One year Erica McKenzie watched a team of sled dogs cross the finish line at the Iditarod barely winded.

"Those dogs looked like they'd been out on a 10-mile walk," said McKenzie. "They were ready to go out again."

They'd just finished running 1150 miles in 9 days.

McKenzie wondered where all their energy came from.

As an assistant professor of veterinary medicine at Oregon State University, McKenzie decided to find out.

She will greet the finishers of this year's Iditarod and take blood samples from 100 dogs. She believes the secret to the dogs' phenomenal endurance may lie in proteins in their blood.

Human marathoners rely on glycogen for long periods of exercise. When they run out of glycogen, they "hit the wall" and cannot continue.

In blood taken from dogs during 500-mile test runs, McKenzie found the dogs run out of glycogen on their first day of racing.

Yet they kept right on running.

Day after day.

"They don't seem to hit a wall and that's very interesting to some of us in exercise physiology," says McKenzie.

Something inside the dogs' metabolism is switching to another source of energy. McKenzie speculates blood globulin may be part of that energy source.

Diet could play a big part in that.

The typical sled dog consumes 4,000 to 6,000 calories a day, far more per pound of body weight than any Tour de France bicyclist.

Tour de France racers consume massive quantities of carbohydrates. Sled dogs consume massive amounts of protein.

"I think the fact they have such different diets is part of the reason they are so capable of performing prolonged endurance exercise like this," McKenzie said.

She wonders if human athletes could change their performance with a similar diet.

"I think it (her study) will contribute to the field of exercise physiology, understanding what fuels are available to the endurance athlete," she said.