They are a mystery to researchers: people who are significantly overweight and yet show none of the usual metabolic red flags. Despite their obesity, they have normal cholesterol levels, healthy blood pressure levels and no apparent signs of impending diabetes.
Researchers call them the metabolically healthy obese, and by some estimates they represent as many as a third of all obese adults. Scientists have known very little about them, but new research may shed some light on the cause of their unusual metabolic profile.
A study in the journal Diabetologia has found that compared with their healthier counterparts, people who are obese but metabolically unhealthy have impaired mitochondria, the cellular powerhouses that harvest energy from food, as well as a reduced ability to generate new fat cells.
Unlike fat tissue in healthy obese people, which generates new cells to help store fat as it accumulates, the fat cells of the unhealthy obese swell to their breaking point, straining the cellular machinery and ultimately dying off.
This is accompanied by inflammation, and it leads to ectopic fat accumulation — the shuttling of fat into organs where it does not belong, like the liver, heart and skeletal muscle. A fatty liver frequently coincides with metabolic abnormalities, and studies suggest that it may be one of the causes of insulin resistance, the fundamental defect in Type 2 diabetes.
In the healthy obese, however, the fat tends to remain in the subcutaneous padding just beneath the skin, where it appears to be fairly innocuous.
“The group that doesn’t gain fat in the liver as they get obese seems to avoid inflammation and maintain their metabolic health,” said Dr. Jussi Naukkarinen, a research scientist specializing in internal medicine at the University of Helsinki. “There is a complete difference in how they react to obesity.”