Blood sugar is mostly the simple carbohydrate glucose. Carbohydrate digestion begins in the mouth, proceeds through the gastrointestinal tract and into the blood, where glucose levels are closely controlled by hormones. In the cells, glucose is either stored or broken down for energy. The energy from metabolism is stored mostly as adenosine triphosphate, or ATP.
Digestion in the Gastrointestinal Tract
Digestion of carbohydrates begins in the gastrointestinal tract with the mechanical processing of food by the mouth. Enzymes such as amylase in the saliva begin to break large chains of carbohydrates into smaller chains. This process continues in the stomach and intestines. Many of the enzymes in the intestines responsible for breaking down carbohydrates come from the pancreas.
Absorption from the GI tract
In the intestines, the carbohydrates are broken down into simple sugars such as glucose. The intestine is folded over many times to vastly increase its surface area. It has little hair like projections called villi, which are composed of cells. The cells are able to absorb simple carbohydrates and deliver them to the bloodstream, elevating the blood glucose level.
Hormonal Regulation
The blood glucose level is regulated by many hormones. The pancreas releases two primary hormones known as insulin and glucagon. Insulin is released in response to rising blood sugar and signals the cells in the tissues of the body to take up the glucose from the blood. Many other hormones work with glucose to regulate various elements affecting blood glucose. In contrast to insulin and insulin-like hormones, hormones such as glucagon act to raise blood glucose.
Metabolism and Storage
Once glucose enters the cells, it is either metabolized or stored. If metabolized for energy, it is first broken down from six carbons into a couple of three-carbon molecules in a process called glycolysis. Most of the energy generated is stored and used as ATP. Glycolysis makes a net of two ATP molecules. The three-carbon molecules then go through a process called the tricarboxylic acid cycle, or Krebs cycle. This cycle generates more energy-containing chemicals and breaks down the molecules from glucose into carbon dioxide. Some of the energy-containing molecules are used in combination with oxygen to generate many more ATP molecules.
If glucose is not broken down, it is typically stored. Many molecules, including carbohydrates can be converted into fat. Alternatively, glucose molecules can be linked together and stored as glycogen, especially in the muscles and liver. (See Reference 1 and 2)
Diseases
Some diseases are associated with carbohydrate metabolism. Type I diabetes results from the pancreas not releasing insulin, so blood sugar rises and cells do not get a supply of glucose. Type II diabetes occurs when the cells become resistant to insulin. Glycogen storage diseases occur when the body is unable to break down stored glycogen. Glycogen storage disease type V, McArdle disease, is caused by a deficient enzyme in skeletal muscle. Exercise causes muscle cramps and can cause muscle breakdown.
References
- "Physiology"; Linda S.Costanzo; 4th Ed 2008
- "Lehninger Principles of Biochemistry"; David L. Nelson and Michael M. Cox; 4th Ed 2004
- "Harrison's Principles of Internal Medicine"; Anthony S Fauci et al; 17th Ed 2008
