Newswise, June 3, 2017 – 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”
Getting to the marrow of the matter
Bone and marrow are more dynamic than you might think. Marrow,
in particular, is a hub of activity, coordinating the formation of bone and
cartilage while simultaneously churning out blood cells, immune cells, and
cancerous cells.
Marrow also produces fat, which has a lot to do with its
vaunted status in cuisines around the world. But the physiological role of bone
marrow fat in the body – and even whether it is beneficial or harmful for one’s
health – has remained somewhat mysterious.
Generally, marrow fat has been thought to comprise 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 new study
offers evidence to the contrary.
Styner’s work also offers fundamental insights on how marrow
fat forms and the impact it has on bone health. Previous studies have suggested
that a higher amount of marrow fat increases the risk of fractures and other
problems.
“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,” said Styner.
“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.”
Vanishing fat cells
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.
“Obesity appears to increase a fat depot in the bone, and this
depot behaves very much like abdominal and other fat depots,” said Styner.
“Exercise is able to reduce the size of this fat depot and burn it for fuel and
at the same time build stronger, larger bones.”
Setting the stage
The research leaves a few lingering mysteries. A big one is
figuring out the exact relationship between burning marrow fat and building
better bone. It could be that when fat cells are burned during exercise, the
marrow uses the released energy to make more bone.
Or, because both fat and bone cells come from parent cells
known as mesenchymal stem cells, it could be that exercise somehow stimulates
these stem cells to churn out more bone cells and less fat cells.
More research will be needed to parse this out. “What we can say
is there’s a lot of evidence suggesting that marrow fat is being used as fuel
to make more bone, rather than there being an increase in the diversion of stem
cells into bone,” said Styner.
But marrow fat, being encased in bone, isn’t easy to study.
The team’s new research represents a leap forward not only in understanding
bone marrow fat but also in the tools to study it.
The group’s previous
work relied
on micro CT imaging, which requires the use of a toxic tracer to measure marrow
fat. In the new study, they took advantage of UNC’s 9.4 TMRI, a sophisticated
MRI machine of which there are only a few around the country.
Using MRI to assess marrow fat eliminates the need for the
toxic tracer and allows highly detailed imaging of living organisms.
“If we want to take this technique to the human level, we
could study marrow fat in humans in a much more reliable fashion now,” said
Styner. “And our work shows this is possible.”
The team also developed techniques to perform a much more
detailed assessment of the number and size of fat cells within the marrow, and
even examined some of the key proteins involved in the formation and reduction
of bone marrow fat.
Styner is now working with collaborators to adapt these
methods for studying the bone marrow dynamics that might be at work in other
conditions, including anorexia and post-menopausal osteoporosis.
