"BMI is trash. Full stop." This controversial tweet, which received thousands of likes and retweets, was cited in a recent Medscape perspective by one doctor on when physicians might stop using body mass index (BMI) to diagnose obesity.
Body mass index (BMI) has for years been the consensus default method for assessing whether a person is overweight or has obesity, and is still widely used as the gatekeeper metric for treatment eligibility for certain weight-loss agents and bariatric surgery.
But growing appreciation of the limitations of BMI is causing many clinicians to consider alternative measures of obesity that can better assess both the amount of adiposity as well as its body location, an important determinant of the cardiometabolic consequences of fat.
Alternative metrics include waist circumference and/or waist-to-height ratio (WHtR); imaging methods such as computed tomography (CT), magnetic resonance imaging (MRI), and dual-energy X-ray absorptiometry (DEXA); and bioelectrical impedance to assess fat volume and location. All have made some inroads on the tight grip BMI has had on obesity assessment.
Chances are, however, that BMI will not fade away anytime soon given how entrenched it has become in clinical practice and for insurance coverage, as well as its relative simplicity and precision.
"BMI is embedded in a wide range of guidelines on the use of medications and surgery. It's embedded in Food and Drug Administration regulations and for billing and insurance coverage. It would take extremely strong data and years of work to undo the infrastructure built around BMI and replace it with something else. I don't see that happening [anytime soon]," commented Daniel H. Bessesen, MD, a professor at the University of Colorado in Aurora and chief of endocrinology for Denver Health.
"It would be almost impossible to replace all the studies that have used BMI with investigations using some other measure," he said.
BMI Is "Imperfect"
The entrenched position of BMI as the go-to metric doesn't keep detractors from weighing in. As noted in a commentary on current clinical challenges surrounding obesity recently published in Annals of Internal Medicine, the journal's editor-in-chief, Christine Laine, MD, and senior deputy editor Christina C. Wee, MD, list six top issues clinicians must deal with, one of which, they say, is the need for a better measure of obesity than BMI.
"Unfortunately, BMI is an imperfect measure of body composition that differs with ethnicity, sex, body frame, and muscle mass," note Laine and Wee.
BMI is based on a person's weight in kilograms divided by the square of their height in meters. A "healthy" BMI is between 18.5 to 24.9 kg/m2, overweight is 25-29.9 kg/m2, and ≥ 30 kg/m2 is considered to represent obesity. However, certain ethnic groups have lower cutoffs for overweight or obesity because of evidence that such individuals can be at higher risk of obesity-related comorbidities at lower BMIs.
"BMI was chosen as the initial screening tool [for obesity] not because anyone thought it was perfect or the best measure but because of its simplicity. All you need is height, weight, and a calculator," said Wee in an interview.
Numerous online calculators are available, such as this one from the US Centers for Disease Control and Prevention, where height in feet and inches, and weight in pounds, can be entered to generate the BMI.
BMI is also inherently limited by being "a proxy for adiposity" and not a direct measure, added Wee, who is also director of the Obesity Research Program of Beth Israel Deaconess Medical Center in Boston, Massachusetts.
As such, BMI can't distinguish between fat and muscle because it relies on weight only to gauge adiposity, noted Tiffany Powell-Wiley, MD, an obesity researcher at the National Heart, Lung, and Blood Institute in Bethesda, Maryland. Another shortcoming of BMI is that it "is good for distinguishing population-level risk for cardiovascular disease and other chronic diseases, but it does not help as much for distinguishing risk at an individual level," she said in an interview.
These and other drawbacks have prompted researchers to look for other useful metrics. WHtR, for example, has recently made headway as a potential BMI alternative or complement.
The Case for WHtR
Concern about over-reliance on BMI despite its limitations is not new. In 2015, an American Heart Association (AHA) scientific statement from the group's Obesity Committee concluded that "BMI alone, even with lower thresholds, is a useful but not an ideal tool for identification of obesity or assessment of cardiovascular risk," especially for people from Asian, Black, Hispanic, and Pacific Islander populations.
The writing panel also recommended that clinicians measure waist circumference annually and use that information along with BMI "to better gauge cardiovascular risk in diverse populations."
Momentum for moving beyond BMI alone has continued to build following the AHA statement.
In September 2022, the National Institute for Health and Care Excellence (NICE), which sets policies for the UK's National Health Service, revised its guidance for assessment and management of people with obesity. The updated guidance recommends that when clinicians assess "adults with BMI below 35 kg/m2, measure and use their WHtR, as well as their BMI, as a practical estimate of central adiposity and use these measurements to help to assess and predict health risks."
NICE released an extensive literature review with the revision, and based on the evidence, said that "using waist-to-height ratio as well as BMI would help give a practical estimate of central adiposity in adults with BMI under 35 kg/m2. This would in turn help professionals assess and predict health risks."
However, the review added that "because people with a BMI over 35 kg/m2 are always likely to have a high WHtR, the committee recognized that it may not be a useful addition for predicting health risks in this group." The 2022 NICE review also said that it is "important to estimate central adiposity when assessing future health risks, including for people whose BMI is in the healthy weight category."
This new emphasis by NICE on measuring and using WHtR as part of obesity assessment "represents an important change in population health policy," commented Powell-Wiley. "I expect more professional organizations will endorse use of waist circumference or waist-to-height ratio now that NICE has taken this step," she predicted.
The recent move by NICE to highlight a complementary role for WHtR "is another acknowledgment that BMI is an imperfect tool for stratifying cardiometabolic risk in a diverse population, especially in people with lower BMIs" because of its variability, commented Jamie Almandoz, MD, medical director of the Weight Wellness Program at the University of Texas Southwestern Medical Center in Dallas.
WHtR vs BMI
Another recent step forward for WHtR came in March with the publication of a post-hoc analysis of data collected in the PARADIGM-HF trial, a study that had the primary purpose of comparing two medications for improving outcomes in more than 8000 patients with heart failure with reduced ejection fraction.
The new analysis showed that "two indices that incorporate waist circumference and height, but not weight, showed a clearer association between greater adiposity and a higher risk of heart failure hospitalization" compared with BMI.
WHtR was one of the two indices identified as being a better correlate for the adverse effect of excess adiposity compared with BMI.
The authors of the post-hoc analysis did not design their analysis to compare WHtR with BMI. Instead, their goal was to better understand what's known as the "obesity paradox" in people with heart failure with reduced ejection fraction: the recurring observation that when these patients with heart failure have lower BMIs they fare worse, with higher rates of mortality and adverse cardiovascular outcomes compared with patients with higher BMIs.
The new analysis showed that this paradox disappeared when WHtR was substituted for BMI as the obesity metric.
This "provides meaningful data about the superiority of WHtR compared with BMI for predicting heart failure outcomes," said Powell-Wiley, although she cautioned that the analysis was limited by scant data in diverse populations and did not look at other important cardiovascular disease outcomes. While Powell-Wiley does not think that WHtR needs assessment in a prospective, controlled trial, she called for analysis of pooled prospective studies with more diverse populations to better document the advantages of WHtR over BMI.
The PARADIGM-HF post-hoc analysis shows again how flawed BMI is for health assessment and the relative importance of an individualized understanding of a person's body composition, said Almandoz in an interview. "As we collect more data, there is increasing awareness of how imperfect BMI is."
Measuring Waist Circumference Is Tricky
Although WHtR looks promising as a substitute for or add-on to BMI, it has its own limitations, particularly the challenge of accurately measuring waist circumference.
Measuring waist circumference "not only takes more time but requires the assessor to be well-trained about where to put the tape measure and making sure it's measured at the same place each time," even when different people take serial measurements from individual patients, noted Wee. Determining waist circumference can also be technically difficult when done on larger people, she added, and collectively these challenges make waist circumference "less reproducible from measurement to measurement."
"It's relatively clear how to standardize measurement of weight and height, but there is a huge amount of variability when the waist is measured," agreed Almandoz. "And waist circumference also differs by ethnicity, race, sex, and body frame. There are significant differences in waist circumference levels that associate with increased health risks" between, for example, White and South Asian people.
Another limitation of waist circumference and WHtR is that they "cannot differentiate between visceral and abdominal subcutaneous adipose tissue, which are vastly different regarding cardiometabolic risk, commented Ian Neeland, MD, director of cardiovascular prevention at the University Hospitals Harrington Heart & Vascular Institute in Cleveland, Ohio.
The Imaging Option
"Waist-to-height ratio is not the ultimate answer," Neeland said in an interview. He instead endorsed "advanced imaging for body fat distribution," such as CT or MRI scans, as his pick for what should be the standard obesity metric, "given that it is much more specific and actionable for both risk assessment and response to therapy. I expect slow but steady advancements that move away from BMI cutoffs, for example for bariatric surgery, given that BMI is an imprecise and crude tool," he predicted.
But although imaging with methods like CT and MRI may provide the best accuracy and precision for tracking the volume of a person's cardiometabolically dangerous fat, they are also hampered by relatively high cost and, for CT and DEXA, the issue of radiation exposure.
"CT, MRI, and DEXA scans give more in-depth assessment of body composition, but should we expose people to the radiation and the cost?" Almandoz wondered.
"Height, weight, and waist circumference cost nothing to obtain," creating a big relative disadvantage for imaging, said Naveed Sattar, MD, professor of metabolic medicine at the University of Glasgow, UK.
"Data would need to show that imaging gives clinicians substantially more information about future risk" to justify its price, Sattar emphasized.
BMI's Limits Mean Adding On
Regardless of whichever alternatives to BMI end up getting used most, experts generally agree that BMI alone is looking increasingly inadequate.
"Over the next 5 years, BMI will come to be seen as a screening tool that categorizes people into general risk groups" that also needs "other metrics and variables, such as age, race, ethnicity, family history, blood glucose, and blood pressure to better describe health risk in an individual," predicted Bessesen in an interview.
The endorsement of WHtR by NICE "will lead to more research into how to incorporate WHtR into routine practice. We need more evidence to translate what NICE said into practice," said Sattar. "I don't think we'll see a shift away from BMI, but we'll add alternative measures that are particularly useful in certain patients," he predicted.
"Because we live in diverse societies, we need to individualize risk assessment and couple that with technology that makes analysis of body composition more accessible," agreed Almandoz. He noted that the Weight Wellness Program where he practices at Southwestern in Dallas has, for about the past decade, routinely collected waist circumference and bioelectrical impedance data as well as BMI on all people seen in the practice for obesity concerns. Making these additional measurements on a routine basis also helps strengthen patient engagement, he noted.
"We get into trouble when we make rigid health policy and clinical decisions based on BMI alone without looking at the patient holistically," said Wee. "Patients are more than arbitrary numbers, and clinicians should make clinical decisions based on the totality of evidence for each individual patient."
Bessesen, Wee, Powell-Wiley, and Almandoz have reported no relevant financial relationships. Neeland has reported being a consultant for Merck. Sattar has reported being a consultant or speaker for Abbott Laboratories, Afimmune, Amgen, AstraZeneca, Boehringer Ingelheim, Eli Lilly, Hanmi Pharmaceuticals, Janssen, MSD, Novartis, Novo Nordisk, Pfizer, Roche Diagnostics, and Sanofi.
Mitchel L. Zoler is a reporter for Medscape and MDedge based in the Philadelphia area. @mitchelzoler
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Image 2: University of Texas Southwestern Medical Center
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Cite this: BMI Is a Flawed Measure of Obesity. What Are Alternatives? - Medscape - Apr 26, 2023.