Despite the American Medical Association’s decision in June to classify obesity as a disease, a study has shown that obese people can be perfectly healthy in terms of cholesterol and blood pressure levels, showing no apparent signs of impending diabetes.

A study in the journal, Diabetologia, has shown that the metabolically healthy obese can generate new cells to store fat in the subcutaneous padding, just beneath the skin, where it appears to remain harmless.

In contrast, the fat cells of the unhealthy obese swell to breaking point and die off. This leads to fat accumulating where it doesn’t belong, such as liver, heart and skeletal muscle, which then causes health issues.

“The group that doesn’t gain fat in the liver as they get obese seems to avoid inflammation and maintain their metabolic health,” says Dr. Jussi Naukkarinen, research scientist specialising in internal medicine at the University of Helsinki, who argues that studying metabolically healthy obesity can teach a lot about normal obesity.

In Dr Naukkarinen’s study, the individuals had become obese at a similar age and had remained so for about a decade, showing no further signs of ill health.

Dr Naukkarinen’s work has involved studying identical twins, one who is obese and one who is not, allowing them to take into account the influence of genetics and environment.

Among other things, the study showed that the fat cells of the unhealthy obese became engorged. Their ability to burn fuel and produce adenosine triphosphate, the body’s energy currency, was reduced.