Metabolic responses to exhaustive exercise in racing sled dogs fed diets containing medium, low, or zero carbohydrate

Eighteen racing sled dogs were assigned to three diets containing protein, fat, and carbohydrate in proportions as follows: diet A, 39:61:0; diet B, 32:45:23; and diet C, 28:34:38 on an available energy basis. The dogs were studied through a 28-week training period and subjected to three special tests, the first after 12 weeks training, the second at 24 weeks, and the third 4 weeks later. Overnight fasting, resting blood samples were taken before exercise, then about 5 or 30 min after exercise in the first 2 tests, or 1,5...30 min after exercise in the third test. Negligible changes in the red cell indices or serum concentrations of total protein, sodium, and urea indicated that there were no major water shifts. Significant decrements were found in serum concentrations of albumin (3% of resting value), calcium (4%), magnesium (13%), and inorganic phosphorus (39%). Significant increments were found in serum concentrations of creatinine (50%) and activities of glutamic-pyruvic and glutamic-oxalacetic transaminases (31 and 52%, respectively). None of the above variables showed differences between diets, exercise bouts, or time after exercise. Significant decrements in plasma cholesterol (D, mg/100 ml) were linearly related to the initial concentration (I mg/100 ml); D - 0.161 I - 17 mg/100 ml. Hyperglycemic responses were exhibited by 14 dogs in the 3rd test, including five dogs on diet A. Resting plasma glucose concentrations, peak values after exercise, and removal rates were the same in dogs fed all three diets. Blood lactic acid concentrations were linearly related to plasma glance (two from each group) had significantly higher peak lactic/glucose ratios tthan the six "worst" dogs, but there was no significant difference between diets in other measures of glucose or lactic acid. Plasma concentrations of free fatty acids, acetoacetic acid and 3-OH-butyric acid reached a maximum 10 min after exercise. Peak values and mean increments of free fatty acids were highest in dogs fed diet A. Also, mean free fatty acid increment was significantly higher in the six best dogs than in the six worst. An enhanced ability to mobilize body fat should confer an advantage in a dog subjected to prolonged strenous exercise in which fatty acid oxidation accounts for most of the oxygen consumption.