The Truth About BMI
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If you've ever set foot in a doctor's office or read a news article, you're probably familiar with BMI. Maybe you've even been told that your BMI classifies you as "unhealthy." But, have you ever stopped to wonder what BMI is or where it came from or how it could tell you whether or not you're healthy?
BMI, or body mass index, was created in the early 1800s by a mathematician named Lambert Adolphe Jacques Quetelet. There are a few issues right from the beginning, in that the equation was created to classify groups and not meant for use on an individual level. The population used in creating the BMI equation was also not representative of the populations where BMI is used today.
Also, I'm not statistician, but thinking logically about how the equation is set up and having read some input from statisticians, it doesn't make much sense to square a person's height in the equation. From what I understand, Quetelet had to square height in order to get the formula to work with the data he had, which is a more than somewhat questionable scientific practice.
All of that aside, BMI doesn't differentiate between muscle, bone, fat mass, and fluid weight. So, a person with a high percent of muscle mass will end up with a BMI classifying them as overweight or obese. This can happen often in athletes. According to our current standards of judging health off of BMI alone, they would be classified as "unhealthy", even though they are physically fit and actually quite healthy.
The misclassification of health based on BMI doesn't just happen in athletes though. In a 2016 study looking at 2005-2012 NHANES data, which is nationally representative date from over 40,000 individuals age 18 and up, it was estimated that over 74 million US adults are misclassified as either cardiometabolically healthy or unhealthy. What this means is that people are either:
classified as unhealthy based on their BMI but their actual health indicators (such as blood lipid levels, blood glucose, insulin resistance, and inflammation) indicate that they are healthy
classified as healthy based on their BMI but their health indicators indicate that they're unhealthy
That is A LOT of people being misclassified! And let's think about the ramifications of this. That means that people who are perfectly healthy are probably being told they need to lose weight, which creates all sorts of shame and stress (which we know are harmful to health). It also likely means that some people who have legitimate health issues aren't having them addressed because their "BMI is healthy".
I get that BMI is a simple and non-invasive tool and that it would be convenient if it provided a valid assessment of risk. However, it doesn't really appear to do that. What it does do is create an overemphasis on weight at the expense of focusing on the development of maintainable behaviors that we know actually improve health.
When we focus only on weight, people go on restrictive diets, only to gain all of their weight back and then some over the long term. They're also likely to take up physical activity (which has been shown time and time again to positively impact numerous aspects of health, regardless of weight), only to abandon it weeks later when the scale isn't moving fast enough. These behaviors contribute to weight cycling, or the repeated gain and loss of weight, which has been linked to adverse cardiovascular outcomes.
Focusing only on weight leads to weight stigma (rejecting those who don't fit society's standards for body size), which is not only stressful but encourages people to avoid seeking medical care for fear of being shamed or scolded for their weight.
What we can do instead is focus on behaviors that actually promote overall health, such as developing effective stress management skills, getting adequate sleep, incorporating physical activity in a way that is enjoyable and realistic, and developing a healthy relationship with food. When you are doing all of these things, your body will settle out within your set point range, or the weight range that is most healthy for your specific body. If you're still curious about set point weight, this is a funny yet easy to follow video that does a pretty good job of covering the basics.
Because of all of the above that we know about weight and health, more and more dietitians and other health providers are moving toward focusing on things like managing stress, adequate sleep, enjoyable physical activity, and developing a healthy relationship with food to help people actually improve their health and find the weight that is healthiest for them as an individual. If you've felt stuck in an endless cycle of pursuing a weight that sits in a certain BMI range without success, then I hope this gives you some hope and encouragement that all is not lost in terms of your health. You have permission to pursue healthy behaviors simply because they're healthy, whether or not they result in a certain weight or BMI.
Bacon L, Aphramor L. Weight Science: Evaluating the Evidence for a Paradigm Shift. Nutrition Journal 10, 2011.
Mann T, Tomiyama AJ, Westling E, Lew A-M, Samuels B, Chatman J. Medicares search for effective obesity treatments: Diets are not the answer. American Psychologist 62: 220–233, 2007.
Tomiyama AJ, Hunger JM, Nguyen-Cuu J, Wells C. Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012. International Journal of Obesity40: 883–886, 2016.
Tomiyama AJ, Carr D, Granberg EM, Major B, Robinson E, Sutin AR, Brewis A. How and why weight stigma drives the obesity ‘epidemic’ and harms health. BMC Medicine 16, 2018.
Tylka TL, Annunziato RA, Burgard D, Daníelsdóttir S, Shuman E, Davis C, Calogero RM. The Weight-Inclusive versus Weight-Normative Approach to Health: Evaluating the Evidence for Prioritizing Well-Being over Weight Loss. Journal of Obesity 2014: 1–18, 2014.