It’s a fat–burning secret anyone interested in bone health should know. For the first time, UNC School of Medicine researchers show that exercising burns the fat found within bone marrow and offers evidence that this process improves bone quality and the amount of bone in a matter of weeks.

The study, published in the Journal of Bone and Mineral Research, also suggests obese individuals – who often have worse bone quality – may derive even greater bone health benefits from exercising than their lean counterparts.

“One of the main clinical implications of this research is that exercise is not just good, but amazing for bone health,” said lead author Maya Styner, MD, a physician and assistant professor of endocrinology and metabolism at the University of North Carolina at Chapel Hill. “In just a very short period of time, we saw that running was building bone significantly in mice.”

Although research in mice is not directly translatable to the human condition, the kinds of stem cells that produce bone and fat in mice are the same kind that produce bone and fat in humans.

In addition to its implications for obesity and bone health, Styner said the research also could help illuminate some of the factors behind bone degradation associated with conditions like diabetes, arthritis, anorexia, and the use of steroid medications.

In her patients, Styner is all too familiar with the chronic toll of osteoporosis and fractures. This new evidence shows it’s possible to use exercise to reverse some of the effects on bones.

“I see a lot of patients with poor bone health, and I always talk to them about what a dramatic effect exercise can have on bones, regardless of what the cause of their bone condition is,” said Styner. “With obesity, it seems that you get even more bone formation from exercise. Our studies of bone biomechanics show that the quality and the strength of the bone is significantly increased with exercise and even more so in the obese exercisers.”

The researchers performed their experiments in two groups of mice. One group was fed a normal diet (lean mice) and the other received a high–fat diet (obese mice) starting a month after birth. 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.

The researchers analyzed the animals’ body composition, marrow fat and bone quantity at various points. Predictably, 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.

Perhaps more 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. The tests also revealed that exercise improved the thickness of bone, and that this effect was particularly pronounced in obese mice.

According to Styner, all of this points to the conclusion that marrow fat can be burned off through exercise and that this process is good for bones.