Anyone who's concerned with their weight, or overall health and well-being, has likely heard of or even measured their own body mass index (BMI). However, there are a lot of BMI myths out there, and studies from the past decade suggest that BMI may not be the most accurate measure of body fat and corresponding health levels. Researchers have found other more accurate measurements to help doctors and patients determine a healthier diet and lifestyle.
What Is BMI?
The Centers for Disease Control and Prevention (CDC) defines body mass index as "a measure of weight adjusted for height, calculated as weight in kilograms divided by the square of height in meters." This measurement is intended to determine how much fat a person has on their body. According to this logic, staying within a certain BMI range indicates whether someone is a healthy weight, according to their height. The measurement is simple, inexpensive, and noninvasive, which has contributed to its increased use.
The CDC defines BMI ranges and their corresponding weight statuses as:
- Below 18.5 -- Underweight
- 18.5-24.9 -- Normal
- 25.0-29.9 -- Overweight
- 30.0 and Above -- Obese
Why It's Inaccurate
The main issue with BMI is that it measures excess weight, rather than excess fat, and doesn't take into consideration where weight is distributed throughout the body. Weight comes not only from fat but also from different bone structures, muscle mass, and water retention. For example, two people could have the same BMI because they're the same weight and height. But one person's weight may come from having muscle, which weighs more than fat, as opposed to excess fat, which is an indicator of potentially poor health.
According to a 2008 study published in the International Journal of Obesity, BMI's accuracy is limited, particularly among certain populations, such as men, the elderly, and individuals in the intermediate BMI ranges. And a 2009 study published in BMC Public Health found that self-reported BMI measurements tend to be inaccurate, with many participants overestimating BMI measures at the low end of the BMI scale (less than 22) and underestimating BMI measures at the high end, particularly measures greater than 28.
More Accurate Ways to Measure Body Fat and Healthiness
A 2008 study published in the International Journal of Obesity reviewed a selection of measures that are more accurate at determining body fat than BMI. These include:
- Skinfolds, where skin and fat thickness are measured away from the muscle to determine body fat levels
- Bioelectrical impedance analysis, which uses electrical signals to measure the body tissue's resistance (fat is more resistant)
- Hip circumference
- Waist-hip ratio
- Sagittal abdominal diameter, which measures the distance between the back and the highest point of the abdomen
- Dual-energy x-ray absorptiometry (DEXA), which uses body scans to measure bone density and fat content
Also, according to the CDC, another simple measure of body fat distribution is waist circumference (sometimes divided by height).
There are a lot of BMI myths out there, but studies show that there's more to the story than this formula takes into consideration. If you have concerns about your weight or health, talk to your doctor about whether you should make any dietary or lifestyle changes to ensure better health and longevity.