Obesity has been declared an epidemic and a disease. More than one third (35%) of adult men in the United States meet the classification for obesity, and 5.5% are morbidly obese. Prevalence among women is even higher—with 40.4% obese and 9.9% morbidly obese, according to a 2016 analysis of data from the National Health and Nutrition Examination Survey.

Obesity is linked to increased rates of mortality, type 2 diabetes mellitus, and cardiovascular disease. And those rates are growing. However, not all obese people have an increased cardiometabolic risk. These individuals have metabolically healthy obesity (MHO), and they don’t seem to have a higher risk of cardiovascular disease than individuals with healthy weight.

How is this possible? The issue is hotly debated, but a recent article in JAMA suggests that physical fitness may be an important factor in overcoming obesity’s higher risk of morbidity and mortality.

What is MHO?

Metabolically healthy obesity was first described in 2001. Since then, researchers have discovered that MHO is prevalent, comprising up to 25% or more of the adult obese population, researchers noted in a 2013 study. Despite its history and prevalence, authorities still haven’t agreed on what defines MHO as a phenotype.

But the jury is still out on whether people with MHO have reduced mortality compared with other obese people. Some studies have found that those with MHO may have a lower risk for mortality and cardiovascular events compared with metabolically unhealthy obese people, but a greater risk than metabolically healthy people of normal weight.

Then again, most studies that linked MHO with increased mortality risk didn’t include assessments of cardiorespiratory fitness or physical activity, noted authors led by Steven N. Blair, PED, professor, Departments of Exercise Science and Epidemiology and Biostatistics, Arnold School of Public Heath, University of South Carolina, Columbia, SC, in a recent editorial in JAMA. (This issue of JAMA was devoted entirely to obesity.)

“When cardiorespiratory fitness is rigorously measured and other important confounders are considered, relatively fit individuals with MHO are not at significantly higher risk of cardiovascular disease morbidity or mortality than metabolically healthy normal-weight individuals,” Dr. Blair and colleagues wrote. 

Cardiorespiratory fitness greatly influences the “obesity paradox”—the counterintuitive phenomenon that obese individuals have a lower risk of mortality for coronary heart disease and heart failure than normal-weight individuals.

“These observations should not be interpreted to suggest that obesity should be promoted,” cautioned Dr. Blair and colleagues. “However, in current society, many people progressively gain weight and lose cardiorespiratory fitness as they age. Conceivably, maintaining cardiorespiratory fitness may be more important than preventing the development of obesity.”

Obesity is not destiny

Indeed, authors of a 2017 article in The Lancet Diabetes & Endocrinology suggested that one practical way to improve the health of obese individuals is to aim, at a minimum, at achieving metabolically healthy obesity. Ideally, “every person with obesity should be motivated to achieve a normal weight in the long term, but more moderate weight loss sufficient for the transition from metabolically unhealthy obesity to metabolically healthy obesity might also lower the risk of adverse outcomes,” they wrote.

This advice underscores the hypothesis that MHO isn’t so much a discrete condition but perhaps more like a point on a continuum. “The metabolically healthy obesity phenotype is a transient condition for many participants; 30.6% of people who initially had metabolically healthy obesity developed metabolically unhealthy obesity over a 10-year period,” the authors noted. Sumo wrestlers, for example, often transfer from a metabolically healthy to an unhealthy state upon retirement from the sport, according to a researcher in Japan.

Fitness vs. weight loss

Physical activity and cardiorespiratory fitness might be the answer—or, at least, the answer for now.

“The harms of obesity have been emphasized for many years, yet the population continues to become more obese,” wrote Edward H. Livingston, MD, JAMA Deputy Editor, Clinical Review and Education, in an editorial in the recent obesity issue of JAMA. Because dieting isn’t effective, he noted, overweight and obese patients should at least be encouraged to exercise more and be in better physical condition.

“It is possible that a different message, one encouraging physical fitness, may do more to improve individual and population health than continuing to advise weight loss when that message is increasingly ignored,” Dr. Livingston suggested.

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