While it has long been held that too much fat in the liver may result in diabetes, researchers appear to have discovered that not all types of fat are harmful.

Writing in the latest issue of Nature Medicine, a group of Japanese scientists described how they changed the fat composition in the livers of mutant mice and fed them exactly the same rich, fatty diet as other mice. But while all the rodents became obese and the normal mice developed resistance to insulin and became prone to diabetes, the mutant group was free from those problems.

"Obesity is a matter of quantity of fats in the body, but it is our new message that the quality of fats could be a new determinant factor for diabetes," said Hitoshi Shimano of the Department of Internal Medicine at the Graduate School of Comprehensive Human Sciences.

Excessive fat intake leads to obesity and overwhelms the storage capacity of fat cells, with surplus fat being stored in the liver. Development of fatty liver can result in insulin resistance and increased glucose levels - hallmarks of diabetes.

"The absolute levels of fat in the liver do not therefore seem to be detrimental to maintaining normal glucose levels. Instead, the types of fat that are present seem to be a more important factor, with shorter fat molecules being healthier than longer ones," the researchers wrote.