Unlocking the mysteries of sled dog metabolismThe sport of sled dog racing took a nano-type leap last year when Lance Mackey did the unthinkable: over 2,000 miles and two 1st place wins in less than a month. His team’s startling feat caused even a non-mushing, x-c ski racer like me to stop and take notice. This win was a benchmark for the endurance sport world that grabbed your attention like a 50 below wind chill.Mackey’s record, I was certain, would be quickly challenged by other mushers.Whether it was my sports nutrition background, or because I was still feeling pretty flat one week after skiing a mere 50 kilometers in the American Birkebeiner, I couldn’t help questioning how dogs could not only run – but race – two ultras less than 10 days apart. If this were to be the future of Iditarod teams, obviously the risk of stress related problems such as musculoskeletal injuries, diarrhea, immune system depression, or chronic fatigue syndrome, would be increased exponentially.I turned to one of the nation’s experts in canine physiology and nutrition, Michael Davis, PhD, DVM, Oklahoma State University for some insight on this intensive type of racing. Davis, who has been studying the cause of fatigue during long distance exercise since 1993, has had help from a group of PhD veterinarians, drawn from several different universities throughout the U.S. Sled dogs have been used as their model to study fatigue because of their fantastic ability to run for extended distances.Follows is a recap of two important studies that Davis’ group has published and our conversation about what it could mean for mushers who attempt to race multiple ultra distance events. The first was published in Medicine & Science in Sports & Exercise, March 2005, and the follow-up study in the American Journal of Veterinary Research, August 2008.Glycogen Replacement: How Much Carbohydrate for Ultra-length Events?The first study Davis’ team put together went something like this: feed a high fat diet to a well-trained dog sled team, run them 80 kilometers at 40% VO2 max (low intensity), rest 8 hours, feed the dogs again and then run back. Do this 160-kilometer per day routine for five consecutive days, testing levels of glycogen by muscle biopsy after each run segment was completed before feeding. The overall purpose of the study was to see the cumulative effects of prolonged sub maximal exercise (800 kilometers total) on muscle glycogen stores. Throughout the test period Davis had the dogs fed a special diet, which was given in four feedings, alternated as a snack or full meal. The snack was a delicious beef broth, which provided 40% of calories from protein, and 57% from fat, almost no carbohydrate. The full meal consisted of a combination of dry dog food and meat combination that provided about 55% of calories from fat, and 17% from carbohydrate. So basically, the dogs were not being given much replacement carbohydrate to rebuild stores for 5 consecutive days. What happened? Well the results were as amazing as Mackey’s wins. Davis shared his thoughts in an email to me: “…As you might suspect our findings relative to the fat/carb issue are are creating some contention in the field. In a nutshell, the studies suggest that carbohydrate supplementation may be of minimal benefit in endurance-racing sled dogs due to their ability to rapidly move away from reliance on muscle glycogen, so much so that they actually replenish muscle glycogen during multi-day runs.Based on a very simplistic “they burn what they are fed” approach (probably valid since in our studies they have not changed body weight or condition), as well as some unpublished data from blood samples, the data indicate that the capacity for fat oxidation rapidly increases in response to exercise, with some elements of the process increasing 3-4 fold in the span of 24-48 hours. The suggestion that the same thing could be accomplished in humans has been met with a cool reception in certain human exercise circles, but enthusiastically embraced in others. Aside from the logistical advantages of burning fat instead of carbohydrate (you can store far more fat than carb), the shift away from carbohydrates seems to blunt two of the more deleterious hormonal responses to exercise, i.e., cortisol and catecholamines (collectively responsible for immunosuppression following strenuous exercise as well as possibly the gastrointestinal mucosalproblems associated with strenuous exercise).I had specifically asked Davis what he thought the optimal snacks and snacking pattern would be for an endurance ultra race. He responded:“… but quite honestly, the answer to the question of “What do you/should you snack?” may come down to “Whatever they will eat.” Any calories are better than none. I preach that a trained endurance dog has comparatively little reserve fuel, and thus they can only convert to miles what they last ate. A rough ballpark estimate is 80 calories/mile, so if you plan on running 100 miles today, you had better get 8000 calories into each dog. Obviously, that number can increase on challenging terrain and/or heavy loads, but it is a useful point for the mushers to keep in mind. 100 miles/day is not a bad pace, but won’t win races. If you want to cover more ground/day, you either have to move faster while moving or run longer and cut into the rest and eating time. I find that most experienced mushers intuitively understand all this, even if they don’t recognize the “math” as such initially. As you have pointed out, solving all the equations simultaneously consistently comes down tofat in the diet. The last point … is that a subject doesn’t adapt to the efficient oxidation of fat overnight. Our preliminary research has found that the ability to rapidly adapt to burning fat while running actually develops over the long haul of training and feeding. Many mushers that I know have moved from feeding a lower fat diet during the off season to maintaining the dogs on the same high fat racing diet year-round, andcontrolling the dogs’ weight by controlling the mass of food provided. Anecdotally, they report that the dogs regain their racing fitness much faster, and are capable of longer faster runs earlier in the season than they previously observed. Whether this is due to the altered feeding strategy, or simply they are discovering that they weren’t really pushing the dogs as hard as they thought, is unknown. However, we have limited experimental evidence to suggest that there is some physiology behind their observations. As a sport nutritionist who has participated and hung around racing all my life, I thought Davis brought up some important points. First, feeding “whatever a dog will eat” implies what experienced racers know only too well, and that is people and dogs have wide variability in taste and tolerance, particularly during exercise. A diet of close to 60% fat was used in his first study, but in the second study, the musher elected to feed more dry dog food, raising the carbohydrate content of the diet to 33%. Despite the availability of carbohydrate, the same high rate of fat was used for fuel (77%).A last question for Davis had to do with the limitations of utilizing fat, particularly during higher intensity level exercise. He responded:I think there is a place for CHO-loading (i.e., exercise intensities that exceed the capacity for fat burning and/or start tapping into the intramuscular substrate reserves). However, endurance sled dog racing, and quite possibly some human endurance activities, don’t fit that criteria. Frankly, the limiting factor for the sled dogs appears to be their ability to shed metabolic heat. Pre- and post-exercise carb loading in endurance dogs probably doesn’t hurt anything, but it likely doesn’t help and therefore may be wasted time, money, and effort. Similarly, there doesn’t seem to be any choke-point phenomenon for transport of fatty acids into the mitochondria, so I would not expect any benefit of L-carnitine.What can we expect for future research?The future grants and research areas are pretty straight-forward: Figure out exactly what is changing to explain those two articles, and then figure out whether that is something that is inherent or develops as part of the conditioning process…as you say, it is a fascinating area and I like working with the dogs and people.Nutrition tips for mid-distance and stage racesA lot of racers, of course, will not be doing ultra-distance races, but the right fuel is just as important. Read what the experts have to say:Joseph Wakshlag DVM, PhD, DACVN Assistant Professor of Clinical Nutrition Cornell University College of Veterinary Medicine: “I think the jury is still out as to whether post-exercise CHO supplements are warranted in distance racing. I thought that they might be most useful during prolonged rest periods like the 24 hour stops and some folks I have talked to use it during the 8 hour layovers. But I still think it is most useful in sprint racing and likely the stage stop racing. I think the work of Arleigh Reynolds and myself has put that issue to rest for the sprinters. Now the big question for the sprinters is do you add protein to the Post exercise carbohydrate supplement or not? We can debate that one for awhile.”Tim Hunt, DVM Iditarod veterinarian, distance and stage musher, and creator of Momentum dog food:“The problem with just feeding extra carbohydrate when you don’t really need it has to do with having all that food in the GI (gastrointestinal tract) causing problems while the dog is running…extra bulk, more stool, diarrhea.”Donna Marlor, RD, BSN, MA is a registered dietician who specializes in endurance sports nutrition and weight management. She lives in Marquette, Michigan with her husband and chocolate lab, Annie. Donna has raced competitively in x-c skiing since college, and is an avid fan of dog sled racing. Donna’s motto: “Change your diet and change your life. Start today.”


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