Dec. 29, 2003 DECEMBER 18, 2003 -- Inactivity is a prime cause of osteoporosis in most animals, including humans. A notable exception is the black bear, even though Ursus americanus spends months every year curled up in a den, hibernating the winter away.
Seth Donahue, an assistant professor of biomedical engineering, has been studying bears to discover why their skeletons retain their strength in conditions that would reduce human bones to the consistency of soda crackers.
Using blood samples taken from five captive bears at Virginia Tech by Michael Vaughan, a professor of wildlife science, Donahue and his research colleagues monitored metabolic markers of bone metabolism throughout the bears' annual cycle. They discovered that, while bone breakdown increases during hibernation, bone production remains constant and may even peak as the bear emerges from hibernation.
Bears don't eat during their long winter's nap, so where does the calcium come from to rebuild their bones?
It turns out that bears are recycling. They don't urinate or defecate when they hibernate, so instead of excreting calcium from their bones, they are essentially putting it back where they found it.
"They don't have a way of getting rid of excess calcium, so the logical place to put it is back into bone," he says.
Age, another risk factor for osteoporosis, doesn't appear to affect bear bones either. Donahue and his colleagues have been testing bear bones' strength, porosity and mineral content, using bones donated by area hunters. They discovered that strength and mineral content increased significantly as the bears get older, while porosity remains constant.
Donahue's work illustrating how black bears' bone breakdown and production balance was published recently in the Journal of Experimental Biology, Vol. 206.
Donahue's team now plans to investigate the structural differences between human and bear versions of two hormones involved in regulating bone metabolism, calcitonin and parathyroid hormone.
The hope is that insights from the bears' hormonal strategy could lead to new therapies for human bone loss. "One component of age-related osteoporosis is inactivity," Donahue says. "Also, there's a decrease in bone formation in the older human population."
According to the Osteoporosis Foundation, the condition is responsible for more than 1.5 million fractures annually. One in two women and one in four men over the age of 50 will suffer an osteoporosis-related bone fracture.
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