Ah, BMI. A controversial topic among dietitians and other health professionals these days. For years, we seemed to blindly accept this scale, but now we are realizing that BMI, or Body Mass Index, is not the best tool for determining health status, or even body mass for that matter. The history of how it started might surprise you. BMI has several limitations, but there are now better alternatives.
A short history of BMI
If you have been wondering where BMI came from, today is your lucky day! You probably didn't know (because I sure didn't) that BMI was invented by a mathematician...not a scientist, doctor, dietitian, or any person working in the field of healthcare. So, that is interesting and a little strange, right?
Adolphe Quetelet was a mathematician and statistician who came up with BMI (originally called the Quetelet Index) in 1832. The term BMI was coined in 1972 and was defined as body weight in kilograms divided by the square of height in meters (wt in kg/ht in m^2). Quetelet came up with the formula as a way to standardize average body sizes in a quick and easy way. Depending on the result of the formula, people would be put into a category of underweight, normal weight, overweight, or obese.
Ancel Keys was the one who coined the term BMI in 1972. He was a physiologist who studied BMI as a way to look at body mass among populations. The keyword there is populations...BMI was not intended to be used when looking at an individual, but that is exactly how BMI is used now. BMI is often used by practitioners as a step in determining a person's health status (although, it probably shouldn't be - more on that later). Insurance agencies also use BMI as a tool, and this started with Ancel Keys when companies started using his analysis and the weight categories when providing coverage to their clients. Keys was apparently annoyed by this, as he maintained that two people could be the same weight but have completely different compositions and fat mass, which is one of the many problems with using BMI.
The problems with BMI
Let's just dive right into the problems with BMI:
BMI does not take body type into account. We are all built differently, as we should be. But for some reason, we lost sight of the fact that healthy bodies come in all shapes and sizes and there is no one "perfect" body. People that are technically overweight or even obese can actually be in excellent health. The BMI formula does not account for the size of your bones (bones are heavy!), the amount of muscle you have (muscles are also heavy!), the areas of your body that store more fat, or the shape of your body.
This means that people with lots of muscles are often considered overweight or obese even though they are in excellent health. For example, Dwayne "The Rock" Johnson is considered obese, but you cannot tell me that guy is not in fantastic shape! On the same note, think of two men who are 6'2" but one has a slender build and one has a large build and large bones. According to the BMI chart, both of these men should weigh about the same amount to be considered a "normal weight", when in reality the man that is genetically built larger should clearly weigh more.
The math doesn't really make sense. Dividing someone's weight by their squared height does not fully work. Other mathematicians argue that doing so divides the weight by inappropriate factors, especially for people who are quite tall or quite short. This is why taller people tend to have higher BMIs. Plus, Quetelet came up with the calculation when he was trying to create a perfect bell curve on a chart showing the BMIs of the population. He, allegedly, was able to get the right curve on the chart by squaring the height, which makes it seem like he was playing around with numbers until he got the results he wanted.
The numbers don't always add up. This sort of rolls into the first bullet point, but people are three-dimensional, not two-dimensional numbers on a piece of paper. It's never really fair to make classifications and assumptions about a person's health based on a number, especially when that number has been assigned to them by an outdated calculation. Recent research tends to agree that there are better ways to measure both health and body mass. One study found that nearly half of overweight individuals, 29% of obese individuals and even 16% of obesity type 2/3 individuals were metabolically healthy. Moreover, over 30% of normal-weight individuals were cardiometabolically unhealthy. These results clearly show that people considered obese can be healthy and people considered normal weight can be unhealthy.
It is used too broadly by some practitioners. There is both scientific and anecdotal evidence of some practitioners not providing the same level of care to patients who fit into the overweight and obese BMI categories. What happens in this scenario is a person with a higher BMI has their health issue disregarded and is told to lose weight to solve their problem. However, weight loss is not always the answer, and telling someone to lose weight without doing more tests or examinations is just poor care. When practitioners rely solely on a BMI number when providing care, then they are not doing their job. They also can cause their patients physical and emotional harm that can lead to eating disorders, body dysmorphia, and more resistance to making positive lifestyle changes.
Some alternatives to BMI
So, if BMI is not exactly useful, what should we be using when measuring body mass?
Well, first, let me digress and ask another question...do we even need to be measuring body sizes? In some cases, yes - like for research, certain athletics, etcetera, but when we measure and compare body sizes we can unintentionally create a stigma around weight and physical appearance. Even the terms used in the BMI scale, like "normal weight" and "overweight", have certain connotations. A person can have all the confidence in the world and then step on a scale or look up their BMI and then lose their self-esteem because we as a society have condemned being overweight or obese. We often forget that people that are (according to BMI) overweight or obese are people too with thoughts and feelings, yet we make jokes at their expense and make people fearful of gaining weight. Being overweight or obese on the BMI chart does not equal being unhealthy. If you want to read more about this whole topic, this article discusses the problems with weight-centered healthcare, various assumptions about weight, and ethics related to the issue.
Back to alternatives to BMI. What can we do instead? There are some better ways to measure body mass and fat that are more accurate and less stigmatizing. People may want to know the amount of fat that is on their bodies to learn if they may or may not be at risk for things like diabetes and heart disease. Again, just because you have a certain amount of fat does not automatically make you unhealthy or prone to health issues. Adopting an overall healthy lifestyle that includes a balanced diet and regular movement has been shown to improve health and reduce disease risk. Here is a quick look at a few BMI alternatives:
The DEXA Scan is considered the "gold standard", but the problem is it is very expensive. The DEXA is like an x-ray machine that can measure body fat, muscle, and bone density. The result is an accurate look at your overall mass and composition.
Relative Fat Mass is another calculation similar to BMI, but more accurate. It looks at the circumference of your waist in relation to your height. It has been researched and the performance of RFM to estimate DXA-measured body fat percentage was overall more consistent than that of BMI among women and men, across ethnic groups, young, middle-age and older adults.
Body Adiposity Index is similar to the relative fat mass calculation, but it looks at the circumference of your hips (instead of your waist) in relation to your height. The thought is that your weight is not necessary for these calculations because hip circumference and height are more strongly correlated with body fat percentage than weight.
Hydrostatic Weighing is an interesting technique that is, unfortunately, just not widely available, so it cannot be used on the general population (like the DEXA scan). You are first weighed dry and on land and then while underwater and sitting in a chair. The results are a highly accurate look at body fat percentage.
Piecing it all together
The gist is that BMI is not accurate and can even be harmful. There are other ways to calculate body fat percentage if needed. It is important to remember that one size does not fit all, and that is especially true when it comes to body size. Just because a number in a chart or on a scale may be higher, that does not mean you are unhealthy or even unworthy. Your worth does not lie in your body weight or waist circumference, and especially not in your BMI.
-Brittany Lubeck, MS, RD
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