Study explains why polar bears are fat yet healthy

When it comes to healthy eating, polar bears break all the rules. They eat mostly fat, but they don't get heart disease the way humans would.

Scientists said the Thursday in journal Cell that the reason lies in their genes.

Some speedy evolutionary tricks, particularly in the genes which handle how fats are metabolized and how fats are transported in the blood, have allowed polar bears to survive in the Arctic, scientists said.

And it all happened within the last 500,000 years, after the polar bear split from its cousin, the brown bear, according to research that compared the two animals' genomes.

Scientists found that polar bears are much younger than previously thought, with past estimates of the divergence time between polar and brown bears ranging from 600,000 to five million years ago.

"It's really surprising that the divergence time is so short," said Rasmus Nielsen, a University of California Berkeley professor of integrative biology and of statistics.

"All the unique adaptations polar bears have to the Arctic environment must have evolved in a very short amount of time," he said.

It's unclear what drove polar bear to evolve into a separate group from brown bears, though it happened at a time that coincides with a warm interglacial period that could have encouraged brown bears to venture further north than they had in the past, researchers said.

Then, when conditions cooled again, a group of brown bears may have become isolated and forced to adapt to a snowy and cold new environment.

Polar bears eat mostly seals, which are rich in blubber, and they nurse their young with a milk that is nearly one-third fat.

About half the bears' overall weight is made up of fat, rather than muscle and bone.

In contrast, a healthy person's body fat percentage could range between eight and 35 percent.

"The life of a polar bear revolves around fat," said Eline Lorenzen, a researcher at UC Berkeley and one of the lead authors on the study.

"For polar bears, profound obesity is a benign state," added Lorenzen. "We wanted to understand how they are able to cope with that."

Researchers compared the blood and tissue samples from 79 polar bears from Greenland to 10 brown bears from Sweden, Finland, Glacier National Park in Alaska and the Admiralty, Baranof, and Chichagof (ABC) Islands off the Alaskan coast.

They found one of the most strongly selected genes was APOB, which in mammals encodes the main protein in "bad" cholesterol, known as LDL (low density lipoprotein) and allows it to move from the blood into the cells.

Changes in that gene hint at how the polar bear is able to manage high blood sugar and triglycerides at a level that would be perilous in people.

Scientists on the study, who hailed from Denmark, China and the United States, said one day, the polar bear's digestive secrets could help boost human health in an age of increasing obesity.

"The promise of comparative genomics is that we learn how other organisms deal with conditions that we also are exposed to," said Nielsen.