When humans engage in highly strenuous exercise day after day, they start to metabolize the body’s reserves, depleting glycogen and fat stores. When cells run out of energy, a result is fatigue, and exercise grinds to a halt until those sources are replenished.
Dogs are different, in particular the sled dogs that run the annual Iditarod Trail Sled Dog Race in Alaska. This is a grueling 1,100-mile race, and studies show that the dogs somehow change their metabolism during the race.
Dr. Michael S. Davis, an associate professor of veterinary physiology at Oklahoma State University and an animal exercise researcher, said: “Before the race, the dogs’ metabolic makeup is similar to humans. Then suddenly they throw a switch — we don’t know what it is yet — that reverses all of that. In a 24-hour period, they go back to the same type of metabolic baseline you see in resting subjects. But it’s while they are running 100 miles a day.”
Dr. Davis, who studied the sled dogs, found they did not chew up their reserves and avoided the worst aspects of fatigue. He is pursuing the research for the Defense Advanced Research Projects Agency, which gave him a $1.4 million grant in 2003 to study the physiology of fatigue resistance of sled dogs.
Dr. Davis, who is teaming with researchers at Texas A&M in a $300,000 Darpa grant, awarded last fall, has been traveling to Alaska for years to learn why the sled dogs are “fatigue-proof.”
“They have a hidden strategy that they can turn on,” he said. “We are confident that humans have the capacity for that strategy. We have to figure out how dogs are turning it on to turn it on in humans.”
Researchers have not demonstrated that ability in other species, but Dr. Davis said migratory mammals or birds could have it. Nor is it similar to the mammalian diving reflex that lets aquatic mammals like seals, otters and dolphins stay under water for long periods of time by slowing metabolic rates.
“The level of metabolism is staying the same,” Dr. Davis said. “It’s not slowing down their calorie burn rate.”
In fact, sled dogs in long-distance racing typically burn 240 calories a pound per day for one to two weeks nonstop. The average Tour de France cyclist burns 100 calories a pound of weight daily, researchers say.
How the dogs maintain such a high level of caloric burn for an extended period without tapping into their reserves of fat and glycogen (and thus grinding to a halt like the rest of us) is what makes them “magical,” Davis says.
If Dr. Davis and the Texas A&M researchers identify the biomarker, or “switch,” that could help the military understand and develop ways to control and prevent the physiological effects of fatigue in strenuous cases like combat.
“Soldiers’ duties often require extreme exertion, which causes them to become fatigued,” Jan Walker of Darpa wrote in an e-mail message. “Severe fatigue can result in a compromised immune system, making soldiers more susceptible to illness or injury.”