With tongue and tail wagging wildly, Larry the lead dog crossed the finish line in March in sunny Nome, Alaska—after running 1,131 miles to win the Iditarod Trail Sled Dog Race for the third year in a row. To most mortals, Larry looks like a happy but nondescript, scrawny mutt. To sled dog mushers, he is a mini legend that simply needs no introduction. To scientists, Larry may hold the key to a physiological mystery.
Specifically, sled dogs seem to flip an internal switch that acutely changes how they burn fat calories, allowing them to keep going and going and going with no obvious pain. Figuring out how that mechanism works may have implications for human diabetics and those battling obesity.
Researchers first discovered the metabolic switch in 2005, when a team headed by Oklahoma State University’s Michael Davis—who has been investigating the metabolic, gastrointestinal, respiratory and blood systems of sled dogs for 10 years—did a controlled study at a professional racing kennel in Alaska. Mushers ran the dogs in mock, 100-mile races for four to five days in a row. Every 100 miles the researchers took matchstick-size samples of leg muscle (about 60 milligrams apiece) from the dogs to test for protein levels, enzyme activity and glycogen, a starchlike compound that stores energy for quick release.
Glycogen turns out to be a crucial piece of the metabolic switch. During the first few days of racing, sled dogs draw energy from glycogen stored inside muscle cells. But instead of depleting glycogen stores and tiring the muscles, the animals suddenly switch to a glycogen-sparing metabolism. They start drawing energy from sources outside of the muscles.
Davis suggests that the muscle cells start extracting fat directly from the blood and somehow transport this fat across the cell membranes and into the cells, where it can be burned as fuel. During race times, fat builds up in a sled dog’s blood, most likely because of the high-fat racing diet. Each 50-pound canine consumes about 12,000 calories daily (typically 60 percent fat and 40 percent carbohydrate and protein).
According to Raymond Geor, an exercise physiologist at Michigan State University, sled dog muscle cells are well equipped to use this fat because they have a higher mitochondrial density—more cellular power plants—than other animals. The mystery is how the blood-borne fat gets into cells in the first place. Increasing evidence suggests that fat is transported into the cells along similar pathways as glucose, Davis says, with the hormone insulin playing a critical role. Researchers are exploring the sled dog’s sensitivity to insulin to better understand this pathway.
Breeding probably had much to do with the development of the metabolic switch. Larry is descended from a long line of racing dogs. “The bloodlines of my dogs date back 100 years,” says Lance Mackey, Larry’s owner and legendary racing musher, the only person to win the long-distance Iditarod and the Yukon Quest in the same year with the same dog team. “They are mixed breeds—mutts—but they’ve been bred to run.”
Selective breeding, though, may not be the whole story. The dogs may have learned to switch metabolic strategies on demand through intense training. If so, then researchers might have an easier time applying what they learn about the canines to humans training for an endurance event or those seeking treatment for diabetes or obesity. Such patients might benefit, for instance, if researchers could pinpoint the mechanisms that boost the body’s sensitivity to insulin or that better utilize fat that builds up in muscle tissue.
This year Mackey won the Iditarod by one of the widest margins ever, finishing a comfortable eight hours ahead of his closest competitor. That Iditarod, however, will be the last one for Larry, after participating for eight of his nine living years. He will officially retire from racing at the end of this year for a well-deserved, if unneeded, rest.
See what we're tweeting about
8 Comments
Add CommentThose of us humans living in cold climates - like South Dakota - experience a similar effect every fall. Our internal furnace "turns on." After adding layer after layer of clothing and blanket after blanket as the weather cools, we suddenly throw off the covers and put our sweaters back in the closet. We also develop a craving for fat. Could both our "furnace" and the dog's endurance be mitochondria mediated?
Reply | Report Abuse | Link to thisDr. Davis findings about dogs becoming robotic-like while running in the Iditarod dont square with the documented facts about how the dogs behave. The following are just a few examples of dogs refusing to run. According to the Associated Press, in the 2007 Iditarod, eyewitnesses reported seeing musher Ramy Brooks kicking his dogs and beating them with a chain when they wouldnt run. A May 22, 2007 editorial in the Anchorage Daily News said, Rare is the musher who hasnt lost it with his or her dogs. Ramy Brooks isnt the first and wont be the last. Steve Fossett wrote in his memoir Chasing the Wind that he bit his lead dogs right ear when he refused to keep running. Iditarod race winner Joe Runyan recommends shocking dogs with a cattle prod when they refused to move.
Reply | Report Abuse | Link to thisIditarod dogs arent machines. They dont want to run when they are sick or injured. What happens to them during the Iditarod includes death, paralysis, frostbite of the penis and scrotum, bleeding ulcers, bloody diarrhea, lung damage, pneumonia, ruptured discs, viral diseases, broken bones, torn muscles and tendons, vomiting, hypothermia, sprains, broken teeth and torn footpads.
The dogs dont want to run when trail conditions are horrid. For instance, in 2007, Shain Perrins told the Alaska Public Radio that Dogs arent wanting to go, because they are going right into the wind. Its probably blowing, gusting up to 50 miles an hour in the valley.
Dogs dont want to run when theyre tired. They routinely suffer from anemia which exhausts them. Kathie Davis told the Spartanburg Herald-Journal that one of her dogs fell asleep in the middle of a run. Martin Busers dog Quebec vanished from his gangline while he was on the move - and Buser had failed to notice. Buser found Quebec curled up in the snow to nap, right where hed come loose.
In Anchorage, several years ago, a mushers dogs got loose from his truck. They didnt run over 1,000 miles to Nome. They didnt even run to the outskirts of the city. They were found curled up, sleeping on peoples lawns about ten blocks from where the mushers truck was parked.
The blood tests dogs receive do not detect all the performance enhancing drugs they may be given. The dogs are not tested for all the common drugs. And there are so many new drugs, including synthetic versions, that its even hard for testers of humans to keep up. But even knowing what drugs to test for isnt enough. Gene doping works on dogs and its impossible to detect.
Margery Glickman
Sled Dog Action Coalition, http://www.helpsleddogs.org
Margery -
Reply | Report Abuse | Link to thisWhile your concerns are certainly valid, this article isn't really about the ethics of dog racing, and nowhere does it mention the dogs becoming 'robotic-like'.
Instead, it focuses on an interesting biological phenomenon that happens in these dogs internally, and what the discovery of this mechanism could mean for science.
Regardless of the ethics involved, it is fascinating that these dogs have developed a way to work their muscles without the muscle becoming tired. Beyond dieting, this would certainly have interesting implications for athletes and other humans involved in labor-intensive jobs. Think about...marathon runners who don't get tired for days!
Evelyn,
Reply | Report Abuse | Link to thisThe dogs do become tired. You should have read what I wrote. In part, anemia tires the dogs. Read below:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16034156&query_hl=3
1: Biol Trace Elem Res. 2005 Summer;105(1-3):87-96.
Sustained strenuous exercise in sled dogs depresses three blood copper enzyme activities.
DiSilvestro RA, Hinchcliff KW, Blostein-Fujii A.
Human Nutrition, The Ohio State University, Columbus, OH, USA.
Studies show mixed conclusions about acute responses of copper status to strenuous exercise. Because copper function involves metalloenzyme activities, which might take days to change, the present study examined the response of three copper metalloenzyme activities to sustained strenuous exercise in sled dogs. A race lasting 12-15 d depressed activities for both plasma ceruloplasmin and erythrocyte superoxide dismutase in dogs consuming commercial dog foods and meats. A shorter, 3-d training run for dogs fed a commercial balanced diet also depressed ceruloplasmin activities but not superoxide dismutase activities. Dogs fed the same diet but that did not run showed no changes in either parameter. Activities of a third copper enzyme, plasma diamine oxidase, also decreased after a 3-d training run. In summary, blood activities of three copper enzymes were depressed by sustained strenuous exercise in sled dogs.
-------------------------------------------------------------------------------------------------------------------------
According to the National Academy of Sciences Board on Agriculture & Natural Resources, copper is a helper in enzymatic reactions. The function of copper includes: Connective tissue formation; iron metabolism; blood cell formation; melanin pigment formation; myelin formation; defense against oxidative damage. Anemia is a sign of a copper deficiency.
- Board on Agriculture & Natural Resources, National Academy of Sciences, website, 2005, http://dels.nas.edu/banr/cd_dog_vit.html
While the article by Krista West doesn't portray the dogs as being robotic-like, Dr. Davis has described the dog this way.
Margery
Some dogs get tired. Some don't. Some love running and winning. Loving it or not, some run themselves to death, or are forced to. (We see the same range of conditions in humans who run endurance events.) Whether that's right or wrong was not the subject of the article and I don't think this is the forum for it. The ability of some dogs to run without tiring is the subject of this article. It's fascinating and very relevant, since they are mammals whose cellular biochemistry is essentially the same as ours. The "robot" or "machine" simile is appropriately used in admiration of an athlete whose performance seems beyond what a biological organism could be capable of.
Reply | Report Abuse | Link to thisScientific American is a prestigious journal and requires it's authors of articles to have credentials. What are Krista West's credentials and why were they not posted under her name in the article?
Reply | Report Abuse | Link to thisLee in Michigan
Our goal is to provide the most accurate and up to date information on all weight loss products, in the web: www.2daydiet-green.com/ , there are many kinds’ goods, (2 day diet Japan ling zhi)|(super slim slimming capsule) | (super slim pomegranate capsule)|(2 day diet japan ling zhi pills)|(meizitang side effects)|(lida daidaihua slimming capsule)|(2 day diet capsule), you can choose, and you will feel so much better about yourself when you are fit, and this will flow over into other positive changes in your life.
Reply | Report Abuse | Link to thisOur goal is to provide the most accurate and up to date information on all weight loss products, in the web: www.2daydiet-green.com/ , there are many kinds goods, (2 day diet Japan ling zhi)|(super slim slimming capsule) , you can choose, and you will feel so much better about yourself when you are fit, and this will flow over into other positive changes in your life.
Reply | Report Abuse | Link to this