Glycogen turns out to be a crucial piece of the metabolic switch. During the first few days of racing, sled dogs draw energy from glycogen stored inside muscle cells. But instead of depleting glycogen stores and tiring the muscles, the animals suddenly switch to a glycogen-sparing metabolism. They start drawing energy from sources outside of the muscles.
Davis suggests that the muscle cells start extracting fat directly from the blood and somehow transport this fat across the cell membranes and into the cells, where it can be burned as fuel. During race times, fat builds up in a sled dog’s blood, most likely because of the high-fat racing diet. Each 50-pound canine consumes about 12,000 calories daily (typically 60 percent fat and 40 percent carbohydrate and protein).
