BMI Can’t Tell The Full Story Of A Person’s Health, So Why Are We Still Using It?

I don’t remember as much about being 18 as I probably should, given it was relatively recent. I have far more distinct memories of my early teenage years. This is probably because, in my later teens, I was very anorexic and doing whatever I could to ignore my constant gnawing hunger. That makes you forget a lot.

Everything blurs and morphs together when your body is in starvation mode, making it hard to verify when or exactly what happened. One memory that has remained, despite becoming somewhat unstuck from time, is the moment I tried to get help for my anorexia on the NHS. I remember being made to weigh myself and describe how I was eating was impacting my life. Finally admitting the way you’re abusing your body is a devastating experience and I remember sobbing, begging for help. Yet at 5ft 10 and the lowest I ever weighed as an adult, my BMI (body mass index) was not low enough. At the point I admitted I needed help with my obsession with my weight, I was told I weighed too much to be helped. Alanis Morissette would have had a field day.

I understand that a cut-off point feels necessary, especially when the National Health Service is straining under the weight of caring for our population with less and less funding. I appreciate all they do to keep the nation healthy, especially as the current crisis shows. But in that moment I was let down. I was fortunate enough to have the option of private healthcare, but I struggle to know where I would have been if I had the NHS alone.

This was 2011. I’ve learned since then that my situation was far from unusual but I’d hoped that things might have changed. But BMI is frequently still the primary factor in determining what healthcare you can access, even while experts call for updated ways of assessing health. In fact, for some medical issues, it’s become even more common, which particularly affects people with eating disorders, gynaecological or "women’s issues", and trans people. The irony is that the body mass index was never designed to measure an individual’s health and by interpreting it that way, you can actually endanger people’s health much more.

The body mass index was invented by Adolphe Quetelet nearly 200 years ago and has been called out by anonymous writer Your Fat Friend for having a "bizarre and racist history". It was created as a means to quantify the characteristics of what Quetelet called l'homme moyen (the average man) whom, to him, represented the social ideal. He developed an equation of height (m)2/weight (kg) from a position as a statistician, not a physician. This means, crucially, that Quetelet's equation was designed to assess population health trends, not individual health. It does not take into account muscle mass, age, sex, fitness, race or any other metric, because it was not designed to do so. Moreover, the participants on which the formula was based were largely Scottish and French which, in the 19th century, meant a largely white and Anglo-Saxon population. As a formula, it was never designed to take racial differences into account. 

The Quetelet Index (as it was initially called) lay dormant for many years, with various other methods being trialled as ways to measure health. That was until it was redefined as the body mass index in 1972, in a paper by Ancel Keys and others. BMI (as it was now known) was said to be "if not fully satisfactory, at least as good as any other relative weight index as an indicator of relative obesity" in comparison to measurement of fat by skin callipers and underwater weighing (body density). Keys ran this analysis on data obtained from Anglo-Saxon populations, leading many to question "the generalizibility [sic] and applicability of the BMI and its cut-off points to other populations" as well as "its sensitivity as a measure of excess fat". The study notes that the findings corroborate BMI as a correlation between body weight and height for all participants "except the small group of Bantu men" (an umbrella term to refer to speakers of the Ntu languages who largely hail from sub-Saharan Africa) but didn’t see that as reason enough to discount it. And so despite these problems, and increasing evidence that there is no one way to be (or look) healthy – and that there are in fact several different ‘types’ of obesity – BMI continues to be the easiest, and therefore most common, way that health is measured in individuals. As Sylvia R. Karasu, MD writes in Psychology Today: "Despite all the progress we have made in science since Quetelet’s 19th century index, we are still far from being able to measure our body’s fat conveniently and accurately in a physician’s office."

Using BMI as a measure of health, according to Katherine Kimber, a registered dietitian who spoke to me on behalf of the British Dietetic Association, is "incorrect and naive". She goes on: "Being of higher weight doesn’t mean someone isn’t healthy and being smaller bodied doesn’t mean the person is healthy… [It] can result in misdiagnosis of the many thin people who get so-called 'obesity associated' disease. It can also result in the perpetuation of stereotypes, the stigmatisation of larger bodies, worsened healthcare quality, and ultimately worsened patient health." This is not just theoretical – BMI overestimates fatness and health risks for Black people, and underestimates health risks in Asian communities. This is not noted on the NHS website – only that "Black, Asian and other minority ethnic groups have a higher risk of developing some long-term (chronic) conditions, such as type 2 diabetes" – implying that you should actually be more mindful of your BMI if you are not white. Then there is the fact that so much of the research around BMI focuses on people assigned male at birth (AMAB), not taking into account the ways that percentage of fat differs for people assigned female at birth (AFAB).

If you are categorised as 'overweight' or 'obese' according to BMI, that then becomes how your health (or supposed unhealthiness) is perceived, irrespective of how healthy you are. Likewise, if you are within the 'healthy' bounds of 20-25 on the BMI scale, you are 'healthy' even if you know you haven’t eaten a full meal in days. When it comes to treatment for eating disorders this can have a direct impact on someone’s route to recovery, as the director of external affairs for Beat, Tom Quinn, tells me. "There is a growing bank of evidence that weight, or BMI, should not dictate how ill someone is deemed to be. You cannot tell whether someone is unwell simply by looking at them, and it is crucial to break the stigma that only very underweight people can have an eating disorder. Healthcare professionals sometimes fail to spot eating disorders and this is often exacerbated in patients who are not underweight – this is particularly worrying as this can lead to potentially dangerous delays in them accessing treatment."

Being under a certain BMI (whether that is 'unhealthy enough' at 18 or 'healthy enough' at below 25) means that weight loss, by any means necessary, becomes the only way you can access some forms of healthcare, without taking into account the impact such a delay could have on your health (both physical and mental). Over recent years there have been several NHS Clinical Commissioning Groups (CCGs) which have changed their policies around access to surgeries based on BMI: either delaying surgeries if a person's BMI is over 30 or blocking access entirely until they lose weight. According to Freedom of Information requests made by The GP in 2015, "83% of CCGs deny patients treatment for infertility, joint replacements and aesthetic surgery if their BMI is considered too high". These blocks are in some places extended to anything considered ‘non-emergency’. If you have a BMI over 30 there is apparently a higher risk of surgical and anaesthetic complications, according to Oxford University Hospitals, but the Royal College of Surgeons (RCS), British Dietetic Association (BDA) and the Association for the Study of Obesity (ASO) all made statements rejecting these policies when they happen. ASO said: "To suggest that individuals with obesity need to lose weight to be considered for surgery goes against the existing scientific evidence base and the founding principles of the NHS." 

This was the case for Poppy (22, they/them) in Norwich, who was denied a laparoscopy, an operation that was the only way they could formally diagnose their endometriosis. After waiting about four months for an appointment with a gynaecologist at their local hospital, they tell me that "the first thing that happened when I walked in was my BMI being taken. After some pretty standard discussion with my gyno who wasn't, shall we say, kind, and a surprise internal examination (not ideal for a sexual trauma survivor!) she essentially told me that what I have is just standard period pains and that she won't refer me for a laparoscopy because my BMI is too high – read: I was too fat. She told me to go on the pill for three months and come back when I had lost 20 pounds."

For Dani (29, she/her) in Edinburgh, it went a step further. She was seeking a referral for breast reduction surgery, an elective procedure to combat her back and shoulder pain: "I have back and shoulder pain, and I wear a bra all the time because if I don't, it hurts. When I exercise I wear two sports bras." She spoke to her GP, who told her she needed to reduce her BMI (without even weighing her) before then weighing her and working out her BMI was 29. "She asked me to list all the things I ate the day before, then when she heard it was healthy, told me to go on the 5:2 diet and that 'sugar is worse for you than smoking'. She also said 'the lower people's BMI, the better they recover from surgery', which I know can't be true!" (This depends on the nature of the surgery.) They talked so much about weight loss that Dani left the appointment unclear if the referral had been made (after calling the surgery she found out it had), and went away and lost weight as she was asked, despite having no desire to lose weight otherwise. "It almost felt coercive – like if I lose the weight then I can have the surgery I need, even if I'm already fit and healthy and not looking to go on a diet out of choice." After losing the weight and waiting 18 weeks, Dani heard from the hospital: "I received a two-line letter saying I 'do not need to be sent an appointment at this time' and if I disagree I need to get a new referral from my GP. They didn't give a reason, and the letter was unsigned/had no contact information."

It is probably not a coincidence that both these procedures would have gone some way to alleviate pain that so many women and non-binary AFAB people experience – the same pain that we know is not taken seriously. It can also apply to trans people who are trying to access forms of medical transition. A high BMI in this case can be interpreted as an easy way to deliberately gloss over that pain and see it as "normal" in Poppy’s case or even, in Dani’s case, something to be envied: "At one point I started crying, and she told me that a lot of women would be envious of the position I'm in (having big breasts, I assume, not crying in my GP surgery on a Monday lunchtime)."

As Katherine points out, there is an immense health risk in making surgeries contingent on weight loss. "There is limited evidence to support the long-term benefits of weight loss. In fact, there is overwhelming evidence that shows any form of intentional weight loss has no long-term success. Regardless of the degree of initial weight loss seen with lifestyle intervention, most weight is regained within a two-year period and by five years the majority of people are at their pre-intervention weight. As such, it is unethical to recommend weight loss to allow patients access to treatment."

In the cases of the people I spoke to, the psychological impact of suggesting or even prescribing weight loss was also not considered. Poppy wasn’t asked at all if they have a history of disordered eating (they do, but are recovered) and Dani was only asked at the end of her appointment "after more than 10 minutes of telling me how to lose weight ('keep a single square of dark chocolate in the fridge at work!')".

Katherine tells me that "healthcare providers actually spend less time with larger bodied patients, because they stereotype those individuals as being non-compliant or lazy." As for both Dani and Poppy, "these interactions are noticeable to the patient, worsen their healthcare experience, keep a proper diagnosis from being made, and often result in the withholding of appropriate treatment – like surgery." This stigma perpetuates the very problems that doctors are trying to correct through dieting. 

As Katherine notes: "Waging a war on fatness will not improve health. Rather, focusing on positive health behaviours and taking the microscope off of body weight and size is a positive alternative approach." That looks largely impossible unless the NHS gets the right amount of funding and there is an overhaul in how patients are treated.

So what's the alternative? The National Institute for Health and Care Excellence is meant to be reviewing the use of BMI, though it's unclear when that will be concluded. Margaret Ashwell has posited that waist-to-height ratio, as opposed to BMI, is a better indicator of early health risks. But perhaps instead of replacing BMI, there should be a radical overhaul of the idea that health can be quantified by any single number. In reducing people to values on a sliding scale, the wellbeing of the individual falls off the agenda, meaning everyone's health is at stake.