Originally Posted by DannoXYZ

Actually, you have to take timing into account here. You DO want as high-GI a carb as possible to charge up the depleted glycogen stores after a workout. The insulin spike is EXACTLY what you want as well. Insulin triggers glut-4 in the cell membrane and transports glucose from the bloodstream into the muscle cells; right where you want it. That's why a 4:1 carb-protein mix is ideal because it generates a higher insulin-spike than carbs alone.

Also the most pressing thing to get is glucose into the muscle-cells, not protein. You've got a 20-30 minute window when glucose-transport occurs much faster than normal. Best to get glucose in at this time. If you do not have sufficient glucose in your blood, the actual muscle tissue is catabolized and converted to glucose to restock the glycogen stores. This muscle-tissue is preferred because it's nearby. Ingested protein DOES NOT ward off muscle-catabolism due to low blood-glucose. So it's more important to get carbs in than protein after a ride.

The muscle rebuilding actually occurs hours later. And the protein that's used for that comes from an inter-cellular amino-acid pool. This supply is an immediate source. The ingested protein, aside from taking a long time to digest, just goes towards replenishing this amino-acid pool later.

Digestion-rate and metabolism is completely independent. Your body only burns calories at a rate that's required to meet the current level of activity. How fast you digest something has nothing to do with your energy requirements. If you're riding at a 500-cal/hr pace, you're not going to be riding faster if you eat high-GI vs. low-GI carbs. The high-GI faster-digesting carbs will just help you suck in more calories per hour so you can ride farther before bonking, but it won't do anything to make you go faster.