We shouldn’t define ourselves by a number, but numbers are sometimes helpful for measuring the risk of developing health problems. For example, if you have a blood pressure that’s significantly higher than 120 over 80, your risk of heart disease, stroke, and kidney disease goes up. We’re all familiar with another number, the number you get when you step on the scale, but body weight alone doesn’t tell you a lot unless it’s extremely high or low.
For determining whether a person is obese, health care professionals have, for years, used BMI, a measure of body mass index. Although the formula to calculate BMI looks a little intimidating when you do it by hand, you can find BMI calculators on the internet to quickly calculate your own body mass index. The problem is: BMI, like body weight, is not a particularly accurate indicator of who’s obese and at high risk for health problems due to obesity.
What is BMI?
BMI as a measurement was “invented” almost two-hundred years ago by a mathematician named Adolphe Jacques Quetelet. The government needed a formula to determine what percentage of the population was obese and, with there being no better formula, adopted Mr. Quetelet’s formula. Since that time, it’s graced the charts of patients who come for a physical, although it’s not without its critics.
What’s wrong with BMI? That becomes more obvious when you see how to measure it. To determine your BMI, divide your weight in kilograms by your height in meters squared. The reason this formula is a little intimidating is because most people in Western countries don’t think in metric units, but, as mentioned, a BMI calculator does the number crunching for you. All you have to do is enter your height and weight. If the calculator spits out a value between 185 and 24.9, you’re “normal. A BMI over 30 is “obese,” while anything between is “overweight.”
Here’s a quick reference chart:
. Underweight – BMI of 18.4 or lower
. Normal weight – BMI of 18.5 to 24.9
. Overweight – BMI of 25 to 29.9
. Obese – BMI of 30 or more
Sounds simple – but is it accurate? The best thing about BMI as a measurement IS its simplicity but it’s lacking as a tool for monitoring health. Since the formula only plugs in weight and height, it says nothing about how much muscle versus fat you have on your frame or how that fat is distributed.
Why is this important? Recent research shows waist size is an important indicator of whether you’ll develop future health risks like heart disease and type 2 diabetes, and the formula for calculating BMI doesn’t take waist circumference into account. This means a bodybuilder or athlete because they have so much muscle could easily have a BMI of over 30 and be classified as obese, despite having a low body fat percentage and being mostly lean muscle. In addition, an individual who falls into the normal or slightly overweight category based on BMI may have a waist size that places them in the same risk category as someone obese.
BMI also doesn’t consider the size of your frame. If you have a large frame, you may have a normal body fat percentage but weigh more simply because your bones are heavier. Because your weight is higher, so is your BMI, but it’s not due to body fat, but bone. Likewise, a small-framed individual may have a deceptively low BMI because their bones are light but still have body fat percentage that places them in the overweight category.
Is There an Alternative to BMI?
As you can see BMI, as a measurement of health and obesity, has serious shortcomings, but is there a better obesity measurement? According to new research, your fat-to-bone ratio is a more accurate way to measure obesity. After looking at more than 2700 chest x-rays, researchers found that dividing the diameter of soft tissue that lies beneath the clavicle bone, or collarbone, by the diameter of the clavicle, is a better indicator of whether a person is truly obese than is body weight or BMI. They also discovered this measurement was a better for predicting the risk of developing health problems associated with obesity, including coronary artery disease, stroke, and peripheral vascular disease.
Despite these findings, it may be awhile before fat-to-bone ratio becomes the standard for measuring obesity. For one, you have to take an x-ray to get the ratio. Not everyone wants to get an x-ray to find out whether they’re overweight or obese. Secondly, we need more research to create fat-to-bone research charts similar to the current BMI charts. Yet this study at least points out what many health and fitness professionals already know – BMI is a flawed measure.
What about Body Fat Percentage?
If BMI is flawed and the fat-to-bone ratio is still in the research stage, what IS the best way to determine whether you’re overweight or obese? How about body fat percentage? A 2000 study published in the American Journal of Clinical Nutrition showed body fat percentage is a better measure of disease risk related to obesity than BMI. The problem with body fat percentage is getting an accurate measurement.
The most accurate methods for determining body fat percentage are water displacement and a DEXA scan but these methods are expensive and more cumbersome than calculating a BMI. Plus, a DEXA scan exposes you to low levels of radiation. Methods you can do at home include skin-fold measurement and a body fat scale that uses bioelectrical impedance. Both methods can be off by as much as 8%. With these limitations, it’s not surprising that BMI continues to be the standard doctors use. That’s because it’s fast and easy to calculate.
The Bottom Line
For now, BMI is still the “standard” for measuring obesity and the risk of obesity-related complications, but waist circumference and body fat percentage are better measures. BMI measurements are typically less accurate if you’re athletic or lift weights. If you want a more accurate idea of where you fall on the continuum, consider getting your body fat percentage measured.
Live Science. “BMI Not a Good Measure of Healthy Body Weight, Researchers Argue”
Medscape Medical News. “Fat-to-Bone Ratio Promising New Measure of Obesity”
WebMD. “Diet and Weight Management: Body Fat Measurement: Percentage Vs. Body Mass”
Centers for Disease Control and Prevention. “About Adult BMI”