Effect of multiday exercise on serum hormones and metabolic substrate concentrations in racing sled dogs

Abstract

Prolonged submaximal exercise relies on the steady delivery of oxidizable substrates to the working muscle, with the sources of those substrates either stored reserves or food absorbed from the gastrointestinal tract during exercise. Fat oxidation could be advantageous for this type of exercise because of potentially greater reserves, but recent studies suggest that athletic dogs remain highly dependent on carbohydrate to fuel exercise despite ingesting a high fat diet. The purpose of this study was to characterise the pattern of exercise-induced hormone and substrate concentrations as they relate to carbohydrate and fat metabolism during prolonged submaximal exercise in dogs. Two studies (a 10-dog pilot study and a subsequent primary study using 54 Alaskan sled dogs) were conducted with the dogs running 160 km/day for 4 or 5 days. Blood samples were obtained within 60 min of cessation of daily exercise and in the second study within 30 min of the start of the next day of exercise. Samples were analysed for key hormones and substrates. Results demonstrated the development of a strong hormonal stimulus for glycogenolysis/gluconeogenesis that coincided with sparing and replenishment of muscle glycogen. The stimulus for glycogenolysis/gluconeogenesis tended to diminish during rest periods in the early stages of the exercise challenge, but remained increased during later rest periods and for several days after the conclusion of exercise. These data support the hypothesis that in the face of a high-fat diet, ultra-endurance racing sled dogs rely on large amounts of hepatic glucose output to support prolonged submaximal exercise.