Bear Bones May Provide Key to Osteoporosis

Bears don’t go to Florida when the snow flies, but they get to do the next best thing: curl up in a den and sleep away the winter.

If people did this, according to the prevailing medical wisdom, we’d wake up in the spring with a colossal appetite and bones the texture of sugar cookies. Bears, however, seem to have a special mechanism that protects them from developing weak bones, even though they sometimes lay around for months at a time.

Dr. Seth Donahue, an assistant professor in Michigan Tech’s Department of Biomedical Engineering, has been trying to figure out how bears manage to avoid osteoporosis, despite their sedentary ways.

The answer may lie in their hormones.

As in humans, bear bone tissue does break down during inactivity. However, while human bones don't rebuild while their owners are inactive, bears seem to be able to recycle their calcium with almost total efficiency. Even mother bears, who give birth and nurse their cubs during winter hibernation, don't lose bone density.

Donahue made this discovery while a doctoral student at Pennsylvania State University, analyzing blood serum from radio-collared bears. The results were published in the journal “Clinical Orthopaedics and Related Research.”

He’s continuing his work at Michigan Tech, trying to determine which hormones—or other molecules—help bears resist osteoporosis. He is also studying the bones of black bears donated by Upper Peninsula hunters.

Donahue and graduate student Kristin Harvey have been taking small sections from bear leg bones and observing them under a microscope to see if, as bears age, the bones are becoming more porous, a sign of osteoporosis. They aren’t. Chemical analysis of the bones also shows that they aren’t losing minerals, which are essential for strength.

Lastly, they have also been subjecting small pieces of bone to varying degrees of mechanical pressure, to determine how much stress is required to cause a breakage. No matter what the bears’ age, their bones are remarkably strong.

“Their teeth have rings, like trees, so we know exactly how old they are,” Donahue said. “Our preliminary results suggest that their bone density and fracture strength remains high, even as they age.”

“Bears aren’t affected by these annual periods of disuse, while other species have negative consequences,” he said.

Donahue thinks the key to bone health in bears may be a hormone that regulates calcium uptake. “It’s possible that the bears’ hormone has just a couple of amino acids out of sequence,” he said. If that’s the case, then maybe it would be possible to produce that hormone synthetically and prescribe it to people suffering from osteoporosis.

“Now, we don't have a treatment that works,” he said. “A few therapies slow down bone loss, but nothing can recover bone that's already been lost.”

One hundred years ago, osteoporosis was rare. But now, as people routinely live into their 70s, 80s and longer, it can degrade the quality of life.

As we get older, broken bones can be slow to heal and can cause chronic pain. Plus, most elderly patients are ill equipped to handle the stress of surgical treatment, and as a result their lives can be shortened. Approximately 24 percent of individuals over age 50 die within a year following a hip fracture.

Plus, the cost of treating and caring for the victims of osteoporosis runs into the billions of dollars.

According to the National Osteoporosis Foundation, osteoporosis is responsible for more than 1.5 million fractures annually (300,000 hip fractures, 700,000 vertebral fractures, 250,000 wrist fractures and 300,000 fractures at other sites).

In 2001, an estimated $17 billion was spent on the care of patients with osteoporotic and associated fractures in hospitals and nursing homes. Public health officials expect all these figures to climb as the numbers of frail elderly rise.

Donahue plans to publish the results of his most-recent research soon. He’s hoping to garner funding to begin studying the calcium-regulating hormone in bears. The calcium-regulating hormones are proteins, and determining their structure is a complicated and expensive process.

But the study of bears’ unique resistance to osteoporosis shows unusual promise, he believes. “It’s a model that shows a lot of potential,” Donahue said. “It’s naturally occurring, so we already know it works.”

Ultimately, all we need to know is whether it could work as well in people as it does in bears.