This is the fifth of a five-part series on Metabolism.
Carbohydrate metabolism involves both anabolism and catabolism. Because your body prefers the glucose provided by dietary carbohydrates for fuel, most glucose that enter cells is catabolized or torn apart to produce energy. Very little is used for anabolism. When not enough glucose is available for fuel, the cells catabolize fatty acids next and only then amino acids.
This is why it is important to eat carbohydrates with protein; the carbohydrates will then be used for fuel and the protein will be spared and used to anabolize new protein and muscle. This is sometimes called the protein-sparing effect of carbohydrate. When only protein is eaten the cells have to catabolize it for fuel instead of making it into new protein and tissue. For this reason most protein powders used as supplements contain some carbohydrate too. That way they can be sure that the protein is used for its intended purpose.
The glucose absorbed through the intestine immediately goes into the portal system, the blood vessels that connect the intestinal blood supply directly to the liver. The liver then has first crack at the glucose absorbing up to two-thirds of it.