brain healthy, then we’ll work on your body’. That was completely unnecessary. I didn’t mention anything about body or food issues.”

Chloe isn’t the only one who has had such an experience. Charlotte, 29, says her doctor has always brought up her weight regardless of what her appointment is for. “Most poignantly was at university when I was told I was two pounds overweight,” she says, which is almost laughable. “I also got told the reason I get terrible migraines during my period is because I was too fat,” she adds. At this, she stopped going to the doctor, and didn’t even register with one for four years when she moved house to a new place.

“Most poignantly was at university, when I was told I was two pounds overweight.”

So is there a missing link between doctors and patients? Or is it a case that some doctors are just not as sensitive as they should or could be when discussing weight? Why the disproportionate amount of interest in fatness, when a smoker, for example, may not even have their smoking habit brought up in an appointment at all?

“Doctors are not allowed to have this sort of response to people being overweight,” says Dr. Jane Woyka, referring to examples of insensitive or unnecessary comments as detailed above. “Nobody wants to be fat. Most people who are overweight have issues which compound their susceptibility to being overweight and the vast majority have tried to lose weight.”

Nonetheless, BMI remains a hugely important part of medical reporting. “It’s the best method we’ve got, and there’s nothing else that comes close to helping us,” she says. “But you have to take it with a pinch of salt. It’s not a brilliant indicator, but it’s the only one used widely in a useful way.”

So how can doctors make use of BMI without compromising the health of people who come into their surgery? Dr. Woyka points out that it isn’t at all easy. “It’s a total minefield,” she says, “and as a doctor you will lose out every single time.” If a doctor doesn’t tell a smoker that they should stop smoking, for example, Dr. Woyka says that would be “negligent”, and yet if you tell them to stop in a “judgemental” way, that can also harm the patient. “We are on a hiding to nothing sometimes,” she says.

“BMI is the best method we’ve got, there’s nothing else that comes close to helping us. But you have to take it with a pinch of salt.”

For doctors, then, it is best to take a patient’s general health into account when analysing their symptoms, and hope a patient will prompt any weight-related discussion themselves, Dr. Woyka says. For example, if someone comes in with polycystic ovary syndrome (PCOS) and they are overweight, doctors can explain that PCOS is linked to metabolic disorders, and ask the patient what they know about it. “People have always done reading around their symptoms, and they’ll come out with things that they you want to work on with them,” Dr. Woyka adds. “You’re not saying ‘you are overweight’, you say ‘associated with this condition is a tendency to put on weight’.”

Giving too much importance to someone’s weight without assessing the symptoms they have come in for, along with their health in general, is dangerous. Conditions can be missed, and it perpetuates societal stigmas against people who are fat, which can have resounding mental health implications. The same applies with someone who is underweight.

While BMI can be a useful tool, the figures it produces should not be taken as gospel, as body shape and build vary wildly among the population. While it is not ideal to be over- or underweight, there needs to be a greater awareness of the wider social implications of not having the ‘perfect’ body – and that an appointment for, for example, an ear infection, may not be the best time to lecture someone about their weight.

Page last updated January 2019