Exercise Burns Bone Marrow Fat, Supports Strong Bones
July 31, 2017
Think back to every diet, home exercise or gym membership commercial you have ever seen. They all tend to focus on one thing: burning fat. And as they do, they show pictures of legs, waists, midsections and all the other places people know and see that we build fat when we take in more calories than we burn.
What they don’t show is the hidden fat you may not know is even there. The average human adult carries one to three pounds of fat inside their bones, as part of their bone marrow, and while that may not be a scale-tipping amount, new research from UNC School of Medicine shows not only that exercising can burn bone marrow fat, but also that reducing that fat may lead to healthier bones.
Researchers found that in mice, running decreased the size of bone marrow fat cells and in obese mice, running also greatly decreased the number of fat cells.
This study, published in the Journal of Bone and Mineral Research, is the first to show that exercise can help burn bone marrow fat and because all the exercising mice showed increases in bone density, suggests that bone marrow fat may be involved in how our bones build and rebuild themselves.
Scientists have known for decades that exercise can help improve bone health while excess fat leads to bone loss, but Dr. Maya Styner, a professor of endocrinology and metabolism at UNC and lead author of the study says role of bone marrow fat in that balance is not well understood.
“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,” Styner said in a press release. “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.”
To study bone marrow fat’s role, Styner and her colleagues took two groups of mice and fed them different diets from birth. One group got a low-fat diet, while the other got a high-fat diet. After 16 weeks, Styner gave half of the mice from each group a wheel to run on, and as mice will naturally run when given the chance, those mice got plenty of exercise over the next six weeks.
Styner measured the mice’s body and bone composition several times throughout the course of the experiment. To measure marrow fat without harming the mice, the researchers used an MRI technique only available at a few points around the country. All the exercising mice showed decreases in the size of their bone marrow fat cells of 30-40 percent but the high-fat mice also lost more than half of their bone marrow fat cells.
All the exercising mice also showed some bone strengthening but Styner says, it was more dramatic in the obese mice, suggesting that bone health, bone marrow fat, obesity and exercise are all intertwined.
Bones are constantly rebuilding themselves. Scientists have known for decades that some cells break bone down while others build it back up. The building up process slows down as we get older, which is one reason why older people lose bone density. Exercise has been shown to give the bone-building cells a boost, like the mice in the study.
Our bone marrow starts as a red substance that rapidly pumps out new blood cells, but as we age, we start developing fatty yellow bone marrow in addition to the red. Until recently scientists believed that this yellow marrow served as a last resort store of energy, but this study shows that it can burn off just like any other fat during exercise. Since previous studies have shown that both bone marrow and regular fat result in decreased bone density, this study raises the question of whether loss of bone marrow fat can also kick bone-building into high gear.
This study does not answer that question, and Styner says more studies will be needed to determine whether the energy from burning that fat goes into building bone, or whether burning fat signals stem cells to make more bone and less fat or if something else is going on.
Whatever is actually happening, if this effect is similar in humans, then the 75 million people in the United States, Europe and Japan with osteoporosis could use exercise to help their bones, and get even more bone benefit if they are obese.
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.