First, what the heck is bone fat? Second, why would anyone want to burn it? The answers to both questions come from researchers at the University of North Carolina, who, working with mice, found that exercise burns bone fat in obese creatures, but not in those who aren’t obese, and also that burning bone fat makes bones stronger. Now back to the the first question. Bone marrow does several things; it produces blood cells and immune cells, and it also produces fat. a UNC news release reports that bone fat has long been thought to be a special fat reserve that is not used to fuel energy during exercise in the same way other fat stores are used throughout the body during exercise. The researchers’ study suggests otherwise. The researchers fed a normal diet to one group of mice (lean mice) and the other received a high-fat diet (obese mice). When they were four months old, half the mice in each group were given a running wheel to use whenever they liked for the next six weeks. Because mice like to run, the group with access to a wheel tended to spend a lot of time exercising. Then the researchers analyzed the animals’ body composition, they found, unsurprisingly, that the obese mice started with more fat cells and larger fat cells in their marrow. After exercising for six weeks, both obese and lean mice showed a significant reduction in the overall size of fat cells and the overall amount fat in the marrow. In these respects, the marrow fat of exercising obese mice looked virtually identical to the marrow fat of lean mice, even those that exercised. What was surprising was the dramatic difference in the number of fat cells present in the marrow, which showed no change in lean mice but dropped by more than half in obese mice that exercised compared to obese mice that were sedentary. Why do we care about than when no one can see how fat our bones are? Because the study also revealed that exercise significantly improved the thickness of bone, particularly in obese mice. And that’s a good thing.