Last Modified 10:41 AM, July 9, 2009

September 21, 2005—

The couch potato lifestyle may appeal to the sedentary beast in all of us, but it can be a major cause of osteoporosis. For black bears, however, it's a different story. Seth Donahue's research has shown that their bones remain strong year round, despite snoozing away for months in hibernation.

Now, his latest studies are showing that this ability isn't limited to black bears. Their big brown cousins, grizzly bears, are as good if not better at keeping their bones strong and healthy.

"The porosity of the grizzlies' bones actually decreased during hibernation," said Donahue, an assistant professor of biomedical engineering. "That's completely the opposite of what happens in humans."

"This confirms what we've studied in black bears," he said.

It turns out that instead of losing the stuff bones are made of, bears recycle it in their sleep. They don't urinate or defecate when they hibernate, so instead of excreting calcium that leaches from their bones, they essentially put it back where they found it.

Now that Donahue has verified that bears have a special mechanism that fights off osteoporosis, the trick is to understand it.

"The hard part is figuring out the mechanism," he said. "There's a strong possibility that parathyroid hormone levels are important." His recent studies on black bears show that parathyroid hormone levels are strongly correlated with bone">bone formation, which comes as no surprise. "A synthetic version is being used in humans to treat osteoporosis," Donahue notes. "But we also believe other molecules are involved, and we're investigating what they might be and the role they play in bone formation."

One thing is certain: There's nothing about hibernation itself that protects an animal's skeletal structure. Other hibernating creatures, such as bats and ground squirrels, do get brittle bones during their long winter's nap.

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.D

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