The most current conventional wisdom among doctors and obesity researchers goes something like this: While people who are obese are at high risk for metabolic syndrome ― a constellation of symptoms that increase the risk for Type 2 diabetes, heart disease and more ― there is also a significant group of obese people who are totally healthy and will remain healthy. This so-called “healthy obese” idea has been somewhat controversial ― a departure from decades of medical science that held obesity will, more often than not, had adverse effects on health.
Now new genetic research from scientists at the Karolinska Institutet in Sweden suggests “healthy obesity” isn’t as clear-cut as it seems.
A genetic analysis showed that people who were obese but qualified as metabolically healthy still had fundamental differences in the way that genes in their fatty tissue express themselves. It’s unclear whether or not these genetic differences correspond to different disease risks, but the fact that genetic expression is altered on a cellular level suggests that there’s a lot more to healthy obesity than we think.
“Insulin-sensitive obese individuals may not be as metabolically healthy as previously believed,” said study author Mikael Rydén in a statement.
Genetic differences between obese and never-obese people
To see how genes expressed themselves in different types of people, Rydén recruited 65 people to participate in the study. Fifteen had never been obese, while the rest were obese people who were part of a clinical study on gastric bypass surgery and preparing for an operation.
Rydén and his team tested the obese patients for metabolic health by checking their insulin response; those who were sensitive to insulin are considered metabolically healthy, while the insulin-resistant are not.
Then the researchers took biopsy samples of abdominal white fat tissue from each participant for genetic sequencing.
Rydén found a big difference between the mRNA in samples from the never-obese and the obese. But abnormal gene expressions in biopsies from the group of obese participants were “very similar,” he said, with almost no difference between insulin-sensitive and insulin-resistant participants.
This suggests that obesity may drive this unique gene expression even in cases when it doesn’t drive insulin resistance, Rydén explained.
Healthy obese people are distinct from the never-obese
While the results don’t offer any clear takeaways for obese people, Rydén says his research could indicate that people who are obese but healthy may still need medical surveillance for obesity-related conditions. Further study is needed to determine what the gene expression differences actually mean for health.
Other researchers praised the study’s design, but agreed its implications are not yet clear. Dr. Caroline Kramer, an endocrinologist and obesity researcher at Mt. Sinai Hospital in Toronto, Canada, pointed out that it remains to be shown whether or not the genetic differences in fat tissue are responsible for or even linked to metabolic differences in people of different weights.
Dr. Samuel Klein, director of the Center for Human Nutrition at the Washington University School of Medicine, felt the study affirmed the current thinking on “healthy obesity” through the insulin response tests.
“It shows clearly that there’s a real subset of metabolically healthy and unhealthy obese people, but it may not be so easily explained by differences in fat tissue biology,” Klein said.
Why the controversy over healthy obesity is so important
More than a third of U.S. adults, or about 79 million people, are considered obese and at higher risk for conditions like Type 2 diabetes, cancer and heart disease.
But research that raises the possibility of metabolically healthy obesity has not yet agreed on a definition of what exactly metabolic health is, which is why studies estimate that anywhere from six to 75 percent of obese people may be metabolically healthy, according to a 2014 review on the issue. Harvard professor Dr. Frank Hu outlined four criteria for healthy obesity in a 2013 op-ed: a normal waist size, sensitivity to insulin, normal blood pressure, cholesterol and blood sugar levels, and good physical fitness.
But to be clear, most scientists agree that people who clock in at 30 or over on the body mass index scale are probably at risk for serious health conditions ― if not now, then perhaps down the road. The reason researchers are so interested in the concept of healthy obesity is that obesity doesn’t appear to be a one-size-fits-all condition. And the more we learn about how obesity affects different types of people, the more precise doctors can be when it comes to recommendations for patients who aren’t sure if they should try to lose weight or just keep exercising and eating well while maintaining their weight.
“Further exploration of metabolically healthy obesity could help us fine-tune the implications of obesity,” Hu told Harvard Health. “It supports the idea that we shouldn’t use BMI as the sole yardstick for health, and must consider other factors.”
Rydén’s study was published in the journal Cell Reports.