You wouldn’t necessarily know it by looking at a polar bear’s ample figure, but the world’s only marine bear is a fat-burning machine.

Polar bears can lose as much as four pounds of pure fat per day, according to Anthony Pagano, a wildlife research biologist with the United States Geological Survey. While dieters around the world would love to know the bears’ secret — to shave off a single pound in a day a human would have to run a marathon without eating any more than usual — their high metabolism is not necessarily a good thing, as Mr. Pagano details in a study published Thursday in the journal Science.

To maintain those stores of fat, the bears have to venture out onto Arctic sea ice to catch seals. But as global warming makes the sea ice more scarce, the bears have to search farther and wider for food, and the more they search, the more fat they burn — putting them at risk for starvation.

“Some of these bears moved up to 250 kilometers over the course of an eight- to 11-day period, so they could really cover quite a distance over a short period of time,” Mr. Pagano said, describing the study.