Let's Talk About Obesity and Body Mass Index (BMI)
Doctors have long used a measure called Body Mass Index (BMI) to diagnose obesity. Now, experts say there are better ways.
For decades, a simple mathematical formula has been used to help doctors diagnose obesity. It’s called Body Mass Index (BMI), and depending on how high your number is, it can result in a stigmatizing label that has implications in terms of insurance, access to various medical treatments, and emotional wellbeing.*
Our Pro Panel
We went to some of the nation's top experts on obesity to bring you the most up-to-date information possible.
Francisco Lopez-Jimenez, M.D.
Chair for the Division of Preventive Cardiology
Angela Fitch, M.D.
Associate Director of the Massachusetts General Hospital Weight Center, Vice President of the Obesity Medicine Association
Sharon Zarabi, R.D.
Registered Dietitian and Bariatric Program Director
Lenox Hill Hospital
New York, NY
For years, doctors have used a measure called Body Mass Index (BMI) to diagnose obesity. To determine your BMI, you divide your weight (in kg) by your height (in meters) squared. But today, experts say there are better ways to diagnose this ubiquitous condition, including waist circumference; waist-to-hip ratio; and electrical bioimpedance analysis.
A BMI of 30 or more is considered obese. Obesity is further divided by class:
Class 1 = BMI of 30.0 to 34.9
Class 2 = BMI of 35.0 to 39.9
Class 3, or severe obesity = BMI ≥ 40.0
Not so much. BMI critics—and there are many—believe it may be a misleading measurement. First, because BMI doesn’t distinguish fat from muscle, bone, or organ, it doesn’t measure body fat percentage, which is strongly linked with a person’s health. Nor does BMI factor in where you carry your weight. Belly fat, or fat that accumulates around the waistline and abdominal organs, greatly heightens one’s risk of heart disease, type 2 diabetes, and other obesity-related complications, as well as death. But a person could easily fall into the “normal” BMI category (18.5 to 24.9) and have a waist circumference that puts him or her in a high-risk health category. There are other criticisms, as well – these are just two.
In men, a waist circumference greater than 40 inches is considered to fall in the obesity range. For women, it’s a waist circumference greater than 35 inches.
*But this number alone doesn’t present the full picture of a person’s health, and experts say there are better ways to assess obesity and offer help for this prevalent public health issue.
What Exactly Is Obesity?
First, it might help to cover what obesity isn’t. Obesity isn’t indicative of laziness, love of ice cream, or lack of willpower. And it certainly shouldn’t be the butt of jokes or the cause for discrimination. Obesity is in fact a diagnosable disease, just like type 1 diabetes or multiple sclerosis. It occurs when your body accumulates and stores excessive amounts of fat, and it’s extremely prevalent.
In fact, 40% of the U.S. population is living with obesity and that number is slated to soar to nearly half of the American population by 2030, according to research in the New England Journal of Medicine. That’s especially troubling because obesity contributes to eight of the 10 leading causes of death in the United States.
How is the disease diagnosed? That’s typically done by calculating a person’s Body Mass Index (BMI), a rough estimate of whether their weight is healthy based on their height.
How Is BMI Calculated?
You’ve likely seen a BMI chart hanging in your doctor’s office at some point. It looks similar to an old-school pantyhose sizing chart, with increasing weight increments listed along the horizontal axis and height along the vertical one. But instead of color-coded boxes indicating what size you should purchase, BMI charts assign you a number that dictates whether or not you have obesity.
That number, your BMI, is calculated via the following mathematical equation (or you can just plug your numbers into this BMI calculator):
BMI = body weight (in kg) ÷ height (in meters) squared
A BMI of 25.0 to 29.9 for adults is considered overweight by most doctors.
A BMI of 30 or more is considered obese.
Obesity is further divided by class:
Class 1 = BMI of 30.0 to 34.9
Class 2 = BMI of 35.0 to 39.9
Class 3, or severe obesity = BMI ≥ 40.0
For instance, someone who is 5’4” and weighs 175 is considered obese (BMI = 30).
The same goes for someone who is 5’9” and weighs 204 pounds (BMI = 30.3).
BMI isn’t a perfect measure. One major reason why: It doesn’t distinguish how much muscle versus fat a person is carrying--therefore, a fit person and a sedentary person could have the same BMI. However, it is a good gauge of whether one’s body fat is in the healthy range.
Why Does BMI Exist?
The equation was developed in the early 19th century by a Belgian mathematician, sociologist, and astronomer named Lambert Adolphe Jacques Quetelet. Clearly an OG statistics geek, Quetelet was interested in calculating a way to describe l’homme moyen (“the average man”) and determined that, aside from the growth spurts that happen following birth and during puberty, “weight increases as the square of the height.” Accordingly, the calculation divides an individual’s weight (in kilograms) by the square of his or her height (in meters).
Because we use pounds and not kilograms in the U.S., the formula is altered to what you see above. (Divide your weight [in pounds] by your height [in centimeters] squared. Multiply that number by 703 = your BMI.)
Obesity existed in Quetelet’s day, but had yet to be linked with negative health consequences. In other words, his mathematical tinkering was not intended as a way to label people as healthy or not. Fast-forward to the late 1980s and early 1990s. Thanks to published studies linking obesity and other causes of death, everyone from epidemiologists to insurance companies began noticing that people with obesity were more likely to die sooner, and a wide range of professionals began using BMI as a measurement of fatness.
In 2000, the World Health Organization officially defined obesity as “the disease in which excess body fat has accumulated to such an extent that health may be adversely affected” and selected a BMI of 30 as the bottom threshold for defining obesity. That BMI, the WHO and other public health experts determined, is the point at which a significant uptick in mortality seemed to begin. That’s correct in the sense that scores of studies have shown that health risks do tend to rise along with BMI level. High BMI often predicts future illness, like:
And because BMI is so easy and inexpensive to calculate, it has been adopted by public health experts as a key diagnostic criteria to use when studying health at the population level, which involves looking at and comparing massive groups of people to determine why one group—those of a certain age, ethnicity, geographic location, or particular health behavior—may be healthier than another. That’s quite different than using BMI to diagnose obesity in individual patients.
The Problems With Using BMI to Diagnose Obesity
Can that same calculation that works so well in studying broad swaths of people also be used effectively to diagnose individual patients? This is a hot-button issue in medicine now. Arguments exist both for and against using BMI as a diagnostic tool. Those in favor appreciate that it’s a fast, easy, no-cost calculation, offering a quick but reliable snapshot of risk. But critics—and there are many—believe it may be a misleading measurement. Here are a few reasons why:
BMI doesn’t actually measure fat.
Obesity is defined as possessing excess body fat. But the number on the scale—the same figure used in calculating BMI—doesn’t distinguish fat from muscle, bone, organs, or any of the other major components of the human body. That means BMI often fails to capture a person’s actual body fat percentage. If you gathered 1000 people, all with a BMI of 25, then used more precise measuring techniques to assess their body fat percentage, you would find they ran the gamut from 15 to 45 percent.
Body fat percentage, not BMI, is largely responsible for a person’s health. Too high a number can pave the way toward obesity-related complications, including diabetes and heart disease. By relying on a person’s overall weight, BMI may overestimate body fat in athletic, muscular individuals or those with large or tall frames, labeling them as overweight or having obesity when in fact they’re far from it. However, the reverse is also true, which brings us to our next, more pressing point...
You can have a “normal” BMI but still have a high percentage of body fat—and the associated medical complications.
It sounds like an oxymoron, but normal weight obesity is a thing. More than half of U.S. adults currently considered to have a normal BMI actually have a high body fat percentage (more than 20 percent fat for men or 30 percent fat for women.) You may have heard this be referred to in the media as being “skinny-fat.”
Men with normal BMIs but more than 23 percent fat had higher rates of high blood pressure, diabetes, and cardiovascular disease compared with those with low body fat percentages, according to a study in the AMA Journal of Ethics. These men with normal weight obesity were also four times more likely to have metabolic syndrome, a constellation of cardiovascular risk factors, including high cholesterol, high blood pressure, and more. Women with normal BMIs but more than 33 percent body weight were seven times more likely to have metabolic syndrome.
BMI doesn’t factor in where you carry your weight.
The way in which fat is distributed in your body has major health implications, yet it’s not taken into account by the BMI calculation. Belly fat, or fat that accumulates around the waistline and abdominal organs, greatly heightens a person’s risk of heart disease, type 2 diabetes, and other obesity-related complications, as well as death. But a person could easily fall into the “normal” BMI category (18.5 to 24.9) and have a waist circumference that puts him or her in a risk health category.
Individuals with “normal,” or non-obese/non-overweight BMIs, but with a large waist circumference (known as central obesity) are much more likely to die prematurely compared with people who are deemed obese via BMI but don’t have a large circumference. What qualifies as central obesity?
A waist circumference greater than 40 inches for men
A waist circumference greater than 35 inches for women
A 2015 study in the Annals of Internal Medicine confirmed this exact phenomenon when researchers looked at more than 15,000 adults between the ages of 18 and 90 years, all of whom had “normal” BMIs. Those with central obesity had up to twice the risk of dying prematurely as participants who were overweight or obese according to BMI but did not have large waist circumferences.
What Are Other Ways of Diagnosing Obesity?
Many leading experts in the fields of obesity medicine and cardiology are calling for a more holistic approach to diagnosing obesity. They believe that BMI can be used as a starting point, correctly identifying a sizable percentage of patients as having obesity or not having obesity. For instance, generally speaking, if your BMI is 30 or higher, chances are you do have obesity. (Olympic athletes and NFL linebackers aside.) But there are other factors to consider, and other tests that can be performed to lend validity to a BMI assessment.
Superior Ways of Measuring Body Fat
If you grew up in the 90s, you’ll remember hydrostatic body fat testing was popular—people paying to be dunked in pools to see how much water they displaced. Today’s methods are far less cumbersome (and drier to boot):
Waist circumference: This measure, combined with BMI, could provide a more accurate measure of obesity. It’s inexpensive, but not foolproof, as one provider may hold the measuring tape in a slightly different position compared with another, skewing results. Waist circumference should be measured at the smallest area of one’s waist, just above the belly button. But in people with large bellies, it can be difficult to determine where the smallest part is.
Waist-to-hip ratio: This number divides waist circumference by hip circumference (measured around the widest part of your buttocks). Anything below 0.9 is considered healthy. A ratio of 1.0 or above is correlated with a two to three times increased risk of dying. Someone with a larger waist than hips will have a waist-to-hip ratio of 1.0 or above.
Electrical bioimpedance analysis (BIA): Using a mild electrical current to measure body fat sounds scary, but we promise it’s painless. It usually involves standing on a scale or monitor, which sends a low-level current through your feet. This current passes more quickly through water (such as that found in muscle) than it does through fat. Depending on how quickly the current makes its way through your body, BIA technology can accurately assess your body fat percentage. Several companies make at-home versions, including Tanita, Fitbit, and Nokia.
Dual-energy X-ray absorptiometry (DEXA): Widely used to measure bone density, DEXA is also considered the gold standard for body fat measurement. It’s essentially an enhanced X-ray. Though highly accurate, it isn’t always covered by insurance and involves a small dose of radiation.
Other Factors to Consider Besides Body Fat
Many professionals believe that a more progressive diagnostic tool for obesity would account for more than excess body fat. Regardless of what brings a patient to the doctor’s office, be it obesity, cancer, infertility, or any of the other thousands of conditions affecting the human body, we are all far more than just a number, and these experts want that to be factored into the diagnosis of obesity. A holistic screening would include:
Checking for the presence of adverse health effects caused by body fat, such as:
Questions about behavioral habits, including:
Questions about medications being taken, including those known to cause weight gain.
While these conversations aren’t happening nearly enough in healthcare providers’ offices, they are starting to become more common. You might find that asking your health care provider for a more holistic obesity exam is just the push he or she needs.
Lastly, remember that being diagnosed as having obesity doesn’t define you as a person. We live in a culture where obesity is highly stigmatized, both by the public as well as members of the medical community. More clinics are requiring personnel to enroll in obesity sensitivity training, to make sure they know the proper, most respectful way to address and work with this population, but until everyone has become educated (and agrees to implement that education), some practitioners will continue to treat their high-BMI patients as numbers. If you feel mistreated by your doctor, nurse, or dietitian because of your weight, it’s time to find another clinician.
- BMI Equation: Nephrol Dial Transplant. (2008). “Adolphe Quetelet (1796-1874)--the average man and indices of obesity.” ncbi.nlm.nih.gov/pubmed/17890752
- Negative Health Consequences: Advances in Chronic Kidney Disease/National Kidney Foundation. (2006). “ A history of obesity, or how what was good became ugly and then bad.” ackdjournal.org/article/S1548-5595(06)00106-6/fulltext
- World Health Organization Officially Defines Obesity: Journal of Obesity. (2016) “For Researchers on Obesity: Historical Review of Extra Body Weight Definitions.” ncbi.nlm.nih.gov/pmc/articles/PMC4904092/
- High BMI Often Predicts Disease: Centers for Disease Control and Prevention. (n.d.). “Body Mass Index: Considerations for Practitioners.” cdc.gov/obesity/downloads/bmiforpactitioners.pdf
- A Key Diagnostic Criteria: American Journal of Public Health. (2003). “What Is Population Health?” ncbi.nlm.nih.gov/pmc/articles/PMC1447747/
- BMI Doesn’t Actually Measure Fat: American College of Cardiology. (2015). “Body Mass Index Fails to Accurately Measure Body Fat.” cardiosmart.org/News-and-Events/2015/07/Body-Mass-Index-Fails-to-Accurately-Measure-Body-Fat
- BMI May Overestimate Body Fat: National Heart, Lung, and Blood Institute. (n.d.). “BMI Tools.” nhlbi.nih.gov/health/educational/lose_wt/bmitools.htm
- Normal Weight Obesity: ScienceDaily. (2008). “Normal Weight Obesity: An Emerging Risk Factor For Heart And Metabolic Problems. sciencedaily.com/releases/2008/03/080327172025.htm
- Risks of Normal Weight Obs: European Heart Journal. (2009). “Normal weight obesity: a risk factor for cardiometabolic dysregulation and cardiovascular mortality.” academic.oup.com/eurheartj/article/31/6/737/418220