Physical activity can cause certain stem cells to become bone rather than fat, research has indicated.
Scientists at McMaster University suggested the findings offer another reason to exercise - as overall health is improved by an enhanced capacity to make blood.
The study of mice showed mesenchymal stem cells, which can turn to bone or fat, were more likely to take the latter path in sedentary animals.
"What we're suggesting is that exercise is a potent stimulus - enough of a stimulus to actually trigger a switch in these [cells]," said leader of the research Gianni Parise.
Fat cells filling the bone marrow cavity can mean blood stem cells are less productive, which can result in conditions such as anaemia, but bone cells encourage the process and can ultimately boost the immune system and make oxygen uptake more efficient.
Last month, research published in the Lancet indicated just 15 minutes of moderate exercise each day could extend a person's lifespan by several years.