Body composition vs. BMI, in a sea of fad diets and nutrition information-overload, it can be difficult—even downright frustrating—to sort out fact from fiction. What’s the difference between body composition and BMI metrics? We all know that nutrition is a key component to health and wellness, so how do we determine which nutrition approach or protocol is right for us? Or which metrics to use to make those decisions?

If you’ve ever visited the doctor, chances are you’re familiar with the term “BMI” (body mass index) and may even know what your individual BMI is. But what exactly is BMI, is it important, and what should you do with the result once you have it? There’s a lot of swirling information (and misinformation) about BMI, body fat percentage, and body composition, but what’s the difference between them? Is one more important or accurate than the other?

There’s a place for BMI, but it’s not the whole story

A few years back upon leaving my doctor’s office, I stared in disbelief at the front page of my printed visit summary. Frankly noted in black and white was one simple word: “OBESE.”

Based on my body weight, I was now branded by a BMI that had reached 30. I struggled to take it in. I knew I had some extra weight on me, but was I really obese? Was I really at a greater risk for disease and chronic illness? Was I even more unhealthy than I let myself believe? I emotionally spiraled as I was confronted with information that felt confusing and loaded. I consulted Google to figure out what it all meant…and quickly discovered that BMI did not present the whole story.

BMI vs body composition: which is which?

BMI stands for “body mass index” and is a measurement of an individual’s weight with respect to their height. It’s a basic mathematical equation that is quick and easy to calculate with just a person’s height and weight metrics, giving it accessibility and popularity within the medical community and beyond. It is not a measure of body fat—a common misconception—but it does often correlate with body fat, in that it provides insight into whether or not you’re likely to be at a healthy body weight for your height (2). It’s important to realize that BMI is a general indicator of health and disease risk rather than a direct measurement of a person’s body composition (1).

Body composition, on the other hand, specifically measures how much of an individual’s total weight comes from lean mass (muscle, bones, connective tissue, and water) and how much comes from fat. Accurately determining an individual’s body composition is generally much less accessible for most people, often requiring the use of expensive equipment and testing through a variety of methods. The clear benefit is that body composition reveals your body fat percentage, offering greater insight into your health and disease risk than BMI alone.

Body composition testing is also more insightful for a greater variety of body types, including individuals who have a healthy BMI but a higher body fat percentage (sometimes referred to as “skinny fat”), as well as those who are very muscular, who might have an unhealthy BMI but who also have a healthy body fat percentage (2).

Using the information to move forward

As I personally discovered, BMI alone doesn’t tell the whole story. The reality of my journey was that I needed to lose some body fat and take responsibility for my poor lifestyle habits, and being confronted with my BMI helped me to do just that.

I did eventually have my body composition tested with a DEXA scan, and after several months of regular exercise and healthy eating, my results revealed that my body fat percentage was in a healthy range, even though my BMI still categorized me as “overweight.” This served as further evidence that it’s important to consider many factors that make up the big picture, rather than getting hung up on a single metric.

We shouldn’t completely throw BMI out the window. As a generalized marker for the majority of the population, it offers some basic insight into your risk for certain obesity-related diseases and chronic conditions. In this way, it can be a significant marker of health in some cases. Do keep in mind that some people are outliers within the scope of BMI, particularly those who are very muscular or those who tend to be tall and lanky. It’s also important to consider the many other factors that might better inform progress toward your individual health and fitness goals. Other factors might include how you feel, how your clothes fit, what you see in the mirror, or your performance in the gym.

Taking the opportunity to have your body composition tested at least once can be a great way to inform you more specifically and accurately about your health. There are many ways to have your body composition checked, with different methods offering several metrics with a varying range of accuracy. For instance, some methods offer insight into the distribution of fat you have in your body, how much visceral fat you have (the fat located around internal organs that puts you at a higher risk for certain diseases), or your bone density. It’s important to do your research on a given method to better understand and interpret your results. As always, it’s a good idea to consult with a professional if you have questions, such as your doctor, your trainer, or the company administering the testing.


Hyatt Training personal trainer intern Elana WittAuthor Elana Witt is a personal training intern at Hyatt Training. She believes all people possess the ability to get stronger and feel better, no matter where they’re starting from. Through learning correct, functional movements, she wants each of her clients to better understand their body and their capabilities while feeling empowered to achieve their goals. Elana is a NASM certified personal trainer. Learn more about Elana, or get in touch with her by emailing us at

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