There is so much controversy surrounding continued use of the body mass index (BMI) in clinical and research settings. It’s well justified too. Because of this somewhat misleading tool a large percentage of people in the United States and abroad have been classified as either overweight or obese. Now that obesity is treated as a “disease”, measurement tools for diagnosing obesity definitely need to change and they need to change fast.
What is the Body Mass Index?
The BMI is a cheap and easy way to categorize body weight. You can calculate your 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:
- Underweight = <18.5
- Normal Weight = 18.5-24.9
- Overweight = 25-29.9
- Obesity = BMI of 30 or greater
When used in conjunction with circumference measurements (waist and hip measurements), the BMI can be a valuable tool for classifying some of the health risks of body weight, albeit underweight or overweight. In fact, this is a primary tool used by physicians for diagnosing obesity.
A major problem with the BMI lies in the fact that it’s based on the concept 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. This is evidenced by the image on the right (originally published in an article on The Daily Mail), which depicts five women of different statures who each weigh 154 pounds.
And there’s another huge problem with the BMI.
The BMI Does Not Distinguish Between Fat Weight and Fat-Free Weight
The BMI is based on your height and your scale weight. However, in spite of widespread usage, scales are quite useless, as they 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.
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.
Why Measuring Your 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 increases the risk of numerous health problems including high cholesterol, high blood pressure, and type 2 diabetes.
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 clearly muscular woman with a BMI classification of “borderline overweight”, I continue to find myself frustrated whenever I’m asked to step on the scale during a medical exam.
I’m sure that many others share my sentiments.
If for whatever reason your BMI has put you in an obesity category, request 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.
Learn what it takes to achieve and maintain good health through weight control. Pick up a copy of Leaving Your Fat Behind.
Disclaimer: The information provided is for educational and informational purposes only and is not intended to diagnose, treat, cure or prevent any disease. Any reader who is concerned about his or her health should contact a physician for advice.
Before starting an exercise training program you should first make sure that exercise is safe for you. If you are under the age of 55 years and generally in good health, it is probably safe for you to exercise. However, if you are over 55 years of age and/or have any health problems, be sure to consult with your physician before starting an exercise training program.