Carbohydrate Stores: Muscle Glycogen, Liver Glycogen, and Glucose

The energy required to power the human body begins with the consumption of food, and the subsequent extraction by the body of the carbohydrate-based sugars, known as glucose and glycogen. The manufacture, storage, and utilization of these sugar compounds for the energy needs of the body is an intricate and multidimensional process.

Carbohydrates are compounds formed from carbon, hydrogen, and oxygen molecules. Carbohydrates are divided into two general groupings: simple carbohydrates and complex carbohydrates. Simple carbohydrates are the simple chemical structures of monosaccharides, or single sugars, such as glucose and fructose. Complex carbohydrates are composed of complex sugars known as polysaccharides, of which glycogen is the most prominent example.

Foods are divided for nutritional purposes into three basic groups: carbohydrates, fats, and proteins. Each of these food groups is a primary source for different materials essential to the growth, development, and maintenance of the human body; individual foods may contain varying amounts of each of these groups.

Seventy-five percent of the energy derived and stored by the body from carbohydrates is ultimately used by the body to sustain brain function; the balance is divided between muscle function and red blood cell production, essential to the transport of oxygen. The carbohydrate food group is composed of foods that are derived from plants (such as whole grain cereals and their byproducts such as breads and pastas); green vegetables, fruits, and dairy products are also rich in carbohydrates. Proteins, which are used primarily by the body to build and repair muscle tissue, are found in meat, soy products, dairy products, and some nuts. Fats, which the body converts into fatty acids, are essential to the absorption of a number of fat-soluble vitamins that are critical to body health, such as vitamins A, D, and E.

Physical health will generally be maintained with a diet that comprises from 60% to 65% carbohydrates, 12-15% proteins, and less than 30% fat. When the intake of carbohydrates exceeds that which can be stored and converted to energy as glycogen or glucose, the body will store the excess carbohydrates as fat, often leading to weight gain.

The body extracts carbohydrates from food sources through a process known as hydrolysis, whereby the warm fluids, commencing with the saliva in the mouth and concluding with the action of the small intestine, break down the carbohydrates in the food into glucose. As it is a simple sugar, glucose is able to be transported through the wall of the small intestine to be stored by the body in the liver.

Once processed, glucose will take one of three pathways into the body. Irrespective of the route through which glucose is directed in the body, it will be metabolized into energy in the same fashion. While it is commonly stated that the body "burns" its stored carbohydrates, the actual chemical process has an additional component. No matter where the glucose is stored, when it is used it creates a compound known as adenosine triphosphate (ATP), which is the actual energy source within the body.

The first and most direct route into the body for recently converted glucose from the small intestine is the bloodstream, where glucose is immediately available to be converted into ATP, in combination with the oxygen received into the bloodstream from the cardiorespiratory system.

The second repository for glucose is the skeletal muscle system. Glucose is converted into its storage form, glycogen, which is a long string of single sugars stored as a starch, a complex sugar. Once stored in the muscles, glycogen cannot be released into the bloodstream, but will be utilized as fuel to produce ATP by the muscle itself.

The most important storage mechanism of processed glucose is performed by the liver. As the largest organ in the body, the liver performs a number of purifying and metabolic functions within the body, one of which is to store glucose in its glycogen form. The liver is capable of containing up to 10% of its volume in glycogen, in contrast to the 1% storage by volume carried on in the skeletal muscles. The liver both releases glycogen when it is needed for energy production, as well as regulates the amount of glucose present in the blood, critical to health (known as the blood sugar level).

The process by which liver glycogen is converted into blood glucose is related to the actions of the pancreas, which monitors blood glucose levels. When the pancreas determines that blood glucose levels are too low, causing a condition known as hypoglycemia, the pancreas produces a hormone, glucagen, to stimulate a release of stored glycogen from the liver, in the form of glucose, into the blood to restore balance. When blood glucose levels are too high, which is, conversely, a hyperglycemic condition, the pancreas releases the hormone insulin to stimulate the liver to release less glucose. Impaired insulin production in the pancreas is the essence of the condition known as diabetes.

When the production of insulin in the pancreas becomes impaired, careful attention must be paid to blood glucose levels, which may be tested on a frequent basis. Regulation of diet, including attention to the amount of carbohydrate consumed, is essential to the maintenance of healthy blood glucose levels. Exercise, although it may place pressure on blood glucose levels through the body's use of carbohydrate stores to produce ATP, is believed to be an important tool in counteracting the serious potential impacts of a diabetic condition on the cardiovascular system.

SEE ALSO Carbohydrates; Cardiovascular system; Glycogen level in muscles; Lactic acid and performance.