There is a distinct type of fat in our bones that doesn’t act like fat at all. These peculiar fat cells, or adipocytes, grow larger, not smaller, when we starve.
“They are fake adipocytes,” says cancer researcher Catherine Muller at the University of Toulouse, France. “They look like adipocytes but they don’t have the main function of adipocytes.”
The main function of fat is to store energy and release it when needed, says Muller. This is what most of the fat in our bodies – found under our skin and around internal organs – does. Besides this white fat, as it is known, we also have small patches of brown fat around the neck that actively burn food to produce heat.
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The fat inside our bone marrow, which accounts for about 10 per cent of all fat in lean people, is usually assumed to be a form of white fat and often regarded as a mere space filler. But animal studies in the 1970s revealed a bizarre property: these fat cells grow bigger when animals starve. Fat cells should behave in the opposite way, by releasing energy and shrinking.
The fat cells in bone marrow appear to behave in the same strange way in humans. In 2010, for instance, MRI scans revealed that than healthy women, despite having far less fat elsewhere in the body.
Muller became interested because breast and prostate cancers often spread to bone marrow, and it appears that bone marrow fat cells help tumours grow. She was surprised to discover that very few studies have looked at bone marrow fat in people.
So her team has analysed samples of bone marrow fat and below-skin fat from people undergoing hip surgery. The researchers found the bone marrow fat cells resemble subcutaneous ones in appearance, but produce a distinctive set of proteins.
Yellow fat
This means they have a different metabolic role to white fat and should be regarded as a distant subtype, the researchers conclude. They propose calling them “yellow adipocytes”, or yellow fat.
Bone marrow fat should indeed be regarded as a different type of fat, says William Cawthorn at the University of Edinburgh, UK, . Recent animal studies by his team and others also show it is distinctive, he says, and Muller’s study confirms this for people as well.
What still isn’t clear is why yellow fat behaves in such an odd way. It may play such a vital role in the production of blood cells in bone marrow that the body can’t afford to use it as an energy store during lean times, Muller suggests, which would explain why the cells don’t shrink when we starve.
Bone marrow fat is involved in many other important processes, too. Cawthorn’s work suggests that some of the benefits of calorie restriction may be linked to increased production of a hormone called adiponectin by bone marrow adipocytes. On the down side, yellow fat increases with age and could play a part in weakening our bones.
Name game
But should it be called bone marrow fat or yellow fat? “I have a big issue with them trying to throw another colour at it,” says Cawthorn. “Calling them yellow adipocytes is actively unhelpful.”
But Muller says the fatty parts of bone marrow actually look yellow. “To me, it really corresponds with what you see,” she says.
It has become customary to ascribe colours to types of fat. Beside white and brown fat there is also beige fat – white fat that can produce heat – and white fat cells in breast tissue that secrete milk during breast-feeding and then turn back to normal fat cells.
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