Carbohydrates are utilized for energy, both instant and sustained. When insufficient carbohydrates are taken in, the body must utilize proteins for energy even to the point of catabolizing muscle tissue for energy. If your body uses muscle tissue for energy you lose muscle mass and become weak.
Carbohydrates: Digestive enzymes in the small intestines break down the carbohydrates into glucose. The glucose can be immediately utilized by the body or stored as glycogen in the muscles and liver. The muscles can store about 20 minutes of glycogen for energy. The bloodstream can hold about an hour of glucose for energy. If glucose levels are maximized and all glycogen storage locations are full then the excess glucose is converted to fat by the liver and stored in adipose tissue or fat cells. There is really no limit to the amount of fat that a body can store.
There are three types of carbohydrates Monosaccharides, Disaccharides and Polysaccharides. Monosaccharides are simple sugars and are the basic unit of carbohydrate. Examples of monosaccharides are glucose and fructose. Disaccharides are composed of two monosaccharides. Examples of disaccharides are table sugar (sucrose) which is composed of fructose and glucose also milk sugar (lactose) which is composed of glucose and galactose. Polysaccharides are composed of multiple monosaccharides. Examples of Polysaccharides are starches (bread, fruit, grain, pasta, rice). These are also called complex carbohydrates.
Fiber: Fiber is a form of carbohydrate. Approximately 20 grams of dietary fiber is required in our diets. Fiber facilitates elimination and decreases appetite as a bulking agent. Fiber also inhibits the absorption of cholesterol into the blood stream. It has also been shown that fiber slows the absorption of sucrose into the bloodstream. This can be important in the treatment of type II diabetes. Too much fiber in the diet can restrict the absorption of necessary vitamins and minerals. Excess carbohydrates are converted into fat by the liver and stored in adipose tissue. Sugar is absorbed into the bloodstream within minutes. Consuming large amounts of sugar prior to exercise can actually inhibit performance. This produces a drastic increase in blood sugar. This causes the pancreas to secrete large amounts of insulin to metabolize the sugar. All this insulin inhibits the metabolization of fat by the muscles. Therefore, the muscles rely more on glycogen which is in limited supply. The insulin reduces blood sugar level which is already being reduced by the muscles utilization of glycogen stores for energy production. The blood sugar level reduces to a level which may not only cause fatigue but dizziness as well. Therefore consumption of excess sugar prior to exercise reduces performance and endurance.
Fats: Fats are digested by the enzyme lipase in the small intestines with the assistance of bile salts as emulsifiers. They are then transported through the bloodstream with the assistance of lipoproteins (fat + protein coating + phospholipids) and stored as triglycerides (glycerol + 3 particles of fatty acids) in fat cells. They are then released into the bloodstream as fatty acids when energy is required.
The fatty acids travel through the bloodstream and are combined with glucose to burn the combination as energy. The combination of fatty acids and glucose is necessary for aerobic energy production. The anaerobic system uses mainly glucose and phosphagen, which is limited in its ability to produce energy. Further, lactic acid is one of the by products causing the burning sensation after a hard workout. Inadequate carbohydrate availability will result in incomplete fat metabolization producing unused lipids called ketones and leading to a chemical imbalance in the blood known as ketosis. Organ and muscle tissue may be metabolized to provide glucose from the breakdown of protein. Most of the weight loss that occurs by severe carbohydrate restricted diets is from water loss as the kidneys attempt to rid the body of the ketones.