BMI may be a flawed metric to assess cardiovascular risk

Guidelines recommend measuring body mass index (BMI) to identify people who are at risk for cardiovascular disease, type 2 diabetes and all-cause mortality. Healthcare professionals almost always follow the suggestions and evaluate BMI.

Ian J. Neeland, MD, and James A. de Lemos, MD, of the University of Texas Southwestern Medical Center in Dallas, however, believe that BMI is a flawed metric and may not be the best way to assess risk. Neeland and de Lemos expressed their views in a Sept. 26 editorial in the Journal of the American College of Cardiology.

They cited a study published in that same issue that found measuring abdominal subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) may be effective in identifying cardiovascular disease risk factors. Increases in SAT and VAT volume were associated with worsening cardiovascular disease risk factors, which persisted after the researchers adjusted for BMI, waist circumference and baseline adipose tissue volume.

During a mean follow-up period of 6.1 years, the patients had a 22 percent increase in SAT and 45 percent increase in VAT.

“It is important to note that the vast majority of abdominal adipose tissue is stored in SAT with the VAT depot usually representing one-third or less of total abdominal fat,” Neeland and de Lemos wrote. “The strikingly discordant relative increase in VAT compared with SAT in the present study should clue us in to a major premise—that the inability to expand the SAT depot in the face of caloric surplus, resulting in the greater relative expansion of VAT, may be the underlying physiological derangement predisposing to worsening [cardiovascular disease] risk factors.”

Neeland and de Lemos identified a few potential flaws with BMI, including that some people who are overweight or mildly obese may not have increased mortality compared with people who have a normal weight. Approximately one-third of obese adults has one or fewer cardiometabolic risk factor and do not have cardiometabolic disease, according to the researchers. They also noted that BMI is not part of the Framingham or Pooled Cohort Equation cardiovascular disease risk scores.

Meanwhile, they wrote that healthcare professionals could identify SAT and VAT with computed tomography or magnetic resonance imaging. They cited studies that found researchers could modify SAT and VAT to select the appropriate intervention and monitor response to therapy.

The study did not model SAT and VAT changes jointly, so Neeland and de Lemos could not determine if the associations with SAT were independent of VAT or if they would be significantly attenuated when they took the VAT’s confounding impact into consideration.

“It will be crucial for future outcomes studies to assess the relationships of specific fat compartments jointly to better evaluate the individual depot contributions to risk,” they wrote.

Although studies have shown adipose tissue imaging provides information about cardiometabolic risk that BMI cannot measure, Neeland and de Lemos acknowledged that the invasive nature of tissue biopsy or cell culture assay may be an issue.

“The ultimate question is whether it is possible to translate evaluation of 'form' (the anatomical measurement of adipose depots) to assessment of 'function,' to unravel the pathophysiology of adipose tissue expansion, and circumvent the inherent limitations of BMI and isolated imaging-based fat quantification,” they wrote.