Life in the fat lane

Hibernating bears have evolved to add as much fat as possible each fall so they can survive several months without eating. But just how do they do that without any health repercussions? This mystery is one researchers at Washington State University are trying to unravel.

A view from above showing just how a bear’s body changes from spring to fall, preparing for hibernation.

“When people gain weight, or add a lot of fat, they can become diabetic or suffer other health consequences,” said Heiko Jansen, associate professor of integrative physiology and neuroscience at WSU. “But bears don’t, they’re perfectly adapted to the hibernation/activity cycle.”

For the first time, WSU scientists were able to confirm that bears are completely resistant to insulin during hibernation, meaning any sugar in their bloodstream just stays there and doesn’t break down. But that insulin-resistant status is reversible as the bears’ metabolism restarts when they come out of hibernation.

“Scientists have guessed this is the case for years, but this is the first time anybody has been able to prove it,” Jansen said.

In addition to the insulin finding, the research team wanted to learn how bears’ appetites change so tremendously throughout the year. They don’t eat at all through the winter without feeling hungry. And when they wake up, they don’t act desperately hungry.

But, in late summer, they become ravenous as they begin to build up their fat stores for the coming winter. By October, they slow their intake down again before going back into hibernation, Jansen said.

Hunger is suppressed in bears, as in humans, with the hormone leptin, he said.

“We found that in August we could give bears leptin and it had no impact,” Jansen said. “They still ate all the time. So, we figured out that it’s because their brains change over time. By late fall, they’re sensitive to leptin again and it works to suppress their appetites for the winter. They have developed a very complex system for adapting to their environment.”

A grizzly bear with her cubs at the WSU bear center.

The findings were published in the article “Life in the fat lane: seasonal regulation of insulin sensitivity, food intake, and adipose biology in brown bears” in the Journal of Comparative Physiology:

Jansen and his colleagues hope to continue their study to see exactly how bears resist insulin during hibernation on the molecular level. If they can figure that out, it could lead to breakthroughs in fighting diabetes in humans.

“We’re still a way off from that, but this study shows there’s hope,” Jansen said.