This study compared the benefits of a high-fat diet (HF – 60% calories from fat) or a high-carbohydrate diet (HC – 60% calories from carbohydrate) in two groups of sled dogs that underwent aerobic and anaerobic exercise tests. Key parameters such as blood pyruvate and lactate and muscle glycogen concentration were measured before and after each test to assess the effect of the diet. Gglycogen storage was greater with HC diet, but it was far exceeded by the utilization during exercise, while the carbohydrate sparing effect of the HF diet was better adapted to endurance exercise.