New US research on animals has found for the first time that exercise can help to improve bone health by burning off the fat found within bone marrow.
Carried out by the University of North Carolina School of Medicine, the animal study looked at two groups of mice, a 'lean' group which was fed a normal diet and an 'obese' group which was fed a high-fat diet 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 wanted for the next six weeks.
At various points during the study the team analyzed the animals' body composition, marrow fat and bone quantity.
As expected, the results showed that the obese group started with more fat cells and larger fat cells in their bone marrow. However, after the six weeks both the obese and lean mice who had exercised on the wheel showed a significant reduction in the overall amount fat and overall size of fat cells in the marrow.
In fact, when looking at these two factors, the marrow fat of exercising obese mice was virtually identical to the marrow fat of lean mice, even those that had exercised.
The team also found that while the lean mice showed no difference in the number of fat cells in the marrow, exercising obese mice showed more than a 50 percent drop in these fat cells compared to obese mice that were sedentary.
Exercising obese mice also showed greater improvements in bone thickness than the exercising lean mice.
According to lead author Maya Styner the findings suggest exercise can burn off marrow fat, building stronger, larger bones, which could be of particular importance to those with obesity who often have poorer bone quality.
"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," added 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."
Although the findings in mice are not directly translatable to humans, the stem cells that produce bone and fat in mice are the same kind that produce bone and fat in humans. The team are still unsure of the exact mechanisms behind how burning marrow fat can improve bone health, and point out that further research is needed to understand the relationship better. However, one idea is that when fat cells are burned during exercise, the marrow uses the energy released to make more bone.
Another suggestion is that because both fat and bone cells come from parent cells known as mesenchymal stem cells, when exercise takes place it stimulates these stem cells to produce more bone cells and less fat cells.
The study can be found in the Journal of Bone and Mineral Research.
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