We already knew that fat could accumulate pretty much anywhere on our bodies, but we kind of thought our bones, at least, were a fat-free zone. Sadly, it turns out that's not at all the case. But just like all our other pudge, bone fat can also be blasted by exercise, according to researchers from the University of North Carolina.

When you think about it, it makes sense that bones have fat; that's what makes bone marrow such a delicacy on some menus. But the way in which the bone marrow fat forms and its role in the body have both been a bit unclear to scientists, says UNC. So a study led by Maya Styner, a physician and assistant professor of endocrinology and metabolism at the University of North Carolina at Chapel Hill, set out to investigate.

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"There's been intense interest in marrow fat because it's highly associated with states of low bone density, but scientists still haven't understood its physiologic purpose," said Styner. "We know that exercise has a profound effect on fat elsewhere in the body, and we wanted to use exercise as a tool to understand the fat in the marrow."

Styner and her team raised two different groups of mice by giving them different diets starting a month after they were born. One group was fed a high-fat diet which turned them into obese mice, while the other received a normal diet that kept them lean. Then, at four months of age, half the mice from each group got a running wheel in their cage. While that might not seem like the most exciting gift to you and I, it turns out that mice really like to run, so it suited them just fine.

The researchers then took a look at the bone marrow fat from all the rodents. They found that in the mice that exercised, the amount of fat and the size of fat cells in their marrow had reduced significantly. In fact the reduction was so significant that fat-wise, the marrow of the obese mice was pretty much identical to those of the lean mice – even the wheel-running lean mice. The researchers also found the mice who exercised had thicker bones and that this thickening was most pronounced in the obese mice.

"Obesity appears to increase a fat depot in the bone, and this depot behaves very much like abdominal and other fat depots," said Styner. "Exercise is able to reduce the size of this fat depot and burn it for fuel and at the same time build stronger, larger bones."

While the researchers were able to draw parallels between exercise and thicker, leaner bones, at this point they're not entirely sure about the relationship between marrow fat reduction and bone health.

One theory is that when fat cells get burned inside the marrow, the energy released could be used by the body to beef up bone composition. Another theory involves cells known as mesenchymal stem cells, which lead to the creation of both fat and bone cells. It could be that exercise tips their production quotas to more bone and less fat. Interestingly, if this second theory turns out to be valid, mesenchymal stem cells also produce bone and fat in human, so the results could translate well.

"If we want to take this technique to the human level, we could study marrow fat in humans in a much more reliable fashion now," said Styner. "And our work shows this is possible."

Details of the study have been published in the Journal of Bone and Mineral Research.

Source: University of North Carolina