Why BMI Is Not an Accurate Measure of Health

Woman weightlifting with barbell
John Barry


The first “real” research I did in college was specifically for a research class in senior year. The purpose of the course was to go through the process of deep study on a topic to learn more about that topic, and not simply to pass a test or write a paper. We learned how to delve deep and build upon current knowledge to look further into studies that have been done on any given topic. Another key skill we picked up was how to screen studies and comb through them to pick out which were more legitimate. As a class, we split up into three groups, were given three separate topics, and would meet weekly to present and discuss what we had come across. My group had the responsibility of determining why any particular BMI is considered healthy or unhealthy.


Let’s fast forward for a moment. By currently working in the healthcare/fitness field, I come across BMI numbers for patients and athletes on a daily basis. A client will come in and say something similar to “my doctor told me that my BMI was to high, obese actually”. Now this is told to me with a sense of urgency and worry. Granted, having a high, or low BMI isn’t great, it’s only part of the big picture.


Height and Weight Proportion


Man weightlifting at the gym
BMI is determined by height and weight proportion. It makes sense, as the ratio of what a person weighs relative to their height, should give us a good idea of wether or not that person is overweight, underweight or healthy. However, the only data used is height and weight. It does not take body composition into account. If a person is well above or below “average” height, their BMI numbers will be skewed somewhat. This will give them a number that doesn’t truly reflect where on the BMI scale they fall.


The formula doesn’t account for muscle mass either. Personally, I carry roughly 20 pounds more of muscle than other men my height. I know this by seeing our relative percent body fat and respective weight. This puts my BMI on the overweight borderline obese range. Knowing that my percent body is fat is normally 9-10%, I might be the first obese person with defined abdominals.


Body Fat Percentage


Woman testing body fat percentage
A better measure is our percent body fat. This can be measured a number of different ways. My personal favorite for its ease of use, accuracy and availability is bioelectrical impedance. These are the fancy scales that have metal discs in contact with the feet or handles with metal plates. They measure the bodies resistance to electricity. That number is then put through a number of mathematical formulas to determine what percent of body weight is fat, versus muscle. This can give us a more accurate look at how healthy a person is.


Optimal fat percentages will vary greatly from one individual to the other. These factors include age, gender, specific sport and overall genetics. Woman tend to have a higher percent body fat then men. This has more to do with child bearing reasons. Speak with your doctor or other medical health professional on why the correct number for you is. Take into consideration both short term and long term goals and health. For instance, a competitive, male bodybuilder, will compete at 4% body fat. While not in competition, that some individual would probably be 10-15%. These are not hard and fast rules, just part of the big picture. In addition to these, also keep an eye on other health factors such as blood pressure Nd cholesterol.


Now if BMI, is the best option, why is it so widely used? The answer is straight forward. It’s a very easy thing to calculate and gives a concrete number that fits into a chart. This way a medical professional or insurance company can quickly tell you how healthy or unhealthy you are. But what does that number mean? Why does a certain BMI label a person as overweight or underweight. Let’s finish where we started.


The Bell Curve


My study group was responsible for this issue in class. We had to determine how those specific numbers were chosen to represent healthy/underweight/overweight etc. All of the research I came across pointed me back to the same source. This source simply stated that the numbers were greed upon by a panel of experts. That didn’t give us much to work with, so my group and I collected height and weight measurements for a group of the population. We retrieved as much publicly available information we could. We then took these numbers and graphed them. It formed a typical bell curve on a chart. We then calculated the BMI for each of these individual numbers. They correlated exactly with the same bell curve.


We found that “healthy” people were in the 50th percentile. People that were underweight were in the 25th percentile. People that were overweight were in the 75th percentile. The further away from the mean, the more underweight or overweight/obese a person is.


So next time you’re told your BMI, just remember, the main thing it’s telling you is how you’re plotted on the bell curve of the population.

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