It is now common knowledge that having too much fat is bad for you, but less well known is the serious threat posed by the “hidden fat” that surrounds our vital organs. New research published in the journal Nature Cell Biology claims to have discovered where this hidden, or “visceral fat,” comes from.

Visceral fat and subcutaneous fat are the two main types of fat in the body. Subcutaneous fat is found directly beneath the skin – it gives our bodies cushioning and insulation and is a store of energy. Visceral fat, meanwhile, develops around the heart, intestines and vital organs.

Previous research has suggested that, while subcutaneous fat is protective, visceral fat increases risk of cancer, type 2 diabetes, heart disease and Alzheimer’s. Understanding how these fats originate has been a key concern for researchers, but previous findings have been inconclusive.

In the new study, a team of researchers from the Medical Research Council (MRC) Human Genetics Unit at the University of Edinburgh, in the UK, used genetically modified mice to show that up to 80% of visceral fat in the body can be traced back to a single cell in the embryo.

The researchers found that these cells expressed a gene called Wt1. They were found in the visceral fat of adult mice, but not in subcutaneous fat. The highest concentrations of the Wt1-expressing cells were in the fat depots around the heart and stomach, which are the most dangerous areas to have large amounts of visceral fat.

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While subcutaneous fat is protective, visceral fat increases risk of cancer, type 2 diabetes, heart disease and Alzheimer’s.

“Determining the origins of good and bad fat has been one of the big unanswered questions in obesity research,” says lead author of the study Dr. You-Ying Chau, from the MRC Human Genetics Unit at the University of Edinburgh. “We’ve now shown that most bad fat comes from cells expressing the Wt1 gene in the later stages of pregnancy.”

These “fat precursor” cells were observed by the researchers to behave in a way similar to stem cells, where they continue to make more fat cells throughout their lifespan.

“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,” Dr. Chau says, adding:

“If we could 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 organs. However, it will take many more years of research before we get there.”

Another discovery made by the Edinburgh team was that visceral fat is protected by a membrane, called a mesothelium. This is a significant finding because mesothelia have not previously been thought to exist in fat. As well as holding together and protecting the visceral fat, the mesothelium also contains Wt1-expressing cells.

Prof. Nick Hastle, leader of the research team and director of the MRC Human Genetics Unit at the University of Edinburgh, says that the discovery of the mesothelium “was a big surprise,” adding that “not only does the mesothelium help produce the cells that make the fat, it also surrounds the fat, making it into a neat little organ. In a way this makes sense because, otherwise, how would your body know to form fat and to package it around your heart or kidneys?”

Next, the researchers plan to investigate why the properties of the fat depots around the heart seem to differ depending on how many Wt1-expressing cells are present. In particular, the team wants to see how the fat depots around the human heart – six of them in total – behave.

Prof. Stephen Hill, chair of the MRC’s Molecular and Cellular Medicine Board, which funded the work, says:

Visceral fat can be a silent killer because it’s possible to have a lot of it without looking fat on the outside. Studies like this one are important because they help us to understand how our genes and other biological factors are involved in regulating visceral fat, so that in future we can devise new ways to prevent or treat the devastating consequences of obesity.”