We are told that an increased Body Mass Index (BMI) makes us prone to a large array of diseases. But, as it turns out, fat located under the skin, especially on the buttocks, seems to actually decrease the chances of developing type 2 diabetes, as signaled by a new research published in the Cell Metabolism journal and carried out at The Harvard Medical School.

Subcutaneous fat seems to have opposite effects to visceral fat, the one wrapped around the organs and closely connected to increased risk of illness, by releasing adipokines, metabolism-boosting hormones.

Working on mice, the team transplanted fat from one location of the animals' body to the other. Subcutaneous fat removed to the abdomen triggered body weight and fat mass losses, and a decrease in blood sugar levels. The rodents also got more responsive to insulin, the hormone controlling the way in which the body metabolizes sugar. Insulin resistance triggers the type 2 diabetes. When visceral fat was placed to other body regions, it induced no effect.

"The surprising thing was that it wasn't where the fat was located, it was the kind of fat that was the most important variable. Even more surprising, it wasn't that abdominal fat was exerting negative effects, but that subcutaneous fat was producing a good effect," said lead researcher Professor Ronald Khan.

Other investigations made on obese patients revealed that those of them having high amounts of both abdominal and subcutaneous fat were more insulin-sensitive than patients with high quantities of only abdominal fat. "It is possible that subcutaneous fat offset the effects of visceral fat," Khan explained.

"The finding cast new doubt on the merits of Body Mass Index (BMI) as a way to assess whether somebody was unhealthily overweight, as it did not differentiate between different types of fat. It is still important that people try to control their weight, as healthy lifestyle choices like a balanced diet and taking exercise will overwhelmingly impact on visceral, and not subcutaneous fat levels," Dr David Haslam, of the National Obesity Forum, told BBC News.

As women usually store more subcutaneous fat, mainly on their legs and buttocks, it would be interesting to see if obese men are more affected by various illnesses than women are.