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Why BMI is Flawed

This post was adapted from episode 11 of the Nourished & Free podcast.

Click here to listen to the full episode.

A black and white scale is pictured against a white wall, representing one of the reasons why BMI is flawed

Body Mass Index (BMI) is a tool often used in medical settings to determine one’s overall weight status. But while it may be used by many medical professionals, BMI is flawed.


If you’ve ever been to the doctor, you’ve likely been told by a health professional what your BMI is. If your BMI was “too high”, my guess is you were told you need to lose weight. Doesn’t matter if all other health markers were perfect—if that one number was off, you were told to fix it.


Additionally, the 2020-2025 Dietary Guidelines encourage Americans to maintain a “healthy body weight” and make a point to highlight that 74% of US citizens “have overweight or obesity”, which is then followed by advisements on how to lose weight.


These terms of healthy, overweight, and obese are unfortunately based on the BMI scale, which is not an accurate measure of health at all. Let’s talk more about why BMI is flawed and how this form of measurement can cause more harm than good.


What is BMI?

BMI is a screening tool and nothing more. It screens for weight status and should be used in conjunction with other screening tools and a complete assessment to determine overall health status.


Unfortunately, it is often valued as much more than just a screening tool. Despite the issues with BMI, results of this tool are now leading to diagnoses and conclusions that should not be made such as: unhealthy, overweight, too large, not eating well, etc.


How BMI is Calculated


BMI takes into account only two data points: weight and height. It is calculated by dividing body weight in pounds by the square of height (in inches), then multiplying the result by 703.


Sound confusing? Somewhat arbitrary? That’s because it is. Looking at the actual math behind the measurement makes it pretty clear that BMI is flawed.


Which BMI is Underweight


If your BMI falls below 18.5, you are considered to be underweight.


Which BMI is Normal


If your BMI falls between the range of 18.5 and 24.9, you are considered to be a normal weight.


Which BMI is Overweight


If your BMI falls between the range of 25 and 29.9, you are considered to be overweight.


Which BMI is Obese


If your BMI falls above 30, you are considered to be obese.


Are BMI and Body Fat the Same?


One of the confusing points of BMI is that it is labeling people as ‘overweight’ and ‘obese’, which is often interpreted into meaning they are fat. If the BMI has the power to place a label such as obese, does that mean it’s also telling us how much fat we have on our bodies?


Absolutely not. In fact, one of the chief problems with using BMI is that it does not take into account what is contributing to the total body weight. In other words, it does not take into account what is muscle vs fat vs water, etc.


A BMI result does not tell us anything about the individuals’ total body fat. For example, a body builder with massive amounts of muscle would be labeled as ‘overweight’ or ‘obese’ according to BMI when they are, in fact, very lean and have very little body fat.


This is one of the many reasons why BMI is flawed and why it is preposterous to tell someone to lose weight based on their BMI result alone.


A woman lifts weights in front of a mirror at a gym after learning why BMI is flawed

Can BMI Measure Health?


There are suggestions in research that a higher BMI is associated with a higher health risk. However, these types of studies cannot automatically establish a cause-and-effect relationship. A high BMI cannot cause diseases (like cardiovascular diseases). Rather, our habits do IN RELATION (this is important to remember) to our genetics and environment.


Just because someone has a BMI, does not automatically mean they need to lose weight “for their health”. The fact that so many people (including some heallth professionals!) correlate BMI with health is a huge problem with BMI.


In reality, the more accurate measures of health are metabolic indicators such as blood pressure, triglycerides, cholesterol glucose, insulin resistance and C-reactive protein data.


In fact, a 2008 study showed that using BMI as a proxy for health resulted in the misdiagnosing of over half (51%) of overweight and nearly 32% of obese U.S. adults as unhealthy, and misdiagnosing nearly 24% of normal weight U.S. adults as healthy.


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Additionally, it was found in a 2016 report that 54 million Americans were labeled “obese or overweight” but were in fact healthy according to metabolic indicators.


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These studies make it clear that BMI is flawed, so why did we start using BMI in the first place?


A man exercises using an exercise band after learning why BMI is flawed

The Birth of BMI


Who created BMI? BMI actually started out as a the Quetelet index, named after it’s creator named Adolphe Quetelet in 1832. Adolphe Quetelet was an astronomer and mathematician (read: not a medical professional) who derived this tool using the data of European men. He noted that this was created as a statistical exercise, not as a calculation that could be used on individuals as it was limited to populations. He also did not intend for it to measure health.


However, a physiologist named Ancel Keys reincarnated the calculation in the 1970’s as the Body Mass Index, a tool to measure an individual’s health status… exactly what Quetelet advised against doing.


The Problems with Using BMI


As mentioned, BMI came from a calculation that was exclusive to the data from European men. Because of this, BMI has been criticized as being racist and sexist.


Consider a black woman being told by a doctor that she needs to lose weight based on her BMI alone. Considering BMI was created without women or those of African descent in mind, it’s unscientific and unethical for a doctor to do this.


This is only one example. Even if it made sense to measure health using BMI (it doesn’t), it only would if we were doing that on males of European descent.


So how in the world did this measurement end up being used to dictate the health status of everyone?


Why BMI Became Medically Relevant


Despite the limitations of BMI, in 1899 the Association of Life Insurance Medical Directors of America decided to use this measure to determine that “the overweights are clearly less desirable than either the normal or underweights”, basing this on mortality records that was data almost exclusively reporting on wealthy white men.


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What does this mean? It means that these BMI categories were adopted by insurance companies to decide who is and isn’t more at risk based on bad science.


These companies then worked hard to present higher-weight individuals as a risk to the medical communities, scouring for any data (whether good data or not) that supported their point.


If there was ever a reason to believe that BMI is flawed, this background is a big one. In fact, modern records that are much more reflective of all races, genders, and socioeconomic status’ actually show that those in the overweight category have the lowest mortality rates.


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Three women wearing bathing suits walk along the edge of the ocean after learning why BMI is flawed

The Shift in BMI Thresholds


If you were around in 1998, you may remember that the National Institutes of Health (NIH) released a report changing the thresholds for each of the BMI categories. The impact was huge.


Overnight, people were recategorized into new, higher BMI categories. Not because they gained weight, but simply because the cutoffs were changed.


Imagine going to bed one day being told you are a ‘normal weight’, then waking up and being told you are ‘overweight’ even though the scale says the exact same number as the day before.


The Ugly Truth of BMI


Why did the thresholds of BMI change? Great question—I can’t wait to tell you.


The thresholds were altered based on a report primarily written by an organization called the International Obesity Task Force (IOTF). The IOTF received their funds mostly from two pharmaceutical companies that were developers of weight-loss drugs - just one more problem with BMI.


If we think about it, lowering the thresholds of BMI is a clever business endeavor for these drug companies. If millions of people think they have weight problem and need to lose weight, then the companies selling weight loss drugs benefit greatly.


If you’re anything like me, you likes to offer up the benefit of the doubt before making a judgement call on anything. Perhaps you’re thinking, “well just because the report was from an organization that would financially benefit from there being more people thinking they need to lose weight, that doesn’t mean that the report was actually a bad one with faulty science.”…


Well, I hate to break it to you, but based on a 2016 review: studies financed by industries related to the study itself are seventeen times more likely to have favorable results than studies funded by non-industry sources. As Christy Harrison, MPH, RD put it in her book Anti-Diet, “When you’re being paid by a company to research its product, you’re probably going to want to produce results that keep the bosses happy.”


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A pile of multi-colored pills is shown on a white table, the result of reasons why BMI is flawed

Why Do We Still Use BMI?


You might be wondering at this point why we have ever referenced the BMI chart when clearly BMI is flawed. You might also be wondering why we continue to use it.


At this point in time, I don’t have an answer for you… but I would love to know what the answer is. There is plenty of evidence that BMI is not accurate as an indicator of health and plenty more evidence that it is biased and the result of bad science.


I agree with many researchers that BMI should be discarded as an outdated and ineffective tool for measuring health.


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BMI Is Flawed - So Now What?


If you’ve looked to the BMI chart to help measure your own weight and health in the past, you might be wondering what measure to use instead to avoid the problems with BMI.


But the truth is, you don’t need another measurement tool. The path to health can actually involve fewer tools, measurements, weight loss tricks, and tracking.


If you're ready for me to help you find a way of eating that makes you happy and frees you from the pressures of BMI, visit yatesnutrition.com/services to see how I can help.

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