Body Mass Index (BMI) Basics and Considerations

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Have you ever gone to the doctor and received a BMI number generated from your height and weight? Do you wonder what this number means or if it’s valid? There are many misconceptions about the body max index (BMI). Let’s look at what BMI is and where it comes from, as well as dive into limitations of using BMI as an assessment for health.

What is BMI?

Around the 1830’s Adolphe Quetelet, an astronomer and statistician, developed the Quetlet Index which is now termed, BMI. This is a calculation using weight and height to generate a numerical value that is then used broadly to categorize someone as normal, overweight, or obese.

However, BMI was not originally created to determine individual weight. It was created to look at the distribution of weight in a larger population. Today, this 200-year-old method is often used extensively as an indicator of health for individuals even though it does not tell one’s full health story.

How can BMI be misleading?

As an assessment for individual health, BMI falls short. Especially when compared to other health assessments like cardiometabolic health, a measurement of blood pressure, triglyceride, cholesterol, glucose, insulin resistance and more. As you may have noticed, the BMI calculation also does not measure your muscle and fat content, age, or sex.

Consider the following:

  • A professional male football player, a linebacker to be specific, is 6’2 and weighs 220 pounds. His BMI, 28.2, will be classified as overweight.
  • A world cup female rugby player is 5’5 and weighs 165 pounds. Her BMI, 27.5 is also considered overweight.

According to the BMI calculation, these athletes are defined as overweight which can have implications for care they receive and attitudes of “healthy” or “unhealthy”. However, we know that professional athletes generally maintain a balanced, healthy diet with high levels of exercise that increase their body muscle content.

This is one of the areas in which using BMI as an indicator for health can be problematic. It does not take into account all facets of health and lifestyle.

A 2016 study of over 40,000 adults published in the International Journal of Obesity found that 29% of obese individuals and about half of overweight individuals, as defined by BMI, were actually healthy. These miscalculations go both ways. Of note, the researchers also found that 30% of individuals classified as normal by BMI were unhealthy (Tomiyama, 2016). This study estimates that over 74 million individuals are misclassified when using BMI as an indicator of health (Tomiyama, 2016).

Why should we care?

Today, many healthcare professionals still use BMI as a general health assessment. BMI classifications of “overweight” and “obese” are often associated with poor health as well as assumed to be an indicator for certain diseases. Healthcare providers using BMI may wrongly assume a patient is in poor health or could inappropriately focus on weight when there could be other root causes at play.

Conversely, someone who is classified as having a normal BMI may be unhealthy and in need of critical diagnoses or care. Although BMI is quick, inexpensive, and a convenient form of measurement, it is not the best way to assess overall health and wellbeing of individuals.

What are the alternatives?

Emerging research and work on other forms of health assessment are in process. Although defining health through various lab tests such as cardiometabolic health is more accurate, the measures are expensive, time intensive, and invasive.

Scientists Orison Woolcott and Richard Bergman have found that using a simple equation called relative fat mass (RFM) has shown to be more accurate than BMI for cases of body fat-defined obesity among men and women (Bergman, 2018).

However, much more work is needed to understand how to define health and what measurements to use. Redefining what health is and what health may look like is an ongoing conversation that we all contribute to. Now that you are more familiar with BMI and its limitations, you can feel more confident advocating for your needs as a patient.

Orison O. Woolcott, Orison O., & Bergman, Richard N. (2018).  Relative fat mass (RFM) as a new estimator of whole-body fat percentage A cross-sectional study in American adult individuals. Scientific reports. 8(1):10980. doi:10.1038/s41598-018-29362-1.
Tomiyama, A.J., Hunger, J.M., Nguyen-Cuu, J., & Wells, C. (2016). Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005-2012. International Journal of Obesity. 2016;40(5):883-886. doi:10.1038/ijo.2016.17.

Lexie Garrity is a graduate student in the Nutritional Sciences Program at the University of Washington graduating with her MPH in Nutritional Sciences in December 2020. Lexie’s passions include employee health and wellness, employee rights, and sustainable communities.

Lexie’s approach to nutrition education is informed by Health at Every Size and Eating Competence theories. Lexie believes in the enjoyment of all foods and strives to further work in inclusivity and food access. In her free time, she enjoys hiking and eating bread.

One Thought on “Body Mass Index (BMI) Basics and Considerations”

On September 3, 2020 at 8:30 AM, Laura said:

What about BSA (the surface area measurement)? My doctor takes that–does it have any relevance on healthy weight?

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