In your bones, there is fat.

Why? Researchers don't know, but they have theories.

How does it get there? They have theories about that, too.

Is it the same sort of fat found in muscle? Not sure.

Is this bone fat a bad thing? Yes. Researchers think it is. But sometimes, they say, it might not be so bad.

"This is a new field," said Maya Styner, MD, an assistant professor of medicine in the University of North Carolina School of Medicine. "We don't know exactly how it's produced or why it's there to begin with. There are a lot of unanswered questions."

But Styner, an endocrinologist, has used a new kind of imaging technique to answer at least two: what do diabetes drugs and exercise -- or the lack of it -- do to bone fat, and why does this matter?

Stains and scans

Our bones are not stagnant, rock-like things. They change. Marrow -- the tissue inside bones -- is full of various kinds of cells. And marrow is also full of fat. The amounts of these cells and fats can decrease or increase over time. And the production of these marrow cells and fat depend on a specific type of progenitor cell called a mesenchymal stem cell.

"These stem cells give rise to both bone and fat," Styner said. "For a long time in the bone world, it's been thought that these stem cells produce bone but then, as we age, they start to produce fat, instead."

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Fat of the bone: Exercise, diabetes affect amount of fat inside bones