This article was written by Associate Registered Nutritionist (ANut) and Content Creator at Dietetically Speaking Sophie Gastman (reviewed by Registered Dietitian Maeve Hanan).
What is BMI?
BMI, or body mass index, is a commonly used measure of a person’s weight in relation to their height, but it often isn’t a good marker of individual health as we will explain in this article.
It’s calculated by dividing an adult’s weight in kilograms by their height in meters squared. For reference, the current BMI categories for both men and women are:
- Below 18.5 – underweight
- Between 18.5-24.9 – healthy weight
- Between 25-29.9 – ‘overweight’
- 30 and above – ‘obese’
A Brief History of BMI
BMI has been used in the UK as a standard method of measuring and defining obesity since the 1990s. But where did this come from?
The weight divided by height squared formula was developed almost 200 years ago in the early 19th century, by a Belgian statistician, Adolphe Quetelet.
However, Quetelet’s goal was to measure average weight at a population level, not individually, and it certainly wasn’t meant to determine an individual’s health status (1).
In 1972, epidemiologist Ancel Keys came along, suggesting that this formula was the most useful indices to measure body shape and renamed it BMI, despite only concluding it ‘slightly better’ at estimating body fat in his research, even though it was only accurate 50% of the time (2).
In fact, even the cut-off points for the ‘overweight’ category were decided arbitrarily with the panelists from the National Institute of Health (NIH) in the US agreeing that ‘an increase in body weight of 20 percent or more above desirable body weight constitutes an established health hazard.’ (3).
The NIH also changed the cut off for the ‘overweight’ category in 1998 from 27.8 to 25, so overnight millions of Americans in the ‘healthy’ category were now classified as overweight.
Despite all of the drawbacks of BMI, it continues to be used today by many researchers and health institutions as a convenient and cost-effective way of classifying body size.
Why Do Health Professionals Still Use It?
You may be wondering why we still use a measure that doesn’t seem accurate or based on much solid scientific evidence? You may even be surprised to know that a more accurate way to measure fat-mass exists.
A DEXA scan (Dual-energy X-ray absorptiometry) is regarded as the ‘gold-standard’ method for measuring fat-mass. It uses X-rays to detect the difference between muscle and bone and fat, so can be used to calculate fat-percentage. However, this process requires specialist equipment, trained technicians and some exposure to radiation and to run, so is far too expensive to use across populations in comparison to BMI, which is just a simple calculation that can be used by anyone.
Other ways of measuring body composition include measuring skin-fold thickness, body density or using a mild electric current (called bioimpedance) or measuring specific parts of the body like the waist, hips and upper arm (4). All of these methods have drawbacks when it comes to accuracy and applicability in clinical settings.
In addition to being a simple measure, BMI categories are widely used in determining public health policies and are often used to set criteria for accessing services. They are also used as indicators for treatment pathways or further investigation, for example, low BMI without a medical cause can be an indicator for an eating disorder (however, it should never be used as the sole indicator as there is more to an eating disorder than body size and people of all sizes develop eating disorders).
BMI does provide a snapshot of the overall picture in some cases. For example, because it is used so widely by health professionals and governmental agencies, we can gain some insight into the epidemiology of body size across the world. Its wide usage also means the public are able to easily interpret research using BMI (5).
Health professionals are also trained to check and use BMI and often don’t have a choice, especially in public settings, where there tends to be policies and standard procedures that include checking weight and BMI.
BMI Isn’t Accurate on an Individual Level
As mentioned earlier, the BMI calculation was never intended to be used on an individual level, simply because there are many factors that contribute to health that BMI does not take into account.
The basic calculation cannot differentiate between muscle and fat.
The strikingly obvious problem with this is that men and women have varying body compositions, with men tending to have more muscle than women, yet the BMI calculation is the same for both genders.
The other issue with BMI not accounting for muscle mass is that muscle is far more dense than fat, putting a lot of athletes and bodybuilders into the ‘overweight’ category, despite being at peak athletic health. It also doesn’t scale well in terms of height, as a taller person with the same body composition as a shorter person will have a higher BMI.
When it comes to different ethnicities, there are huge gaps in the research around BMI.
Ancel Keys reserach back in 1972 mainly focused on White, male American and European populations (with the data drawn from African communities being discounted), which clearly does not give the full picture or account for sex and race differences.
For example, a review into the evidence for BMI and waist circumference thresholds in the UK by NICE found limited evidence to suggest the ‘overweight’ cut-off point was lower for Chinese and South Asian populations than the European populations, but noted significant gaps in the evidence were found and concluded that the evidence was ‘insufficient to justify the development of new BMI or waist circumference thresholds for black, Asian, or any other ethnic minority groups in England’ (6). This suggests that much more research is required as other studies have demonstrated the relationship between BMI and body fat percentage differs among ethnic groups due to the influences of the population’s ethnic and environmental characteristics (7).
BMI Doesn’t Equal Health
Although BMI can give a rough snapshot of body size, our BMI does not tell the whole story about our health.
Using a measure that was never intended to determine individual health is unreliable and leads to bad healthcare.
Weight stigma is a huge problem in the healthcare system and research has demonstrated those with higher BMIs often find primary healthcare providers only focus on their weight and medical issues are incorrectly seen as weight-related problems (8).
In fact, half of ‘overweight’ people and almost a third of people labelled ‘obese’ in the US are cardiometabolically healthy but are likely to have their healthcare provider prescribe weight loss above anything else (9). Some research has also shown that having a higher BMI is associated with lower mortality risk (10).
On the flipside, people with a ‘healthy’ BMI can still be unhealthy, but won’t experience the same stigma purely because they are in a smaller body. For example, almost a third of people with a ‘healthy’ BMI are actually unhealthy and using BMI to assume health can lead to people being overlooked (11).
All of this research highlights that there is much more to health than just weight or size. Health and wellbeing, including fitness, mental health and blood glucose control can improve with more movement and stress management, even if a person’s weight stays the same (12).
Why Language Matters
We were all taught as children to ‘never judge a book by its cover’ but unfortunately, this doesn’t seem to extend towards people with obesity’ across the general population or within the NHS. As with anything, the language we use to describe people has a massive impact. In the context of different body shapes and sizes, this leads to stigma and discrimination which can prevent people from seeking health care or from carrying out their normal everyday lives.
The panel at the Annual International Weight Stigma Conference put it perfectly “Part of the problem is that the very act of labeling is a process of othering, one that creates a distinction between us and them… within the medical setting, the main reason to create a separate category for larger bodies is because they are to be treated differently than slimmer patients.” (13).
Research has demonstrated just how stigmatising that the label of ‘overweight’ can be. A double-blind experiment randomly informing participants that they were ‘normal weight’ or ‘overweight’ found normal-weight participants told they were “overweight” reported greater body dissatisfaction and overweight participants informed that they were ‘‘overweight’’ rated themselves as heavier than overweight participants informed that they were ‘‘normal weight’’ (14).
Finally, assigning the label of ‘normal’ or ‘healthy’ to a BMI category that was initially randomly decided seems questionable as you cannot define what is ‘normal’ or ‘healthy’ based entirely on BMI.
On top of this, the majority of the UK and worldwide population fall outside this ‘normal’ category, with 64.2% of adults in England classified as ‘overweight’ or ‘obese’ (15). So really, how normal is it?
Using BMI as a universal indicator of health clearly comes with a lot of issues.
It fails to consider many aspects of health, like age, gender, muscle mass, ethnicity, specific health markers etc.
Plus, the current way BMI is used as a sole predictor of health by some health care providers is incredibly stigmatising and leads to further health inequalities.
Whilst it may be a useful starting point in some cases, BMI isn’t a helpful metric for many people to focus on.
- Fletcher, I., 2013. Defining an epidemic: the body mass index in British and US obesity research 1960-2000. Sociology of Health & Illness, 36(3), pp.338-353.
- Keys, A., Fidanza, F., Karvonen, M., Kimura, N. and Taylor, H., 1972. Indices of relative weight and obesity. Journal of Chronic Diseases, 25(6-7), pp.329-343.
- Health Implications of Obesity. NIH Consens Statement Online 1985 Feb 11-13 [Accessed 21st June 2022]; 5(9):1-7.
- Duren, D., Sherwood, R., Czerwinski, S., Lee, M., Choh, A., Siervogel, R. and Chumlea, W., 2008. Body Composition Methods: Comparisons and Interpretation. Journal of Diabetes Science and Technology, 2(6), pp.1139-1146.
- Gutin, I., 2017. In BMI we trust: reframing the body mass index as a measure of health. Social Theory & Health, 16(3), pp.256-271.
- National Institute for Health Care Excellence (2013). BMI: preventing ill health and premature death in black, Asian and other minority ethnic groups. (NICE guideline PH46).
- Wollner, M., Paulo Roberto, B., Alysson Roncally, S., Jurandir, N. and Edil, L., 2017. Accuracy of the Who’s Body Mass Index Cut-Off Points to Measure Gender- and Age-Specific Obesity in Middle-Aged Adults Living in the City of Rio De Janeiro, Brazil. Journal of Public Health Research, 6(2), pp.jphr.2017.904.
- Persky, S. and Eccleston, C., 2010. Medical student bias and care recommendations for an obese versus non-obese virtual patient. International Journal of Obesity, 35(5), pp.728-735.
- Tomiyama, A., Hunger, J., Nguyen-Cuu, J. and Wells, C., 2016. Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012. International Journal of Obesity, 40(5), pp.883-886.
- Flegal, K., Kit, B., Orpana, H. and Graubard, B., 2013. Association of All-Cause Mortality With Overweight and Obesity Using Standard Body Mass Index Categories. JAMA, 309(1), p.71.
- Gujral, U., Vittinghoff, E., Mongraw-Chaffin, M., Vaidya, D., Kandula, N., Allison, M., Carr, J., Liu, K., Narayan, K. and Kanaya, A., 2017. Cardiometabolic Abnormalities Among Normal-Weight Persons From Five Racial/Ethnic Groups in the United States. Annals of Internal Medicine, 166(9), p.628.
- Lantz, P., Golberstein, E., House, J. and Morenoff, J., 2010. Socioeconomic and behavioral risk factors for mortality in a national 19-year prospective study of U.S. adults. Social Science & Medicine, 70(10), pp.1558-1566.
- Himmelstein, M., Puhl, R. and Quinn, D., 2018. Weight Stigma in Men: What, When, and by Whom?. Obesity, 26(6), pp.968-976.
- Essayli, J., Murakami, J., Wilson, R. and Latner, J., 2016. The Impact of Weight Labels on Body Image, Internalized Weight Stigma, Affect, Perceived Health, and Intended Weight Loss Behaviors in Normal-Weight and Overweight College Women. American Journal of Health Promotion, 31(6), pp.484-490.
- Commons Library Research Briefing (2022). Obesity Statistics. Number 03336 [accessed June 2022 via: https://researchbriefings.files.parliament.uk/documents/SN03336/SN03336.pdf]