Imagine this:
You have caught a cold that has been going around. After trying to fight it on your own for a week, you finally manage to get in to see your doctor for something stronger than Panadol. Within minutes of entering their office you have been weighed, measured, and had your blood pressure taken. After pressing some keys on their computer, they turn back to you and deliver the news: “you need to lose weight; your BMI is too high”. Nothing about the reason for your visit; instead you leave feeling worse. You now have a cold and a “health issue”.
If you related to this situation, I am sorry. Truly, I am. I am sorry someone thought that BMI was a valid way of assessing your health. But it isn’t. The BMI is flawed. As such, that number should mean as little to you as it does to me.
Let me explain why.
What is BMI?
BMI or Body Mass Index has been used for decades to assess weight status in adults. It is calculated by dividing your weight (in kilograms) by your height squared (in metres). The result is then used to classify you as healthy or unhealthy using the following ranges:
- a BMI of <18.5 is considered “underweight”
- a BMI of 18.5-24.99 is considered “normal”
- a BMI of 25-29.99 is considered “overweight”
- and a BMI of >30 is considered “obese”.
At a population level, BMI can be a great tool for measuring trends in weight and identification of those who may be at an increased risk of disease. However, at an individual level, the BMI is flawed.
Why is the BMI Flawed?
There are two people before you: Jack and John. Jack lives a sedentary life as an accountant and does no exercise. John is a professional athlete. Both are 1.8m tall, with Jack weighing 92kg and John 100kg.
So, who is healthier? Jack or John?
According to the BMI, if you picked John, you are wrong. The system classifies John, an athlete, as obese, while Jack is only overweight. How can this be?
The BMI does not recognise or consider anything other than height and total body weight. It does not look at fat mass or fat distribution, muscle mass or bone mass. And it certainly doesn’t look at cardiovascular fitness or strength.
The same report that encouraged universal acceptance of the reference ranges above, also acknowledged that there is wide variation in body form and BMI does not correspond to fatness in populations due to body proportions. This finding was further explored by Professor Nick Trefethen, who said
“as a consequence of this ill-founded definition, millions of short people think they are thinner than they are, and millions of tall people think they are fatter”.
So how did this flawed system come to be?
BMI Backstory
In the 1830s, Quetelet, an astrologer, mathematician, statistician, and sociologist, set out to find “average”. It was his desire to obtain a bell-shaped curve or normal distribution to find the “average man”. In 1835, he found a simple formula that noted a relationship between body mass and height. The formula was kept simple as it was a time without computers, calculators, and electronic devices. However, even he noted at the time, the formula should be used for population studies, not on individuals, due to its simplicity.
In 1972, despite advances in technology, Dr Ancel Keys used the same formula and called it the ‘Body Mass Index’. His study found correlation between body-fat density, fat thickness (under-the-skin fat), and the relationship between body weight and height.
In 1985, the National Institutes of Health started using BMI to define obesity in the United States. Initially, the cutoffs were more liberal, with a “normal” or “healthy weight range” of 20-27. However, by 1998 the universal categories, as we know them today, were implemented upon recommendation by the World Health Organization, as it would be easier for research groups to compare populations – not because it was a more accurate way to measure health.
BMI Distribution in A Normal Population
Usually, with a bell-shaped curve (or normal distribution), most of the population sits in the middle. However, even from studies published as early as 1923, the distribution was found to be above the middle, with an average BMI of 25! This trend has continued, with 42.4% of American adults classified as obese today.
Australian statistics are very similar.
67% of Australian adults were overweight or obese in 2017-2018. That is, more people sat outside the “healthy weight range” than within it. Today, one-in-three people are classified as obese.
But with more people classed as obese, do we really know how healthy they are? In short, no.
Measuring health with BMI
While studies have found a correlation between disease risk and body weight, the same correlation has been found between disease risk and age. The cutoffs were designed using a mostly white European group of men. However, we all know that gender, race, and ethnicity changes how our bodies look. People come in all shapes and sizes. Ignoring this fact does no-one any favours. As such, why should an Asian woman be compared to a Polynesian man?
Furthermore, we know health is multifaceted. A 2016 study found that the BMI misclassified the metabolic health of individuals. When looking at blood pressure, triglycerides, cholesterol, glucose, and insulin resistance, 29% of “obese” individuals were found to be metabolically healthy, while 30% of “normal” weighted individuals were found to be metabolically unhealthy.
What Does All of This Mean for you?
It means that the results of a formula do not define you.
The only thing it tells you is that you perhaps need a new healthcare team, if they are pushing you to change based on an arbitrary number.
To truly understand your health status, a combination of factors needs to be measured. Measure your blood pressure, cholesterol, blood sugars, insulin resistance, exercise patterns, sleep quality, sleep duration, diet, stress levels and overall happiness. Only then you can truly assess how healthy you are.
When it comes to BMI, however, it is no more relevant to your health status than your age or hair length. In fact, it is less useful – to me anyway.
If you are looking for individualised support with your health, our team are here to help. We strive to provide a safe space to discuss all things nutrition to help you achieve your goals.
References
Brazier, Y., & Lal, M. (2021). Body mass index (BMI): Is the formula flawed? Medical News Today. https://www.medicalnewstoday.com/articles/255712
Deurenberg-Yap, M., Schmidt, G., Van Staveren, W. A., & Deurenberg, P. (2000). The paradox of low body mass index and high body fat percentage among Chinese, Malays and Indians in Singapore. International Journal of Obesity 2000 24:8, 24(8), 1011–1017. https://doi.org/10.1038/sj.ijo.0801353
Everyday Health. (n.d.). BMI Flaws, History, and Other Ways to Measure Body Weight and Fat. Everyday Health. Retrieved October 6, 2022, from https://www.everydayhealth.com/diet-nutrition/bmi/bmi-flaws-history-other-ways-measure-body-weight/
Flegal, K. M., & Graubard, B. I. (2009). Estimates of excess deaths associated with body mass index and other anthropometric variables. American Journal of Clinical Nutrition, 89(4), 1213–1219. https://doi.org/10.3945/AJCN.2008.26698
Gonzalez, M. C., Correia, M. I. T. D., & Heymsfield, S. B. (2017). A requiem for BMI in the clinical setting. Current Opinion in Clinical Nutrition and Metabolic Care, 20(5), 314–321. https://doi.org/10.1097/MCO.0000000000000395
James, P. T., Leach, R., Kalamara, E., & Shayeghi, M. (2001). The Worldwide Obesity Epidemic. Obesity Research, 9(S11), 228S-233S. https://doi.org/10.1038/OBY.2001.123
Michaëlsson, K., Baron, J. A., Byberg, L., Höijer, J., Larsson, S. C., Svennblad, B., Melhus, H., Wolk, A., & Lemming, E. W. (2020). Combined associations of body mass index and adherence to a Mediterranean-like diet with all-cause and cardiovascular mortality: A cohort study. PLoS Medicine, 17(9). https://doi.org/10.1371/JOURNAL.PMED.1003331
Nevill, A. M., & Holder, R. L. (1995). Body mass index: a measure of fatness or leanness? British Journal of Nutrition, 13, 507–516. https://doi.org/10.1079/BJN19950055
Norgan, N. G. (1994). Population differences in body composition in relation to the body mass index. European Journal of Clinical Nutrition, 48 Suppl 3(SUPPL. 3), S10-25; discussion S26. https://europepmc.org/article/med/7843146
Nuttall, F. Q. (2015). Body Mass Index: Obesity, BMI, and Health: A Critical Review. Nutrition Today, 50(3), 117. https://doi.org/10.1097/NT.0000000000000092
Popkin, B. M., Du, S., Green, W. D., Beck, M. A., Algaith, T., Herbst, C. H., Alsukait, R. F., Alluhidan, M., Alazemi, N., & Shekar, M. (2020). Individuals with obesity and COVID-19: A global perspective on the epidemiology and biological relationships. Obesity Reviews, 21(11). https://doi.org/10.1111/OBR.13128
Shah, N. R., & Braverman, E. R. (2012). Measuring adiposity in patients: The utility of body mass index (BMI), percent body fat, and leptin. PLoS ONE, 7(4). https://doi.org/10.1371/JOURNAL.PONE.0033308
Swainson, M. G., Batterham, A. M., Tsakirides, C., Rutherford, Z. H., & Hind, K. (2017). Prediction of whole-body fat percentage and visceral adipose tissue mass from five anthropometric variables. PLoS ONE, 12(5). https://doi.org/10.1371/JOURNAL.PONE.0177175
Tomiyama, A. J., Hunger, J. M., Nguyen-Cuu, J., & Wells, C. (2016). Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012. International Journal of Obesity 2016 40:5, 40(5), 883–886. https://doi.org/10.1038/ijo.2016.17
Trefethen, N. (2013, January 5). On obesity, gun control, Syria, bankers, marriage. The Economist. https://www.economist.com/letters/2013/01/05/on-obesity-gun-control-syria-bankers-marriage
World Health Organisation. (2010). A healthy lifestyle – WHO recommendations. World Health Organisation. https://www.who.int/europe/news-room/fact-sheets/item/a-healthy-lifestyle—who-recommendations
World Health Organisation. (2000). Obesity: Preventing and Managing the Global Epidemic. World Health Organisation. https://books.google.com.au/books?id=AvnqOsqv9doC
