Carbohydrate as a source of energy

Fats and carbohydrates are equally important sources of energy when the body at rest and at low-levels of exercise, such as a slow walks.

During short term, intense exercise, carbohydrates are the main source of energy. Carbohydrates are made of carbon, hydrogen and oxygen and are classified as either simple or complex, based on the number of sugar molecules present.

Carbohydrate in the form of glycogen is the most readily available energy source. Compared to fat, carbohydrate provides energy more quickly, can be used whether or not oxygen is present and serves as the sole source of energy for the central nervous system.

During prolonged exercise, fats become the main energy source, and the shift from carbohydrate to fat utilization (carbohydrate sparing) is enhanced by physical training.

For the people of many countries, 80% of their total daily calories come from complex carbohydrates.

Carbohydrate such as starches and sugars are an efficient and readily available energy source especially for the brain. They are also necessary for fat and protein metabolism. Carbohydrates provide energy with 4 calories per gram.

Plentiful stores of carbohydrates in the body are important for excelling in physical activity, especially endurance exercise. During exercise utilization of carbohydrate accelerates an increased release for glucose from the liver is functionally important to maintain blood glucose homeostasis and to possibly attenuate muscle glycogen glycogen depletion.

If inadequate carbohydrate is available, the body burns protein for energy. This protein comes from food and from the body’s own muscle tissue.

Therefore, in cases of inadequate carbohydrate and protein intake, not only would muscle wasting occur, but adequate proteins would not be available for repair of body tissue.
Carbohydrate as a source of energy 

Glycogen in human nutrition

During and immediately after a meal, glucose is converted in the liver into the storage polysaccharide glycogen by a process known as glycogenesis.

Although the total quantity of glycogen in the human body is low, considerably less than one-tenth percent of the total body weight, its role is primarily that of a storage carbohydrate, similar to the role of starch in plants cells. It occurs predominantly in the liver where it is important in the homeostatic mechanism regulating glucose level of the blood.

Glycogen is a branched chain polymer of 6,000 to 30,000 glucose units, that contains two types of glycosidic linkages, extended chains of alpha1-- 4 linked glucose residues with alpha 1-- 6 branches spaced about every four to six residues along the alpha 1-- 4 chain. It is similar to amylopectin in structure but is more highly branched. The average chain length is only 10 to 24 glucose units with 3 to 4 glucose units between branching points.

The highly branched structure of glycogen makes it possible for several glucose residues to be released at once to meet energy needs. 

Glycogen is stored in two tissues. In the liver, glycogen is stored for the short-termed maintenance of blood glucose. In muscle, glycogen is stored as a source of energy. Muscle glycogen is estimated to have a molecular weight of about 1000000 where as the liver of glycogen molecule is much larger, approximately 5 X 1000000. Both molecules, however, constantly change in size as glucose molecules are added or removed.

Glycogen plays an important role in the glucose cycle. The release of glycogen stored in the liver is triggered by low levels of glucose in blood. Liver glycogen breaks down to glucose-6-phosphate, which is hydrolyzed to give glucose.

The release of glucose from the liver by these breakdowns of glycogen replenishes the supply of glucose in the blood.

The most common disease in which glycogen metabolism becomes abnormal is diabetes, in which, because of abnormal amounts of insulin, liver glycogen can be abnormally accumulated or depleted.

Several hereditary defects have been identified in the synthesis and catabolism of glycogen including: Gierke’s disease, Type II glycogen disease, Type III glycogen, Type IV disease, and McArdle’s disease.
Glycogen in human nutrition