BMI ‘Vastly Underestimates’ True Obesity

Twice as many US adults have obesity based on assessment of their fat volume by DEXA scan compared with measurement of body mass index (BMI), a finding that highlights the shortcomings of BMI and adds to the growing case that BMI alone should not be the default gauge for obesity.

“BMI vastly underestimates true obesity,” Aayush Visaria, MD, said at ENDO 2023: The Endocrine Society Annual Meeting.

His findings highlight that “BMI should be supplemented with other measures of obesity” for the management of individual patients, with assessments that could include a bioelectrical impedance scale or waist circumference, said Visaria, who is a researcher at Rutgers Robert Wood Johnson Medical School in New Brunswick, New Jersey.

Visaria cited a new policy issued by the American Medical Association a couple of days before his presentation, as reported by Medscape Medical News, which advises that BMI “be used in conjunction with other valid measures of risk such as, but not limited to, measurements of visceral fat, body adiposity index, body composition, relative fat mass, waist circumference, and genetic/metabolic factors.”

“We’re at the start of the end of BMI,” declared Visaria during a press briefing at the ENDO meeting.

He said that dual-energy X-ray absorptiometry (DEXA) is not practical nor cost-effective for obesity screening in routine practice. Therefore, he predicts that waist circumference, often expressed as waist-to-height ratio, will be measured more often, although he acknowledged that waist measurement can be difficult. However, better physician training on the measure should help it become the norm.

Another useful tool for obesity measurement he foresees quickly becoming widespread is bathroom scales that record both weight and body fat percentage using a small electric current to make a bioelectrical impedance measure of adiposity.

Bioimpedance scales will provide more standardized measurements than waist circumference and “revolutionize how we measure obesity,” Visaria predicted. They are “very accessible and cheap,” he noted, with many models sold for less than $100. 

Obesity Prevalence of 74%

The study by Visaria and colleagues used data from 9784 US adults ages 20-59 years (average age, 39 years) collected in several National Health and Nutrition Examination Surveys in 2011-2018. All these participants underwent DEXA assessment of their total body fat as well as a BMI calculation.

Using standard obesity cutoffs for both BMI and total body fat, Visaria found that DEXA rated 74% of participants as having obesity based on body fat compared with 36% based on BMI.

Among the 64% of the study group who were not obese by BMI, DEXA scans showed 53% of this subgroup did have obesity based on body fat content. Among those with a normal BMI, 43% had obesity by DEXA result.

Further analysis showed that when Visaria added waist circumference to BMI to enlarge the diagnostic net for obesity it cut the percentage of adults missed as having obesity by BMI alone nearly in half.

Additional analyses showed that the rate of missed diagnoses of obesity by BMI was only most common among people of Hispanic or Asian ethnicity, with both groups showing a 49% rate of obesity by DEXA among those with normal-range BMIs.

The rate of missed obesity diagnoses was highest among all women, with a 59% prevalence of obesity by DEXA among women with a normal-range BMI.

The study received no commercial funding. Visaria has reported no relevant financial relationships.

ENDO 2023. Abstract OR10-01. Presented June 16, 2023.

Mitchel L. Zoler is a reporter for Medscape and MDedge based in the Philadelphia area. @mitchelzoler

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