Carbohydrates are essentially sugars. The primary sugar of the body is glucose. Glucose is the first line or preferential source of energy for the body. The body needs glucose in certain tissues and cells in order for them to function correctly. When the blood glucose (or ‘blood sugar') is low, the body will breakdown its glucose energy reserves to replenish the concentration of the blood glucose to the proper level. Glucose is stored in the body as a longer molecule called Glycogen. Glycogen is stored mostly in the liver and in much smaller amounts in muscles. When the blood sugar level drops, glycogen is broken down into its smaller units (glucose) and is released into the blood stream.
If the body does not have enough blood glucose, and has used up all of its glycogen storage units, the body will make glucose. This process is complex and results in new glucose molecules being produced so certain cell types can continue to function correctly. This recreation of glucose occurs at the expense of proteins in other tissues, usually muscles. In other words, muscle tissues are broken down, if necessary, to keep the blood sugar up to the correct level.
The minimum amount of carbohydrate required to prevent the starvation activation systems (ketosis) of fasting is 100 to 150 grams per day. Carbohydrate is needed to satisfy the glucose requirement of the brain (120 Grams per day), red blood cell (30 grams per day) and wound healing (20 to 60 grams per day). These tissues primary fuel is blood sugar or glucose. Muscle tissue is preserved by providing 180 to 200 grams of carbohydrate per day. This is the minimum carbohydrate requirement without the body resorting to breaking down other tissues to build new glucose.
In simplistic terms, a person eats carbohydrates and the body absorbs the sugars for cellular function. If you eat enough carbohydrates, the body will keep its reserves in place. If you eat too many carbohydrates, the body will make the glucose storage molecule called glycogen. If you eat excessive amounts of carbohydrates, and the liver is full of storage molecules, the body will take the carbohydrates and make fat. If you do not eat enough carbohydrates, the body will break down its reserves from the liver. When the liver is ‘empty' of carbohydrate stores, it will make more blood glucose at the expense of other tissues, primarily muscle. So in short, too much sugar and you get fat, not enough sugar and you will loose muscle. As a last resort, the body will burn its fat reserves for energy, and will do so also using carbohydrate to burn the fat.