The Why of BMI

June 2, 2023

Tad Taggart

BMI or Body Mass Index has long been the go-to metric used to determine whether an individual falls within a "healthy" weight range. But after decades of using it a standard practice in the health and fitness world, there is now a growing understanding that BMI is actually fraught with error and inconsistency and is rarely a great measure of health. (If you didn’t already know this, read on to find out why!) If so many of us, including doctors, acknowledge that BMI is so flawed, why is it still around? Let's dive into some of the complexities of evaluating health beyond a simple number and discover why BMI is not as helpful as previously thought.

Understanding BMI's history

To understand BMI, we have to go start at its roots. Our first stop takes us back in time to Belgium in the 1830s and involves a man by the name of Lambert Adolphe Jacques Quetelet. Lambert was a statistician, astronomer, sociologist, and mathematician. In short, this was a man interested in numbers and one of Lambert's obsessions was pondering “l’homme moyen” (or “the average man”). Simply put, he was curious about what the average size of a man was. To collect this data, he collected the heights and weights of various individuals in his area, primarily white European men. To simplify the litany of data, and based on mathematical patterns he identified, he chose to set weight (in kilograms) divided by height (in meters) squared. This number became known as the “Quetelet Index”, but today we know this as BMI (or Body Mass Index). BMI today is used to classify one’s body size (e.g. “normal weight”, “overweight”, “obese”, etc.) and is used in many doctors’ offices to determine one’s “weight-associated health risks”. Despite Lambert Quetelet’s extensive resume, “healthcare professional” is not a title or role he ever held and  the Quetelet Index was never created with the intent of defining one’s health.  So how did BMI become so central to our healthcare practice?

Fast-forward just over one hundred years, when a physiologist and researcher by the name of Ancel Keys conducted research and identified that individuals at higher BMI ranges tended to have higher incidences of chronic health issues. In other words, people in higher BMI brackets tended to have heart disease, diabetes, and certain cancers more often than those in lower BMI brackets. With this discovery, BMI became a focal point of research to identify health trends.

Later, in the late 90s, the World Health Organization (WHO), with the help of the International Obesity Task Force (IOTF), sought to set standard ranges of BMI to classify health risks. Despite research to this point describing health risks increasing above a BMI of 40 kg/m2, the WHO and IOTF set “overweight” starting at a BMI of 25 kg/m2 and “obesity” starting at 30 kg/m2. These ranges of “high risk” being set so far below the research-backed risk threshold is a head-scratcher…until you know that two major funders of the IOTF were pharmaceutical companies with the only weight-loss drugs on the market, Abbott and Roche. If being at a higher weight is suddenly pathological, prescribing medication to lose weight is suddenly much easier.

The BMI scam

What does this mean? It means that BMI was never a helpful standard for identifying health risks and was pushed as a healthcare tool by pharmaceutical companies who wanted to make more money. BMI does not account for or adjust for differences between genders, ethnicities, body composition, or lifestyle factors. In addition, BMI’s very roots are not founded on defining health. Further, the application of BMI to health is based on bias and flawed science.

But what about the trend mentioned earlier? What about people with higher BMIs experiencing more chronic illnesses?

It is indisputable that observational research has shown being at a higher weight/BMI is ‘associated with’ higher risk/occurrences of various chronic diseases. On the flip side, various observational research has also concluded that weight loss reduces this risk. But to conclude that the weight increase or decrease is causing these changes in risk is where many are making a fundamental error. In the research world, we like to use the phrase “correlation does not equal causation”. It is neither the high weight nor low weight deciding the health risk, but the health behaviors and lifestyle factors that can influence one’s weight. In other words, your weight doesn’t decide how healthy you are–your lifestyle does.

Shark attacks and ice cream sales

To illustrate this point, I want to give a parallel example. Consider shark attacks and ice cream sales. If one were to track both throughout the year, one might find that both shark attack rates and ice cream sales are relatively low in the early months of January, February, and March, with a steady climb through April, May, and June, then peak in July and August. One would then likely find that both steadily decline throughout September, October, November, before returning to a low rate in December. With these two rates being seemingly correlated, does that mean that buying ice cream causes shark attacks? Or does that mean that there are possibly other factors at play? Perhaps the time of year and temperature influence peoples’ desire to purchase ice cream and desire to go swimming in the ocean. Relating this back to health and weight, perhaps it is possible that it is not weight gain that makes someone "less healthy" nor weight loss that makes someone "more healthy" but the lifestyle changes that accompany each?

To offer some real-world facts that offer even more perspective on the huge amount of nuance that BMI fails to consider:

  • A large study published by The Journal of the American Medical Association found that higher BMIs tend to suggest better health for black people and don't necessarily show a significant rise in mortality risk until a BMI of 37
  • A 2020 study published in The New Zealand Medical Journal found that potentially harmful fat surrounding the height of white New Zealanders correlated with a higher BMI, the same was not true for those of Maoris descent
  • It has been well-documented that there are significantly higher incidences of metabolic disorders at a lower BMI for people of South Asian descent compared to other races and ethnicities.
  • One of the greatest predictors of health is actually socioeconomic level. Having access to health care, adequate medical treatment, healthy foods, and opportunities for exercise play a huge role in health—much more so than simply our weight.

As you can see, BMI fails to consider the context in which health and wellness exist across race, gender, culture, location, genetic predisposition, and socioeconomic status.

Ditch the BMI and focus on lifestyle

Long story short, your BMI can’t tell you if you’re healthy or not.

Don’t get distracted by the numbers and weight classifications measured by centuries-old systems that were never meant to measure health in the first place. Don’t get discouraged by what direction the scale moves or what you’re “at risk for” based on your BMI level. If you are taking action to change your lifestyle to incorporate healthier behaviors, rest assured that you’re doing something good for yourself. Regardless of what direction the scale moves.

If you find that this leaves you curious as to what your “healthy weight” is without BMI being a reliable measure, please consider checking in for my next Wellness Corner blog, where I will continue educating on health, weight, nutrition, and specifically the controversial “Health at Every Size®” (HAES®) approach to health and weight.

If you're itching to learn more right away, let's connect! Learn more about our nutrition services and how you can talk with me about your health and wellness goals by visiting our Nutrition Services webpage or emailing me at

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