The case for WHtR
Concern about overreliance on BMI despite its limitations is not new. In 2015, an American Heart Association 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, which sets policies for the United Kingdom’s National Health Service, revised its guidancefor 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 Dr. 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.
Waist circumference and WHtR may become standard measures of adiposity in clinical practice over the next 5-10 years.
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 UT Southwestern Medical Center, Dallas.
WHtR vs. BMI
Another recent step forward for WHtR came 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 8,000 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 Dr. 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 Dr. 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, Dr. Almandoz said in an interview. “As we collect more data, there is increasing awareness of how imperfect BMI is.”