As many as a third of obese adults are considered metabolically healthy, meaning they have normal cholesterol and blood pressure levels and show no signs of developing diabetes. Still, they're considered a medical mystery, but new research has shed some light on why some people can be healthy at any size, while others cannot.
It has to do with fat cells, according to a new study in the journal of Diabetologia. Compared to obese people who are healthy, those who are metabolically unhealthy have "impaired mitochondria" and a "reduced ability to generate new fat cells."
What researchers found was that in a healthy obese person, new cells are generated to help store fat as it accumulates, whereas the cells of an unhealthy obese person "swell to their breaking point," making their fat cells larger than any other group.
They were swollen and riddled with inflammation. The breakdown and mobilization of their fat stores was suppressed, and a closer look showed that their mitochondria were malfunctioning. Their ability to burn fuel and produce adenosine triphosphate, or ATP, the body’s energy currency, was reduced.
It leads to ectopic fat accumulation, meaning that fat gets into organs like the heart and liver. (A fatty liver is linked to Type 2 diabetes.)
However, for a healthy obese person, the fat doesn't travel throughout the body, and remains just beneath the skin, where it doesn't seem to cause any physical harm.
A study that appeared in the journal Diabetes Care in August found that metabolically healthy obesity is more frequently found in younger adults, but it may be a transition state, and that "some, if not many, people in this category will eventually develop the expected metabolic disturbances."
Dr. Jussi Naukkarinen, the lead researcher in the fat cell study, said that anti-inflammatory drugs have been shown to "protect mitochondrial function and improve diabetic symptoms and glucose metabolism." He also suggests that high glycemic foods (like sugar and white flour) play a role in spiking blood glucose and insulin levels.
But ultimately, he believes that studying healthy obese people will help those that are unhealthy.
"People haven’t really paid that much attention to metabolically healthy obesity, but I think it can teach us a lot about usual obesity,” he said. “It’s only recently that people studying depression have done happiness studies showing what goes right, and I’m thinking about the metabolically healthy obese phenomenon in the same way.”