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.

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