By GRETCHEN REYNOLDS
For those requiring additional reasons to show up at the running path or at the gym in the dreary heart of winter, science has come up with a compelling new motivation. Exercise can, it appears, keep your bone marrow from becoming too flabby.

This idea is the focus of a series of intriguing recent experiments by Janet Rubin, a professor of medicine at the University of North Carolina and other researchers. For the work, scientists removed bone marrow cells from mice and cultured them. The cells in question, mesenchymal stem cells, are found in bone marrow in both animals and people, waiting for certain molecular signals to tell them to transform into either bone cells, fat cells or, less commonly, something else.

After a stem cell differentiates, of course, it can no longer be anything else: once a fat cell, always a fat cell; once a bone cell, etc. So the fate of marrow stem cells determines the strength and quality of the bone. If a stem cell becomes a fat cell, then the portion of the skeleton to which it might have migrated as a bone cell will be that littlest bit punier. In a study published late last year by researchers at the University of Southern California, the femurs of healthy adults, some in their 20s, others past age 55, were scanned with magnetic resonance imaging. The researchers found that, in both young and old, the amount of fat in the leg’s bone marrow was inversely related to the amount of bone. The more fat in the marrow, the less bone in the thigh…continue to full article.