‘Metabolically Healthy Obesity’ Rising, but Still Uncommon

The proportion of Americans with “metabolically healthy obesity” has risen along with the general obesity increase over the past two decades, but the “healthy” numbers remain low, new data suggest.

Figures from the National Health and Nutrition Examination Survey (NHANES) indicate that “metabolically healthy obesity” — defined as obesity without any associated metabolic disorders, such as hypertension or raised blood glucose — had more than doubled in the overall population from 1999 to 2018, but still remained a small proportion of the overall rising population with obesity.

Moreover, the rise in “metabolically healthy” obesity occurred primarily among people aged 60 and older, men, and White adults, as well as those with higher income, private insurance, or class 1 obesity.

And during the study period, the overall prevalence of obesity rose from 28.6% to 40.9% of the population, along with the corresponding increase in “metabolically unhealthy” obesity.

“We observed a low prevalence of [metabolically healthy obesity] and a large, increasing burden of [metabolically unhealthy obesity]….These results highlight the need for effective strategies to…prevent obesity-related complications among people with obesity, especially among vulnerable subpopulations,” say Jiang-Shui Wang, MBBS, of Huazhong University of Science and Technology, Wuhan, China, and colleagues, who published their findings March 9, 2023, in JAMA Network Open.

Asked to comment, former Obesity Society President Steven B. Heymsfield, MD, told Medscape, “Metabolically healthy obesity is sort of like a warning light — you’re overweight and very lucky.…It’s not just a law of nature that the minute you’re overweight or obese all these bad things happen. You’re bringing obesity onto your genetic makeup, and that can be highly variable between people.…You can also have metabolic syndrome and absolutely normal weight, too.”

Is ‘Metabolically Healthy Obesity’ a Useful Concept?

Heymsfield explained that the concept of “metabolically healthy obesity” is of limited usefulness since a person’s risk factor profile can shift with time.

“You’re basing a definition of health on what someone looks like at that moment and what those various risk factors are.…Obesity is a disease with a long gestational period. It remains subclinical for many years.…When people get older you begin to see things manifest clinically that you didn’t see when they were 20 or 30…not in everyone, but in a lot of people.”

Moreover, he noted, many people can have a high body mass index (BMI) due to large bone structure and/or large muscle mass, with relatively low fat mass. “That’s not so uncommon in the BMI range around 30 kg/m2, especially in younger people. At a BMI of 30 the range of body fatness is very wide.”

For that reason, he advises clinicians to measure not only waist circumference, which is recommended but not often done clinically, but also hip circumference to determine the waist-hip ratio (WHR).

This measure is supported by the World Health Organization (WHO) as both a more accurate assessment of obesity than BMI as well as a better predictor of cardiovascular risk (WHR ≥ 0.90 cm in men and ≥ 0.85 cm in women identifies significant metabolic risk, according to the WHO), said Heymsfield, of Pennington Biomedical Research Center, Baton Rouge, Louisiana.  

So in the case of a person with a BMI of 30 kg/m2 and a normal waist circumference or waist-to-hip ratio with no other metabolic risk factors, “that person is probably not obese or overfat. They’re probably muscular. That may not be totally clear on the surface,” he observed.

Metabolically Healthy Obesity Rises, but Prevalence Remains Low

The NHANES is a series of nationally representative US surveys conducted in 2-year cycles, and the researchers examined data from 1999 to 2000 and from 2017 to 2018.

“Metabolically healthy obesity” was defined as a BMI of 30 kg/m2 or higher without any metabolic disorders, such as high blood pressure, increase in fasting plasma glucose, or changes in lipids, based on established cutoffs.

Included were 20,430 adult participants, of whom 7386 had obesity. For the entire study population, the age-standardized prevalence of obesity increased significantly from 28.6% in 1999-2002 to 40.9% in 2015-2018 (P for trend < .001).

At the same time, age-standardized prevalence of metabolically healthy obesity approximately doubled, rising from 3.2% to 6.6% (P < .001).

And among just those with obesity, the increase in prevalence of the metabolically healthy increased from 25.4% to 34.3% (P < .001).

“Our results suggest that the overall increase in metabolically healthy obesity was driven primarily by the decrease in dyslipidemia among adults with obesity,” possibly as a result of increased awareness and treatment, say Wang and colleagues.

Elevated blood pressure remained stable and elevated fasting plasma glucose increased, with the researchers stressing that “priority should be placed on reinforcing glucose management and reducing insulin resistance among individuals with obesity.”

The study was supported by grants from the National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities, and the Shanghai Municipal Science and Technology Major Projects. The authors report no further relevant financial relationships. Heymsfield is on an advisory board for Novo Nordisk, owns stock in Merck, and receives research funding from Pfizer and Versanis.

JAMA Network Open. Published online March 9, 2023. Full text.

Miriam E. Tucker is a freelance journalist based in the Washington, DC, area. She is a regular contributor to Medscape, with other work appearing in The Washington Post, NPR’s Shots blog, and Diabetes Forecast magazine. She is on Twitter @MiriamETucker.

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