Imagine you’re a serious athlete who trains your butt off nearly every day and is in tremendous overall shape. Now imagine you’ve just been told you’re obese. Thanks to BMI (Body Mass Index), this scenario isn’t just a hypothetical—it’s a reality for many athletes.
BMI is currently the most common way for someone to receive statistical feedback on the health of their body composition, and it’s defined by the National Institute of Health as a “useful measure of overweight and obesity.” But for all its popularity, BMI is far from perfect. Here’s why BMI is meaningless for many athletes and fitness-minded people.
The BMI was first created in the early 19th Century by a mathematician named Adolphe Quetelet, who believed the average healthy weight for a person increased along with his or her height.
Thus was born the “Quetelet Index,” a basic formula designed to help people of all sizes determine if they were maintaining a healthy weight. With some slight variations and a name change (in 1972) to the “Body Mass Index,” Quetelet’s creation lives on today.
Although the vast majority of people use online BMI calculators, which require you simply to punch in your height and weight to calculate your BMI, the actual formula behind BMI exposes its biggest issue. The BMI formula is as follows:
Weight in Pounds x 703
Heights in Inches2
To spell it out, the BMI formula is your weight in pounds times 703 divided by your height in inches squared.
As you can see, BMI is based on two simple factors—height and weight. It takes no account of body-fat percentage, muscle mass, bone thickness or genetic predisposition to a certain frame.
“BMI doesn’t take into account above-average amounts of lean muscle mass. It assumes everyone has the same percentage of lean tissue and fat tissue,” says Tony Bonvechio, strength coach at Cressey Sports Performance. And therein lies the main problem with using BMI for athletes. Most athletes have much more lean muscle mass than the average person, and that simple fact throws a gigantic wrench into the basic assumptions behind the BMI formula.
That’s why insanely in-shape athletes like Rob Gronkowski and Clay Matthews are classified as obese based on their BMIs, 30.6 and 31.9, respectively. Do you want to tell this guy he’s technically obese? Me neither.
And not just hulking football players are classified as having an unhealthy weight. Athletes like Giancarlo Stanton and Russell Westbrook are both “overweight” according to the BMI formula. When an athlete finds out he or she is judged “overweight” or “obese” under the BMI system, he or she might panic and drastically change his or her training in an effort to lose weight—even though their body composition may be quite good as it is. This is a destructive pattern of behavior in terms of performance!
The NIH does admit one of BMI’s limitations: that it “may overestimate body fat in athletes and others who have a muscular build.” That information is not immediately apparent on their website (or on countless government pages similar to it), but it’s hard to put lay too much blame on them for encouraging people to care about their BMI. Their mission is to educate the nation on health, and for the majority of people out there, BMI works pretty well.
STACK Velocity Sports Performance coach Aaron Bonaccorsy says, “BMI is one of those tests that works well for the general population, and on a large scale is probably fairly accurate. However, athletes are the outliers of the large scale.”
The big reason why BMI has become the most common measurement of body fat is because it does work for the majority of people—because the majority of people are not athletes. But when you routinely see studies concluding that a high BMI indicates poor health and is associated with a higher rate of mortality, it can be easy to get nervous—even if you’re an athlete.
Luckily, studies that use more exact body measurement methods are finding that body fat percentage is a more accurate predictor of undesirable health outcomes—not BMI. For example, recent research conducted by UCLA found that higher levels of muscle mass were associated with lower risk of death.
Your BMI is simple to discover, non-invasive and fairly accurate for most of the population. But it doesn’t work for many athletes or fitness-minded people, due to their above average level of muscle mass. And given the fact that many younger people are devoted to fitness and interested in adding muscle, it seems like the usefulness of BMI will continue to decline in the years to come.
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BMI is not a reliable measurement of body fat for athletes, but other methods can offer more accurate results. “Percent of body fat is a far better indicator of both health and athleticism of an athlete than BMI,” Bonaccorsy says.
Body fat percentage can be measured in several ways.
The BOD POD, a machine that uses air displacement technology to measure overall body composition, is used to analyze prospects at the NFL Combine. However, using one requires you to plunk down roughly $45, and finding a location with a BOD POD can be a significant challenge.
Hydrostatic weighing, also known as underwater weighing, is about as accurate as the BOD POD, but it also has issues of price and convenience. The most accessible method for many people is to use body fat measuring calipers to perform a skin fold assessment. A pair of calibers costs around 10 bucks, and if you can find someone who has prior experience using them to measure body fat, they can be highly accurate. This video teaches you more about performing a skin fold assessment using calipers.
It’s also important to realize that if you’re already performing at a high level and putting in the right kind of training, your body composition should not be an especially high priority. “If an athlete is performing well, and that’s their number 1 priority, they shouldn’t worry about their body composition unless it poses a real health risk,” Bonaccorsy says.
But if you believe your body composition is significantly holding you back, it’s worth trying to make some changes. “Having that extra 10 percent of body fat means every step you take, every drill you run, every time you move your body, you are carrying an extra 10 percent that your body is working harder to carry. It adds up,” says Ben Boudro, owner of Xceleration Sports.
The bottom line: if you’re performing well in your sport, and you and your coach are happy with your body composition, there’s no need to obsess about your BMI number. Continue to train hard and eat right, and your body will become increasingly optimized for your sport. On the other hand, if you or your coach believes your body composition is preventing you from being your best, and that belief is backed up by a reliable measure of body composition (preferably something other than BMI), making changes to your routine and diet makes sense.