The research, published Thursday in the journal Science, found the bears are not capable of slowing their metabolism by going into "waking hibernation" when food is scarce.
That contradicts a long-standing hypothesis that polar bears could adapt to shrinking sea ice by conserving energy if they couldn't hunt.
"Bears cannot use a hibernation-like metabolism to meaningfully prolong their summer period of fasting and reliance on energy stores," the study says.
"Our findings suggest that bears are unlikely to avoid deleterious declines in body condition, and ultimately survival, that are expected with continued ice loss and lengthening of the ice-melt period."
Researchers tracked polar bears in the Beaufort Sea north of Alaska for two years. They measured their body temperature and observed their activity.
The hefty mammals depend on winter hunting to build up enough fat to carry them through the lean summer months on land. But Arctic waters now don't often freeze up until early December and thaw much earlier in the spring. That leaves polar bears with less time to bulk up on fatty seal meat.
Polar bears can lose at least one kilogram of fat a day when they aren't on the ice. Given they can be off the ice for up to 150 days, the hefty bears can lose well over 100 kilograms, leaving some emaciated by the winter freeze-up.
"Polar bears are indeed threatened by loss of sea ice," said Steven Amstrup, chief scientist at Polar Bears International and co-author of the paper. "They have no greater ability to survive a period of food deprivation in summer than other mammals do."
The study did find that polar bears are much more efficient swimmers than previously thought.
When bears swim in frigid water, they cool their outermost core tissue, which creates an insulating shell that prevents hypothermia and protects internal organs, the study suggests.
"It's not been observed in very many species, so it's a really interesting scientific discovery," Amstrup said. "It appears to be a great adaptation, but it doesn't mean that polar bears can swim forever."
Canada is home to about two-thirds of the world's polar bears, but experts say climate change could make the western Hudson Bay population extinct within a few decades.
Andrew Derocher, one of the country's leading polar bear experts based at the University of Alberta, said when he first began researching the mammals in the 1980s, summer sea ice was within view of the shore.
Now, he said, the ice is between 600 kilometres and 800 kilometres from shore — an incredible distance to swim. This most recent study gives researchers a better sense of the polar bear's limits, he said.
"It would only take one or two really bad years in the Hudson Bay system to really tip those bears over the edge."
Brandon Laforest, senior specialist of Arctic species and ecosystems with the World Wildlife Fund, said the study underlines that the "No. 1 threat to polar bears" is climate change.
"Action can still be taken," he said from Iqaluit, Nunavut. "The effects of climate change on polar bears can be lessened by a concerted effort to reduce emissions."