Why BMI is not the best indicator of health
I have had so many questions from blog readers and social media followers over the past couple of months relating to BMI (Body Mass Index), body weight, and overall health. The idea of not focusing on body weight as a primary indicator of health and wellbeing, and ditching diet culture for good is an important first step for anyone who wants to work on bettering their relationship with food and their body, and learn to eat more intuitively. But here’s the thing, it can be really difficult to look past a number on the scale, a BMI value, or a pant size as a marker of your worth if you have been dieting for years or have had others (including healthcare professionals) point out that yours is ‘not ideal’.
Despite being used as a screening tool in healthcare, is BMI really a good indicator of someone’s health? Should it be used as a diagnostic tool? And should individuals be treated differently by healthcare providers depending on their BMI? Hopefully by the end of this post you will have more clarity as I answer these questions in some detail. I would love to hear from you if you have any questions about anything that I share here, agree or disagree with any of my points, or would just like to discuss the topic further, so feel free to pop a comment below or send me an email!
What is BMI?
BMI is a ratio of weight (kg) to height (m) that is used to categorise people in different groups based on their body size. It was first introduced in the early 19th century by a Belgian mathematician named Lambert Adolphe Jacques Quetelet, who produced a formula that provided a quick and easy way to measure the degree of ‘obesity’ or ‘fatness’ in the general population to assist the government in allocating resources and as a screening tool for populations . It is widely used today to place individuals people into different weight categories - underweight, normal weight, overweight, and obese - in order to determine risk factors for various chronic diseases and overall good health. Although it can be useful for tracking trends in population body weight and size, it has many limitations when applied to individuals .
Should BMI be used as a diagnostic tool?
It is important to note that BMI should NOT be used as a diagnostic tool (something that is used to identify or determine the nature and cause of a disease or injury), despite the fact that it is commonly used by healthcare providers for this exact purpose. Here’s why:
BMI doesn’t take into consideration muscle mass and composition, bone density, fat distribution, a person’s activity or fitness levels, genetic factors and more - it merely considers a person’s weight and height, and provides a very crude measure of body size. It also doesn’t take into consideration gender differences in body shape, body weight, and muscle and fat distribution.
Much more important than a number is the actual state of a person’s health - both physical and mental. BMI doesn’t take into consideration things like health choices and behaviours, which can affect overall health and wellbeing independent of a person’s weight.
Without the context of other measurements, such as blood pressure, blood lipids, inflammatory markers, thyroid hormones, glucose control, other blood markers, BMI means nothing, and if all of these other markers of health are fine, a person’s body size should not lead to assumptions being made otherwise.
BMI categories are relatively arbitrary. For example, if your BMI is 24.5 you are considered to be ‘healthy’ and will generally be treated as such, but if it moves up to 25.5 you are considered ‘unhealthy’ and will often be treated differently? Interestingly enough, I heard @fionawiller once mention in a podcast episode that BMI categories changed at one point overnight, which suddenly left previously ‘normal weight’ individuals ‘overweight’ without any change in their body size or health status (listen to the episode here, it’s a REALLY good one.
By focusing on BMI, healthcare practitioners can often neglect fully assessing a patient. When someone is in a larger body, often weight loss will be prescribed as a solution by pracitioners, even though the condition that the patient is seeking help for has nothing to do with weight. This means that practitioners might overlook health concerns that would otherwise be diagnosed and treated differently in patients with a smaller body. Likewise, practitioners may be more reluctant to properly screen ‘normal weight’ individuals for things like diabetes, thyroid conditions, and heart disease because they aren’t considered ‘at risk’.
At the end of the day, BMI does not tell you anything about how healthy or sick a person is, it merely tells you something about their size. You may have a higher BMI than someone else, but could have healthier blood markers, higher bone density and muscle tone, healthier sleep patterns, better controlled stress levels, and better mental health. Often, those in the ‘overweight’ and ‘obese’ categories will be prescribed weight loss when they consult healthcare professionals. But if dieting doesn’t work in the long-term, and yo-yo dieting is associated with negative health outcomes, should we really be encouraging people to lose weight? Is it ethical to focus on the number on the scale, using it as a diagnostic tool, rather than their overall health? Or is this just a bit lazy?
Should a person’s BMI affect their quality of care?
No, it shouldn’t, but unfortunately often it does. I conducted a very crude poll to find out how those of you who follow me on Instagram thought about this:
85% of those who answered agreed that some healthcare providers do treat people unfairly based on their weight/body size
53% have experienced poor care themselves as a result of their weight/body size (whether ‘larger’ or ‘smaller’
86% wish that they knew how to better advocate for themselves when visiting the doctor
So what do we know about weight bias in healthcare? A 2015 review by Phelan et al. (2015) found that many healthcare providers do indeed hold strong negative attitudes and stereotypes about individuals that fall into the ‘overweight’ and ‘obese’ BMI categories. We also know from research done in this area that these attitudes influence person-perceptions, judgment, interpersonal behaviour and decision-making as a healthcare professional, and can impact the quality of care that healthcare professionals provide . Those on the receiving end of such treatment are more likely to have expectations of poor treatment that can cause anxiety, stress, avoidance of care, and mistrust of healthcare providers . The goal of good healthcare should always be to improve patients' health, longevity and quality of life through the provision of patient-centred care, but unfortunately screening for ‘obesity’ using BMI as a primary indicator of health has become mandatory in many healthcare settings, and this has arguably placed an increased focus on body weight that has led to the stigmatization, devaluation, and humiliation of many patients.
For example, two people of different body weight and size may see a doctor for a knee injury exacerbated by physical activity. The lower weight individual may receive anti-inflammatories and a referral to see a physiotherapist, whilst someone at a much higher weight is more likely to receive a recommendation to lose weight. Another example could be one from the eating disorder field, where someone with binge eating disorder (BED) may be overlooked by a doctor because they are of a ‘normal’ weight, whilst in reality they may be experiencing trauma and anxiety because they are overlooked due to their body size. Similarly, an individual in a ‘larger’ body may not be screened for an eating disorder due to their body size, but may very well be suffering from an active eating disorder that requires just as much care and attention as one that someone in a smaller body is suffering from.
A study by Mulherin et al. (2013) documented the experiences of 627 Australian women who had been pregnant and had received pre-service maternity care. The researchers found that those with a higher BMI were more likely to have had negative experiences of care during pregnancy and after birth than women of a lower body weight. Of the 248 healthcare providers who took part in the study, most perceived ‘overweight’ and ’obese’ women as having poorer self-management behaviours and reported less positive attitudes towards caring for these women compared with their ‘normal’ weight counterparts.
For another great resource documenting how and why weight stigma is harmful to health, over and above BMI, check out this article by Tomiyama et al. (2018).
But what about ‘health’?
We are often told that ‘obesity-related’ conditions, such as heart disease, stroke, type 2 diabetes, some cancers, are the leading causes of preventable death, however much of the research backing up such statements are based on associations and correlations (a link), not causation (a direct cause), between weight/BMI and these conditions. Another thing to consider is the fact that many of the epidemiological studies backing up such statements don’t usually control for things like:
Fitness and activity
Body image 
Weight cycling (which is associated with inflammation, hypertension, insulin resistance, hyperlipidemia) 
When it comes to the association between BMI and death, there is actually a U- or J-shaped association between BMI and mortality, with a BMI of 25-35 exhibiting the lowest relative hazard of mortality after adjusting for age, gender, smoking, and alcohol consumption . What we do know is that healthy behaviours such as regular, joyful movement, sufficient sleep, eating more in line with innate hunger and fullness cues, eating an overall healthy balanced diet, and managing stress, have a positive impact on a person’s health independent of their BMI/body weight.
For a more in-depth lecture covering the science underlying the associations between weight and health, head over to this link
BMI is a ratio of weight to height that tells you something about a person's body size, not their health status
It is useful for tracking changes in population body weight and size, but has many limitations when applied to individuals because it doesn't take into account things like body composition, bone density, fat distribution, health habits, fitness levels, smoker status, mental health, socioeconomic determinants of health, and important biomarkers
Weight bias - negative weight-related attitudes, beliefs, assumptions, and judgements towards people based on their body weight/size - is prevalent in healthcare settings and often influences the care that doctors, nurses, dietitians, and other healthcare professionals provide
Those on the receiving end of weight stigma - prejudiced attitudes and discriminatory actions towards an individual based on their body size - are more likely to expect poor treatment from healthcare providers, which can result in anxiety, stress, avoidance of care, and mistrust of healthcare providers
Weight stigma is harmful to health (Tomiyama et al., 2018)
 Phelan SM, Burgess DJ, Yeazel MW, Hellerstedt WL, Griffin JM, van Ryn M. Impact of weight bias and stigma on quality of care and outcomes for patients with obesity. Obes Rev. 2015;16(4):319-26. Available from: https://journals.lww.com/nutritiontodayonline/Fulltext/2015/05000/Body_Mass_Index__Obesity,_BMI,_and_Health_A.5.aspx
 Nuttall FQ. Body Mass Index: Obesity, BMI, and Health A Critical Review. Nutrition Today. 2015;50(3):117-28. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4381543/
 Mulherin K, Miller YD, Barlow FK, Diedrichs PC, Thompson R. Weight stigma in maternity care: Women's experiences and care providers' attitudes. BMC Pregnancy and Childbirth. 2013;13(1):19. Available from: https://www.researchgate.net/publication/235003055_Weight_stigma_in_maternity_care_Women%27s_experiences_and_care_providers%27_attitudes
 Schafer MH, Ferraro KF. The Stigma of Obesity Does Perceived Weight Discrimination Affect Identity and Physical Health? Social Psychology Quarterly. 2011;74(1):76-97. Available from: https://www.researchgate.net/publication/258189753_The_Stigma_of_Obesity_Does_Perceived_Weight_Discrimination_Affect_Identity_and_Physical_Health
 Montani JP, Viecelli A, Prévot A, Dulloo AG. Weight cycling during growth and beyond as a risk factor for later cardiovascular diseases: The 'repeated overshoot' theory. Int J Obes. 2006;30(Suppl);S58-66. Available from: https://www.researchgate.net/publication/51374438_Weight_cycling_during_growth_and_beyond_as_a_risk_factor_for_later_cardiovascular_diseases_The_%27repeated_overshoot%27_theory
 Hotchkiss JW, Leyland AH. The relationship between body size and mortality in the linked Scottish Health Surveys: cross-sectional surveys with follow-up. Int J Obes. 2011;35(6):838-51. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3117149/