Understanding Sugar and Your Body

Sugar has a bad reputation today because too much consumption of refined sugar is associated with obesity and diabetes. However, sugar is a term that refers to a chemical composition and not all sugar is bad. Everyone needs glucose which is an energy source for cells and tissues, and glucose is sugar.

What exactly is sugar though? What is sugar’s relationship to carbohydrates and glucose? Actually, sugar and glucose are terms used interchangeably in conversation (for example, people say blood sugar level or blood glucose level). Sugar or glucose comes almost entirely from carbohydrates derived from both plant and animal dietary sources. What most diabetics know is that insulin is needed to transport the glucose to cells for fuel, but carbohydrates play a role also.

Sugar is chemically structured from different combinations of carbon, hydrogen, and oxygen molecules (carbon and water, in other words). These organic substances typically originate from consuming sugars and starches.

What is Sugar?

Sugar is a type of carbohydrate, and carbohydrates are categorized by their chemical structures. It is a substance that is water soluble and crystalline. Certain foods contain starches or sugars or both which means they contain carbohydrates. Not many foods are totally carbohydrate free. Carbohydrates are found in fruits, vegetables, breads, milk, pasta, grains and other foods. Though carbohydrates tend to be viewed in a negative context, the fact is the body needs carbohydrates and the other nutrients in carbohydrate containing foods.

As mentioned, there are different types of carbohydrates (sugars).  Carbohydrates are defined according to the number of molecules they possess. As a result, they are divided into specific types using their chemical structure. Based on the number of molecules linked together in chains, carbohydrates of particular interest to diabetics are named monosaccharides, disaccharides, and polysaccharides: [1]

  • Monosaccharides are made of a single sugar chain and represent the simplest carbohydrate. Consequently, they are referred as simple sugars. This group cannot be metabolized to a simpler form and remain a sugar. Common examples of monosaccharides are glucose and fructose. Glucose is obtained from starch. Fructose is derived from fruit or cane sugar. Another simple sugar, galactose, comes from the lactose in dairy products.
  • Disaccharides are comprised of two sugar chains linked together. The structure may or may not consist of the same two monosaccharides. Common examples are sucrose (table sugar), lactose (milk sugar) and maltose (malt sugar).
  • Polysaccharides are called complex carbohydrates and are made up of ten or more simple sugar chains (monosaccharides). Common examples include glycogen, fiber and starches.
  • Oligosaccharides consist of anywhere from 2 to 10 monosaccharide molecules and are normally formed from polysaccharides that have been broken down. The best example of this type of carbohydrate is fructo-oligosaccharides found mostly in vegetables.

Sugar, Digestion and Glucose

Diabetics are concerned with all types of carbohydrates, but glucose is the most important monosaccharide. It is used by the body in several ways.

  • Glucose is used in the cell’s mitochondria to produce adenosine tri-phosphate (ATP). ATP is referred to as the “currency of energy” for the body.  This sugar is characterized by rapid oxidation during the process of metabolism and rapid energy conversion, making it an important energy source for cells.
  • Glucose is also used by the central nervous system (CNS). Though the CNS does not expend as much energy as other parts of the body, it nevertheless does need a constant and consistent supply of glucose.
  • Glucose is necessary in the formation of proteins that are formed from amino acids.

The process of converting carbohydrates into sugar or glucose begins in the digestive tract. In order to permit absorption, all carbohydrates you consume are initially reduced to a monosaccharide – a simple sugar – in the digestive tract. Once the carbohydrates in food reach the small intestine, they are broken down into maltose, sucrose and lactose and then into the simple sugar – glucose. At that point the glucose is absorbed through the intestinal wall into the bloodstream.  So the first phase of carbohydrate processing is the simple reduction to its basic chemical structure.[2]

After absorption into the bloodstream through the intestinal wall, the glucose is delivered to the liver where it is either distributed to the body’s cells or stored for future energy needs. It is released as required to ensure the blood glucose level is approximately 100 mg/dl. Any excess amounts of glucose are stored in the liver and muscles. The liver can store, at most, about 90 gm, while muscle storage capacity is 300 gm. For glucose to become blood sugar, it must be processed with the help of insulin into fuel for the cells.

When the storage capacity of the liver and muscles is reached, the excess glucose is converted into glycerol. This substance combined with fatty acids will form triglycerides and are stored in adipocytes or fat cells.[3]

Sugar Metabolism

In summary, sugar metabolism works as follows:

  1. Glucose within the cell is converted into energy
  2. Excess glucose is then stored in the liver as well as muscles in the form of glycogen
  3. When the capacities of these storage units are reached, any further excess is converted into fat
  4. Fat has a very high storage capacity

Insulin is required when it becomes necessary to release glucose for energy needs. The basic process is as follows:

  • The alpha cells in the islet cells of Langerhan on the pancreas release the hormone glucagon
  • Glucagon converts glycogen to glucose
  • After the available supply of glycogen becomes exhausted, the energy stored in fat cells is accessed.  In the body fat, the glycogen source has been converted into glycerol and fatty acids.
  • The process is then reversed. The glycerol is converted to glucose, while the fatty acids are broken down and produce a by-product called ketones.

Essentially, glucose cannot be used by cells as fuel without the presence of insulin. There are exceptions to this rule, but generally insulin must be present in adequate quantity for glucose to work. When the cells can’t get enough glucose for energy, the body then begins to burn fat instead of burning glucose. The ketones that are produced from the breakdown of fat can accumulate in the urine and blood. This produces the medical condition of ketoacidosis which can lead to a diabetic coma.

Relationship of Glucose and Glands

Glucose and the adrenal endocrine gland are also involved in the processing of carbohydrates. The adrenal glands, also known as the suprarenal glands, are components of the endocrine system. There are two of them and are found on the top of each kidney. The adrenal glands perform a number of functions, but they are best known for their ability to secrete specific hormones. These include the following:

  1. Adrenaline: This hormone combines with glucagon to convert glycogen into glucose. It also acts to stimulate the manufacture of glucose from protein. Adrenaline can also decrease the physical absorption of glucose by both the muscles and liver. Furthermore, adrenaline assists with breaking down fat into fatty acids.
  2. Cortisol: This hormone is capable of increasing the amount of glucose by stimulating the breakdown of both protein and fat. Cortisone also decreases physical absorption of glucose by cells that require insulin.

The pituitary gland is also an endocrine gland and is attached to the brain. It is one of the most critical components of the entire endocrine system. The anterior pituitary gland secretes a number of hormones including a hormone that helps to regulate blood glucose levels (ACTH) and a growth hormone.[4]  Disorders of the pituitary gland can affect the absorption of glucose in both the liver and muscles.

Clearly carbohydrates are important to the body, and should not be shunned as dietary evils. For type 1 and type 2 diabetics though, the type and amount of carbohydrates consumed is important to managing the disease. Understanding the relationship of carbohydrates and glucose creates a good foundation for understanding dietary choices.

References

[1] Murray, RK;  Bender, DA; Botham, KM; Kennelly, PJ; Rodwell, VW; and Weil, PA (2009). Harper’s Illustrated Biochemistry 28th ed. New York: Lange McGraw Hill.

[2] Guyton, AC; and Hall, JE (2011). Textbook of Medical Physiology, 12th ed. Philadelphia: Saunders.

[3] Marcovitch, H (2006). Black’s Medical Dictionary 41st edition. Lantham, Maryland: Scarecrow Press.

[4] Guyton and Hall, op. cit.

This article was originally published July 12, 2012 and last revision and update of it was 9/10/2015.