Is body mass index the best measure of obesity?
For decades, public health officials and doctors—including myself—have relied on Body Mass Index (BMI), a number calculated using height and weight, to categorize people as overweight to obese. That’s because we know in most cases BMI correlates highly with body fat and serves as a good gauge for assessing one’s risk for diseases, high blood pressure, type 2 diabetes, and several other diseases.
However, what you may not know is that BMI is not a perfect measure. In some cases it may actually overestimate body fat—especially if you’re an athlete or body builder with a lot of lean muscle mass. If you’re older or have low bone density, it may actually underestimate body fat. In fact, some researchers believe the use of BMI may be leading us to underestimate the already out-of-control obesity rates here in the U.S.
A recent study revealed that almost half of adults whose BMI placed them in the overweight category (BMI 25–29.9) actually would be considered obese if their body fat percentage were taken into account!
So how do you test for your body-fat percentage? There are several ways to determine this number, which may very well be the best way to assess where you fall along the overweight-to-obese continuum. Here are the top four methods:
- Hydrostatic under water weighing is considered to be one of the most accurate methods (the “gold standard”), but has its limitations. For one, it requires that you exhale as much air from your lungs as possible before being immersed for 10 to 15 seconds under water, and two, it’s not widely available.
- Bioelectrical impedance is based on the fact that lean body tissue is much more conductive than fat tissue because of its higher water content. A bioimpedance meter is attached to the body and measures the body’s ability to conduct current. It’s noninvasive but less accurate than underwater weighing.
- Skin-fold measurements also are less accurate, but are commonly used by nutritionists and dietitians. Several skin-fold measures are taken on different sites on the body using a caliper device, then used to calculate body-fat percentage. Look for someone well trained in this method as large inaccuracies can occur due to technician error.
- DXA, known best for its utility in measuring bone density, also has been used in clinical and research settings to measure body composition. Today you may find it used in weight loss clinics, with professional sports teams, and even on reality TV! NBC’s The Biggest Loser ™* has used this method to chronicle the dramatic weight loss of the show’s contestants.
So, although not perfect, BMI is a useful starting place to assess your body weight. Combine this with body-fat testing, and together you’ll have a more accurate picture of whether you are at a healthy weight (and body fat), or if you are overweight or obese. You can’t attack a problem if you don’t know you have a problem, so let’s all be accountable for our weight since it is such an important predictor of health. And remember, too, that if you are overweight, even modest changes in body weight (5%–10%) can help lower your risk of weight-related diseases and improve your health.
*All trademarks are the property of their respective owners.