Medical Mystery: Why Are Some Obese People 'Metabolically Healthy'?

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Obesity often brings with it a host of health problems, such as high blood pressure, diabetes and risky cholesterol levels. But a lucky few appear to buck the trend: They are obese, and yet don't have any of these typical risk factors for heart disease or diabetes, a new study finds.

Researchers analyzed information from about 1.3 million U.S. adults who were either overweight or obese. None had previously been diagnosed with diabetes. The researchers looked to see whether these participants had any of four common risk factors for heart disease and diabetes: High blood pressure, high levels of fat in the blood, low levels of "good" cholesterol or elevated blood sugar levels.

Among those who were obese, 10 percent did not have any of these four risk factors.

It's not clear why some people with obesity are able to avoid these problems. In the past, researchers have dubbed this group the "metabolically healthy obese."

However, people who fall into this group may still not be totally healthy, said study researcher Gregory Nichols, a senior investigator at Kaiser Permanente Center for Health Research in Portland. Obesity also increases the risk of other conditions, such as cancer, joint problems and kidney disease, he said.

"They might be metabolically healthy, but that does not necessarily mean they are healthy overall," Nichols told Live Science. What's more, although these participants were free of metabolic risk factors at the time of the study, they could soon develop them in the coming years, he said. Some previous studies have found that even "metabolically healthy" obese people are at higher risk of developing type 2 diabetes, compared with people of normal weight.

Thus, people who are obese should still aim to lose weight, even if they appear otherwise healthy, Nichols said. "Weight loss could improve other types of health [problems], and might reduce the likelihood of developing cardiometabolic risk factors," he said. [The Best Way to Lose Weight Safely]

For the study, the researchers analyzed electronic health care records from members of four health care systems that together serve 12 million people in 11 U.S. states and Washington, D.C. They defined being overweight as having a body mass index (BMI) of 25.0 to 29.9. Obesity was defined as having a BMI of 30 or more, while morbid obesity was a BMI of 40 or more.

They found that 18.6 percent of the people who were overweight did not have any of the four metabolic risk factors, and 9.6 percent of those who were obese did not have any of the four. Looking at only those who were morbidly obese, they found that 5.8 percent did not have any of the four risk factors.

Being "metabolically" healthy was more common among those who were younger — about 30 percent of all adults ages 20 to 34 in the study did not have any of the four metabolic risk factors, compared with just 6.3 percent of those ages 65 to 79.

Several factors could explain why some overweight people and some obese people remain metabolically healthy. "Diet and exercise almost certainly play a role," Nichols said. However, the new study did not assess these factors.

In addition, the distribution of a person's fat can also affect their risk of cardiovascular disease, with fat stores in the belly area (visceral fat) posing a greater risk to health than fat found just beneath the skin (subcutaneous fat) in other parts of the body. Some previous studies have found that obese people who are metabolically healthy have less visceral fat than obese people who aren't metabolically healthy. However, a person's BMI measurement, used in this new study, cannot distinguish between visceral fat and subcutaneous fat.

Ultimately, future studies are needed to follow metabolically healthy obese people forward in time, to see if they remain metabolically healthy over a long period, or even a lifetime, Nichols said. Such studies could determine whether metabolically healthy obesity "is even a real thing, or merely a matter of timing," Nichols said.

In addition, studies should look at the order in which people develop metabolic risk factors, and whether this order affects their risk of developing subsequent heart disease and diabetes, he said.

The study was published March 9 in the journal Preventing Chronic Disease.

Original article on Live Science.

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.