BMI Isn’t the Best Measure of Obesity or Health: So Why Are We Still Using It?

Wonder why you’re getting on the scale at every single doctor’s office visit? Whether for a routine medical exam or a general sick visit, your body weight is almost always one of the first things measured. This measurement is sure to be followed up with some type of height assessment. Once both your height and weight measurements are obtained, these numbers are either plotted on a chart or inserted into a simple equation in order to determine what’s known as your body mass index (BMI).

For decades, the BMI has been the medical community’s preferred method of estimating body fatness. It’s also used as a rough indicator of one’s health status and disease risk. But, BMI isn’t always the most accurate method. In fact, an abundance of research indicates that clinicians routinely misclassify people as either overweight or obese in accordance with BMI estimates.

And, based largely on BMI, obesity is currently treated as a disease and linked to just about every other type of chronic disease from heart disease and high blood pressure to type 2 diabetes and even cancer.

This is definitely a problem—Particularly since your BMI could eventually cost you in higher insurance premiums and penalties despite a potential misclassification. To further clarify the problem, I’m going to offer a bit of additional insight into the BMI. I’ll also discuss a more favorable method of estimating body fatness that you may or may not have already heard about.

What is the Body Mass Index?

The BMI is a cheap and easy way to categorize body weight. You can calculate your own BMI using your height and weight by way of a simple formula: Weight (kg) / Height (m)2. Once your BMI is calculated you can then categorize your body weight in accordance with the following scale:

BMI Classifications
  • Underweight = <18.5
  • Normal Weight = 18.5-24.9
  • Overweight = 25-29.9
  • Obesity = BMI of 30 or greater

That pretty much sums up what happens during each visit to the doctor’s office.

A major problem with the BMI lies in the fact that it’s based on the notion that everyone’s body weight is proportional to his or her height. This is a bad deal for people with short statures, as their BMIs are almost always overestimated.

There’s another huge problem with the BMI.

The BMI Does Not Distinguish Between Fat Weight and Fat-Free Weight

In and of itself, BMI is based on height and scale weight. Problem is, scales don’t distinguish between fat weight and fat-free weight.

Fat weight is comprised of adipose (fat) tissue that lies underneath the skin (subcutaneous) and surrounds spaces between vital organs (visceral), blood vessels, muscles, tendons, and joints. In general, fat tissue functions as an insulator to preserve body heat while also protecting and cushioning organs.

For these reasons, maintaining a certain amount of fat tissue in your body is critical for overall good health.

Fat tissue also serves as a reservoir for storing excess energy obtained from our diets in the form of calories. This is a primal physiological mechanism intended to prevent starvation during times of famine. You can think of this fat as ‘storage fat’, the kind we’re always trying to lose.

Related Article: Understanding the Process of Weight Management

On the flip side, fat-free weight includes all ‘non-fat’ tissues and fluids in the body such as muscle, bone, organs, blood, and water. Since these tissues have a great deal of real estate in the body, variations in fat-free weight can make a whole lot of difference in your overall BMI.

To further explain fat-free weight, I’ll focus on muscle because it’s the only component that is most adaptable to change.

Muscle has numerous functions in the body but it’s especially important for long-term weight management, as it’s very closely related to metabolism. The more muscle you have, the higher your metabolic rate tends to be at rest and during any types of physical activity.

Compared to fat tissue, muscle is very dense. Due to its dense nature, increasing muscularity will make it appear as if you are heavier on the scale and even higher on the BMI when in actuality you are just leaner. This essentially renders the BMI as a totally inaccurate measurement tool for athletes and physically fit individuals.

Related Article: Appearances Can Be Deceiving: Why Being Thin Doesn’t Equate to Being Healthy

Measuring Body Composition is Better

The relative distribution of your fat weight and fat-free weight is called your body composition. This is by far the better way to classify the health risks of body weight. A healthy body composition is one that encompasses a low percentage of fat weight and a high percentage of fat-free weight relative to your overall body weight. Your body composition classification is typically based on your ratio of body fat as shown.

Body Fat % Classifications for Men
  • Athlete = <10
  • Lean = 10-15
  • Normal = 15-18
  • Above Average = 18-20
  • Over-Fat = 20-25
  • Obese = 25+
Body Fat % Classifications for Women
  • Athlete = <17
  • Lean = 17-22
  • Normal = 22-25
  • Above Average = 25-29
  • Over-Fat = 29-35
  • Obese = 35+

It’s very important to understand the difference between your BMI categorization and your body fat percentage.

Even if your BMI classification is “normal weight” you may still be classified “over-fat” in terms of your body fat percentage and this is indeed a health issue. Having excess body fat is actually what increases chronic disease risk.

The BMI alone doesn’t actually reflect that.

Related Article: A Better Weigh: Measuring Success Beyond the Scale

And, luckily, there are many ways to have your body composition tested including hydrostatic weighing, dual-energy X-ray absorptiometry (DXA), Bod Pod, calipers, and bioelectrical impedance devices. Some of these tests can be costly but the information obtained is well worth it. Most universities and colleges offer these tests at low prices or free if you participate in research.

The Bottom Line

Widespread use of the BMI as an index of health status will continue to be an issue of debate—That is until medical professionals and researchers alike start to accept the simple fact that we’re not all of the same stature. As an athletic and muscular woman with an “overweight” BMI classification, I continue to find myself frustrated whenever I’m asked to step on the scale during a medical exam.

Perhaps you feel the same way.

Related Article: The Best Weight Loss Plan for “Big-Boned” Women

If for whatever reason your BMI has put you in an obesity category, I recommend requesting to have your body composition clinically assessed. This is an especially important step if your status is leading to inflated insurance premiums or penalties. Personally, I always have a copy of my most recent body fat percentage readings during doctor’s appointments.

I recommend you do the same, as this is a critical first step towards changing the current standards.