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Why BMI Calculation Is Flawed, and How This Scale Came to Define Obesity

On September 20, 2023   /   Uncategorized   /   Leave a comment

Posted from: https://www.everydayhealth.com/diet-nutrition/bmi/bmi-flaws-history-other-ways-measure-body-weight/

Many people have been there: You go to the doctor’s office, get your blood pressure, height, and weight taken, and the doctor relays the unfortunate news: Your BMI is too high, and you need to lose weight. BMI, or body mass index, has long been used as a way to assess body weight in the United States. The federal government uses the calculation to track obesity rates in the country, and according to this scale, 42.4 percent of American adults age 20 and older are obese, per the Centers for Disease Control and Prevention (CDC) . Adults can measure their BMI by taking their body weight in pounds, dividing that value by their height in inches squared, and multiplying the latter value by 703. Or check out the CDC’s BMI calculator if math isn’t your thing. Here’s what your number means, per the CDC : Below 18.5 is underweight. From 18.5 to 24.9 is normal. From 25 to 29.9 is overweight. And 30 and over is obese. If this formula seems complicated and somewhat arbitrary, that’s because it is. And many experts have started to question BMI’s accuracy. BMI is far from perfect, and as the years have passed, more and more science has emerged that reveals the flaws of this approach. Before delving into what exactly those imperfections are, it’s important to understand not only BMI’s history but also what your BMI number suggest about your health — and why you shouldn’t necessarily jump into panic mode if your doc says your number is too high.
BMI History in the United States History BMI initially came about because researchers, medical professionals, the government, and insurance companies needed a simple way to track health risk among people in the United States. In 1972, a researcher by the name of Ancel Keys coined the term “body mass index” in a paper he published titled “Indices of Relative Weight and Obesity.” That’s according to one article . In the study, he looked at about 7,400 men from five European countries and analyzed their adiposity-body density and subcutaneous fat thickness, two measures of body weight. Using a weight-to-height index devised by Adolphe Quetelet back in 1832 (called the “Quetelet index”), Keys came up with the body mass index as a straightforward way to measure body weight in relation to height. As people became more overweight and the health risks associated with being overweight became clearer, epidemiologists across the world began using Keys’s body mass index approach as a way to track disease risk factors in the general population. In 1985, the National Institutes of Health (NIH) started using BMI to define obesity in the United States. At first, the thresholds were more conservative, but by 1998, the NIH started using the aforementioned easier categories we know today to encompass all ages, both sexes, and every culture. The NIH set that standard in 1998, according to research .
How Experts Use BMI, and What Your Number May Say About Your Health Uses That simple number can be tied to your healthcare. A high BMI may compel your doctor to give you additional health screenings, prescribe certain drugs, and require additional follow-up appointments to monitor your weight. In this way, your BMI affects your healthcare costs. High BMI and Chronic Disease After all, research suggests that BMI is closely tied to disease risk. People who are considered overweight or obese are at a greater risk for chronic diseases such as heart disease, type 2 diabetes, stroke, and even cancer, according to the National Institute of Diabetes and Digestive and Kidney Diseases . High BMI and COVID-19 What’s more, in a study on adults in the United Kingdom published in August 2020 in PNAS , researchers concluded that as BMI increased, the risk for hospitalization from COVID-19 did, too. That increased risk may have been due to impaired glucose and lipid metabolism, the study suggested. Another study, in the August 2020 Obesity Reviews , found that people who have obesity are 48 percent more at risk of death from the infection than non-obese individuals. The bottom line? BMI in the traditional sense is used as a measure for health risk. It cannot, however, provide an indication of someone’s health habits. BMI does not tell you what someone eats, how they move their body, the quality of their sleep, or their stress levels — all things that influence health. What’s more, health habits go a long way in protecting you no matter your weight. For instance, in population-based research on middle-aged and older men and women, obese individuals who adhered to the Mediterranean diet did not have a higher overall mortality risk that is normally associated with a higher BMI, according to a study published in PLOS Medicine in September 2020 . That said, those who had a higher BMI did have an increased risk of death from heart disease. Study authors pointed out that lifestyle habits can mitigate some (but not all) of the health problems associated with being at a higher weight.
Why BMI Isn’t a Perfect Measure to Identify Overweight and Obese People Imperfections Despite its wide use, BMI is flawed. Increasingly, the scale has come under scrutiny as more and more people are being placed into its high-risk categories of overweight or obese. “Body mass index doesn’t show the difference between muscle and fat, so it doesn’t always accurately predict when weight could lead to health problems,” says Kim Larson, RDN , a health and wellness coach at Total Health in Woodinville, Washington. Groups for Which BMI May Be Inaccurate BMI is not always the best measure of body fat and overall health, especially if you’re in any of the following groups. Asian people The BMI scale is not a good measurement for individuals who are Asian, who may see increased health risks before their height and weight calculation qualifies as overweight (BMI of 25 or higher), per research . As the American Heart Association points out, Asian people are more likely to hold fat around their waistline, and this abdominal (called “visceral”) fat is especially dangerous. Because of this, doctors are encouraged to use a different BMI scale for Asians, with lower BMI cutoff points, per the Joslin Diabetes Clinic . Athletes People who are extremely active have heavier bones and more lean mass (think muscle), resulting in higher body weights and higher BMIs, according to a study . But lean muscle mass can actually increase metabolism, as well as help ward off conditions like heart disease and type 2 diabetes, according to another study . Thus, an athlete with a high BMI may not necessarily be unhealthy. Women who are pregnant or nursing Women typically have a higher body weight and body fat percentage when nursing or pregnant, per the CDC. This is so they can supply nourishment to the baby; it’s not an indication of long-term health risks, as the CDC notes. Nonpregnant women Compared with men, women generally have a higher percentage of body fat, according to research. People over age 65 A BMI of less than 23 in people older than 65 is associated with a higher relative health risk. And according to a previous meta-analysis, a BMI of 27 is the best in terms of decreased risk of mortality among this age group, per one paper . The reason for this is not fully known, but most likely it’s multifactorial. Those with a higher BMI tend to carry more pounds of muscle compared with those who have a lower BMI, which has a protective effect in terms of overall functionality, fall risk, and overall immunity. The CDC warns healthcare providers to not measure BMI with intentions to use it as a diagnostic tool. And yet there is an ICD-10 (diagnosis code) for overweight and obesity, suggesting some may still do so, according to the American College of Obstetricians and Gynecologists . The agency also points out that BMI doesn’t measure excess body fat, which may be a better indicator of your health state. Think of it this way: When you step on the scale, the number recorded (which is used to calculate your BMI) doesn’t account for how your body weight is composed or where on your body it’s distributed.
Is the BMI Scale Racist? Racism There is an increasing awareness, or spotlight, on the racist underpinnings of BMI. After all, this was a scale developed after following white European men — how can this also apply to everyone else, especially women who are part of the Black, Indigenous, and People of Color (BIPOC) community? “There is definite truth that the BMI was tested and validated in a white male population,” says Sara Bleich, PhD , a professor of public health policy at the Harvard T.H. Chan School of Public Health in Boston. Yet Dr. Bleich explains that BMI is still useful as a tool because of the way it has evolved over time. “If BMI were only used in its original form and not changed, then we’d have to worry,” says Bleich. “There are so many different studies that have looked at elevated BMI risk in many different populations and what it uniformly shows is that a higher BMI is bad for you. A major challenge is that, statistically, it’s normal in this country to be overweight, but each day when you’re walking around with excess weight, it’s increasing your risk for chronic conditions,” she says, adding that’s true no matter your race. What’s also important to point out is the stigmatization that results when one has a higher BMI across the board. “If you’re someone who has obesity, you tend to delay seeking care because you feel stigmatized. It becomes this vicious cycle where health conditions escalate, and by the time you’re seen, you’re in a worse place,” Bleich adds. As most physicians lack education and training for addressing obesity, they’re more likely to give unhelpful advice (“Exercise and eat healthy!”) that does not address the needs of patients. Racism itself may contribute to health issues. Structural racism is associated with several health disparities, including a higher BMI among Black people, according to a study published in the October 2020 American Journal of Preventive Medicine . (Structural racism, the authors explain, is “institutional policies and actions that result in detrimental treatment for members of a particular racial group.”) This link began to first emerge a decade ago, as research noted that while the rates of obesity were increasing, it was not happening in the same way for everyone. Educated white men were becoming obese at the slowest rate, while uneducated Black women’s obesity rates were rising the fastest. White people often have better access to health education, food, and physical activity; racism itself may also create chronic stress and inflammation that drives disease, the researchers note.
How Inaccurate Is BMI? The CDC says the correlation between BMI and body fatness is “fairly strong.” There have been a couple of studies, however, that have looked into BMI accuracy and how often the number successfully identifies obesity and when it fails. One study looked at more than 40,000 people across all population groups and reported that more than 30 percent of folks in the normal BMI category are cardio-metabolically unhealthy based on their blood pressure readings and metabolic labs, such as HDL (“good”) and LDL (“bad”) cholesterol, triglycerides, glucose, and C-reactive protein. In addition, nearly half of overweight people and 29 percent of obese people were healthy based on their health markers. The study authors estimated that as many as 74 million people who are considered to be unhealthy based on their BMI are, in fact, healthy according to these other parameters. Another PLoS One study found that nearly half the women participants and a quarter of the male participants who were obese based on body fat percentage did not qualify as such based on their BMI. In total, 39 percent of the study participants were obese based on biomarkers and duel-energy X-ray absorptiometry but found not to be obese by BMI alone. Sometimes the inaccuracy of BMI is due to misreporting of height. According to a different study , 29 percent of the study participants underestimated their height and nearly 15 percent overestimated it. That said, while BMI may have its flaws, obesity experts say that it is a useful tool. “Overall, the research shows that as a person’s BMI goes up, so, too, does their risk in all sorts of negative health outcomes, including type 2 diabetes, heart conditions, and hypertension,” says Bleich. “That’s compelling evidence that with increasing BMI, there are also increasing health risks.” Distinguishing Between Fat and Muscle Weight Another major problem with BMI is that the number accounts for your weight but there’s no telling whether the weight comes from fat or muscle or bone density, according to a previous article . This creates issues with BMI accuracy because muscle is denser than fat, so very muscular people may technically qualify as obese even though they don’t have a lot of fat on their bodies, notes the Harvard T.H. Chan School of Public Health . As mentioned, muscle is healthy, metabolically speaking. Where you store your fat on your body also needs to be considered when determining whether the fat will be detrimental to your health. People who have visceral fat tend to be at the highest risk for disease because the fat may be encasing vital organs. This abdominal fat is dangerous because it increases the risk of cardiovascular disease, diabetes, and major organ disease, notes a study .
Why Do Doctors Still Use BMI if There Are Flaws? As the CDC points out, BMI has some merit. It’s a simple, inexpensive, and noninvasive way to assess body weight. It can be calculated quickly and has been widely used, which is why it’s an easy way for statisticians to define obesity in general populations. Health practitioners should be aware of BMI’s downsides and should recognize the limitations. Even if doctors measure BMI at routine appointments, it shouldn’t be the only factor that’s used to diagnose someone as overweight or obese.
Alternatives to BMI Alternatives Most health professionals would agree that health risk should be measured by several factors. Among them are age, gender, medical history, any abnormal blood laboratory tests, and family history. It’s common for doctors and the general public to measure BMI, but the question, then, is this: Is there a better approach to help paint a picture of your health? Here are some contenders. Waist Circumference The National Heart, Lung, and Blood Institute recommends that your waist circumference be less than 40 inches for men and less than 35 inches for women. Waist-to-Height Ratio A waist-to-height ratio of more than 0.5 may put you at higher risk for heart disease and diabetes, per research . Waist-to-Hip Ratio The World Health Organization (WHO) categorizes high risk as a ratio above 0.85 for women and more than 0.9 for men. You can calculate your waist-to-hip ratio by taking your waist circumference and dividing it by your hip circumference. Body Fat Percentage You can measure this value with various methods, including skinfold, bioelectrical impedance (BIA), underwater weighing (hydrostatic), dual-energy X-ray absorptiometry (DXA), and isotope dilution. Skinfold and BIA values are easy to obtain but may be inaccurate. Meanwhile, hydrostatic and DXA are more accurate, but they can be costly, and the tools used to determine these values are less prevalent in clinical settings. Finally, isotope dilution is generally inexpensive and accurate, according to the Harvard T.H. Chan School of Public Health . Larson prefers waist circumference and waist-to-hip ratio for health risk screening. “I think the best way to increase BMI accuracy is to add waist circumference and waist-to-hip ratio to the mix,” she says.
Summary Summary BMI is one among many screening tools, and should not be used on its own to assess a person’s health risk. A BMI that’s considered too high doesn’t necessarily mean your health is doomed. All factors — including your personal and family health history as well as more detailed body measurements combined with the factors accounted for in BMI — need to be considered for a better picture of your present and future health. Additional reporting by Moira Lawler and Jessica Migala .

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