Canine metabolism is unique. Mammalian muscle fibers have been classified into types I, IIa and IIb based on their metabolism. Type I fibers contain less ATPase activity compared with type II fibers. Types I and IIa are characterized by oxidative metabolism, whereas type IIb fibers are characterized by anaerobic glycolytic metabolism. Canine muscle contains mainly oxidative fibers (Armstrong et al. 1982, Gunn 1978a, Snow 1987). Guy and Snow (1981) describe some low oxidative muscle fibers in dogs but acknowledge that the activity of the oxidative enzyme succinate dehydrogenase in the low oxidative fibers was still greater than that in type IIb fibers from other species. Relative to metabolic body size, dogs also metabolize free fatty acids at twice the rate observed in humans (de Bruijne 1981). Dog muscle is, therefore, more adapted to use fat than human muscle and conclusions derived from human experiments may not be valid in dogs.

In humans, the maximum rate of energy expenditure varies for each substrate, but the amount of substrate limits the length of time that this maximum rate of expenditure can be maintained (Hultman et al. 1994). Only creatine phosphate and ATP are able to sustain a 100-m sprint, but because stores are small, this high rate of energy expenditure can be maintained for only a few seconds. Carbohydrate oxidation supports the intermediate speed of marathon runners for 20 miles until all glycogen stores have been used; then, fat oxidation becomes the only source of available energy and runners are unable to accelerate, i.e., they "hit the wall." Stamina is limited by the amount of glycogen in muscle.

In humans, high carbohydrate diets increase stamina because they increase muscle glycogen (Hultman et al. 1994). In dogs, however, high fat/low carbohydrate diets increase stamina. Beagles ran for 20 miles (140 min) when fed high fat (53-67% of energy) diets but became exhausted after only 15 miles (100 min) when fed a moderate fat (29% of energy) diet (Downey et al. 1980). A high fat/high protein diet containing no carbohydrate resulted in better performance and less evidence of exertional rhabdomyolysis when fed to sled dogs (Kronfeld 1973). A high carbohydrate (59% of energy) low fat (16% of energy) diet fed to sled dogs resulted in higher resting muscle glycogen concentrations compared with a high fat (62% of energy), low carbohydrate (14% of energy) diet, but glycogen was used more rapidly during a race; thus the final muscle glycogen concentration was unchanged (Reynolds et al. 1996).

In dogs, as in other species, fat oxidation provides most of the energy at low rates of energy expenditure (60% at 40% of maximal oxygen uptake; VO2 max). As exercise intensity increases, glucose oxidation increases, whereas fat oxidation remains constant so that glucose oxidation is the principle source of energy at high rates of energy expenditure (80% at 85% VO2 max) (Weibel et al. 1996). In dogs, however, the amount of energy from fat oxidation at rest and during exercise is twice that in less aerobic species such as humans and goats (McLelland et al. 1994, Meyer and Doty 1988). Albumin binds more free fatty acids in dogs than in less aerobic species; thus the concentration of free fatty acids in the blood is higher and delivery of free fatty acids to the tissues is enhanced (McLelland et al. 1994). Muscle glycogen and fat stores are larger in dogs than in less aerobic species (Weibel et al. 1996). High fat diets increase resting serum triglycerides and free fatty acids, mitochondrial volume and maximal energy expenditure (Reynolds et al. 1996).

. . . i have heard that dogs who eat raw do not get bloat - is this true?

.... explain why dogs don't need sugars and starches the way we do for energy. I will do a lousy job!

 Quote: Jennifer Mullen

. . . i have heard that dogs who eat raw do not get bloat - is this true?

Yep, it's true. My healthy 8-yr-old GSD died several years ago from bloat. He had been raw fed for about 5 yrs. Vet had no idea what caused the bloat. I have my own theories, but neither of us thought it was diet or exercise/activity related.

Mike