The number of overweight and obese adults in the developing world has almost quadrupled to around one billion since 1980
People can still carry visceral fat around their abdominal organs without it showing in their weightReuters

Scientists have discovered the origin of the "bad fat" found in the body, which is linked to obesity-related illnesses although those carrying it may not be overweight.

Visceral fat, which has previously been linked to heart disease and cancer, builds up around abdominal organs such as the heart, kidneys and intestine. It has been dubbed the "silent killer" because it is possible for people to have the fat without looking overweight.

The fat is more harmful than the subcutaneous fat layers stored beneath the skin. The new research has revealed the fat stems from a different source, as it derives from a certain type of cell found in the developing embryo when it is still in the womb.

You-Ying Chau, lead author of the study at Edinburgh University, said: "Determining the origins of good and bad fat has been one of the big unanswered questions in obesity research. We've now shown that most bad fat comes from cells expressing the Wt1 gene in the later stages of pregnancy."

Chau added: "We also found that cells expressing Wt1 continue to act as a source of visceral fat into adulthood where they may be influenced by external factors such as diet. If we would find a way to control the regulation of these cells, we might be able to stop the body laying down any more bad fat around the organ."

Researchers hope the discovery will eventually lead to new ways of controlling how this type of cell contributes to the bad fat surrounding the vital organs. This information would lower the chances of developing the kind of obesity that raises the risk of serious disease.

A study on genetically modified mice revealed that 80% of the visceral fat of the abdomen comes from embryonic cells known from a gene that they express, called Wt1. This gene is linked to kidney tumours. The latest research has revealed subcutaneous fat, however, has a different origin and does not express the Wt1 gene.

Professor Nick Hastie, of the Medical Research Council's Human Genetics Unit in Edinburgh, said the visceral fat originates from embryonic cells in the womb which are also involved with forming the protective mesothelium membrane around the six visceral fat depots of the abdomen. The "bad fat" is formed after birth.

He added: "We found strong evidence for the existence of a mesothelium, which was a big surprise because nobody thought this membrane existed in fat. It seems that not only does the mesothelium help to produce the cells that make the fat, it also surrounds the fat, making it into a neat little organ."

The study was published in the journal Nature Cell Biology.