Hibernate? Humans? Health lessons we're learning from bears

ANCHORAGE — Gain a few hundred pounds and lie around in bed for months, and you are likely to develop a host of ailments, from diabetes and heart failure to muscle loss, osteoporosis and bedsores.

Unless, that is, you happen to be a bear.

Scientists have puzzled for decades over the evolutionary tweaks that have allowed bears and other hibernating animals to lie still through the winter, forgoing food and water, yet emerge with their health intact come spring. Researchers believed that if they could better understand how the animals did it, they might apply the insights to humans, developing new drugs or medical treatments, for example, or ways for astronauts to survive long space flights in a hibernation-like state.

But progress has been slow, the bear den holding its secrets tightly. And in December, the field suffered a further setback, when a highly publicized hibernation study was retracted after one of the authors was found to have manipulated the data. Yet the advent of technologies like gene sequencing and sophisticated imaging techniques over the last few decades has given investigators hope that they will eventually be able to harness aspects of the bear's exceptional physiology for human use.

Last month, in a session on hibernation and human health at the 24th International Conference on Bear Research and Management in Anchorage, scientists presented more than a dozen studies, including research on bears' cardiovascular system, muscle chemistry, kidney functioning, fat storage and metabolism.

[Want to know how a changing climate is affecting polar bears? Look at what they're eating.]

"I really think a lot is going on at the moment," said Dr. Peter Godsk Jorgensen, a cardiologist at Gentofte Hospital in Copenhagen, Denmark, who in his talk discussed studies that used ultrasound imaging and speckle tracking — a method of quantifying the movement of heart muscle — to look at heart function in bears during winter hibernation and summer activity.

Confirming work by other researchers, Godsk Jorgensen and his colleagues found that bears' heart rate slowed sharply during hibernation, from about 75 beats a minute to as few as 10, with pauses that sometimes lasted 19 seconds or more.

"I once had a patient with a pause of 13 seconds," Godsk Jorgensen said. "When you have that, you go around and you faint and hit your head."

The researchers also identified clusters of blood cells on the bear's ultrasound, called "smoke," that are seen in humans who have severe heart failure or atrial fibrillation, a condition that raises the risk of blood clots and stroke.

Dr. Ole Frobert, a cardiologist at Orebro University Hospital in Sweden who led the research team on the ultrasound study, said work on hibernation in bears was attracting more interest at a time when some scientists have grown disillusioned with studies using mice as subjects.

"Medical research in a way is in a crisis," Frobert said, "because we do a lot of research and publish a lot of papers, but there are very few breakthroughs."

Mice can be more easily kept in a laboratory than bears, of course, but they are far from perfect as a model for humans and often fail to respond to drugs or other treatments as humans do. Bears, although more difficult to study, Frobert said, allow scientists to look at several physiological systems at once, and provide a natural model of evolutionary solutions to questions that continue to befuddle scientists.

In six years of research, carried out in collaboration with the Scandinavian Brown Bear Research Project and other researchers in Europe and the United States, Frobert and his colleagues have found startling differences in bears' physiology during periods of hibernation and periods of activity.

Hibernating bears, grown fat from summer feasting, do not eat, drink, urinate or defecate while they are hibernating. But they lose no muscle mass from inactivity. Platelets in the bears' blood become less sticky, acting as a natural blood thinner, the researchers found, perhaps to counteract blood clots that could form during long periods of immobility. The bears' metabolism drops to 25 percent of its normal state and their kidneys stop functioning, yet do not have kidney failure.

The research team collected blood and tissue samples for the studies from free-ranging brown bears in Sweden in February and again in June. The bears, all teenagers, were tranquilized, and although some of the animals later changed dens, they seemed otherwise unaffected by the research, Frobert said.

The new wave of hibernation research is of particular interest to scientists studying obesity, which has become epidemic in the United States over the past few decades.

Obesity in humans is associated with resistance to insulin, a hormone that regulates glucose in the blood, and Type 2 diabetes. Bears also show insulin resistance, studies find, but do not develop diabetes in the classic sense.

"Obese bears are healthier; in fact, they are more reproductively fit," said Heiko T. Jansen, a professor at the College of Veterinary Medicine at Washington State University who presented at the meetings. "They have all the advantages, which is so counterintuitive to human biology."

In research financed in part by the pharmaceutical company Amgen, Jansen and his colleagues found that bears' handling of insulin appears to vary with the seasons, with resistance increasing during hibernation and sensitivity increasing in summer.

Fat cells of hibernating bears treated with a blood serum from "summer" bears become more insulin sensitive, the researchers found.

"There's clearly something important in serum that's important for multiple things," Jansen said.

He and other researchers said they hoped such studies would eventually lead to drugs to treat diabetes or cure obesity.

But practical applications are still uncertain. And it may take some time for researchers to figure out how bears do naturally what people cannot.

"We have to learn and relearn and relearn that nature has solved these problems," Jansen said. "And it's our job as primates to figure this out."