glycogen

glycogen In the 1840s and 50s, Claude Bernard was applying his great scientific mind to the problem of ‘sugars’ in the body, in particular how the liver could apparently make sugars and ‘squirt them into the blood … in a regulated manner’ when he had fed an animal only on protein. In 1855 he coined the term ‘matière glycogene’ — sugar-making material. He removed a liver, washed it out with water, and found that there was still sugar in a subsequent wash-out. He concluded that the sugar-forming substance was stored in the liver, and was not water soluble. Eventually he succeeded in isolating the ‘emulsive material of the liver’, found it to be similar to starch, and listed its properties in an account so complete as to be valid to this day. It was to be over 70 years before the medical significance of glycogen storage came to light, when defective storage in liver, kidney, and heart became recognized. Another 70 years on, and the several associated diseases are well understood, mainly as enzyme deficiencies, whilst deliberate boosting of glycogen storage in muscle before a marathon run is common knowledge.

Glycogen is the form in which carbohydrate is stored in the body. Each molecule of glycogen is formed by the linkage in branching chains of many thousands of glucose molecules. Thus, glycogen is a natural polymer, a polysaccharide, which has a similar structure to the starch which is found in plants.

Most tissues of the body are able to store small amounts of glycogen, but the main sites of glycogen storage are the liver and skeletal muscles. In both cases, glycogen is made from glucose within the cells in which it is stored, and the synthetic process is stimulated by the hormone insulin. When glycogen is stored within muscle and liver cells, it retains water along with it (approximately 3 g of water for each gram of glycogen), so changes in glycogen content can cause quite noticeable changes in total body weight. For example, in the first few days of starvation, glycogen is used by the liver to maintain blood sugar and by muscle metabolism, and the associated water is excreted from the body in the urine, accounting for a major part of 1–2 kg loss of weight.

There are important differences between the major functions of liver and muscle glycogen. The main role of liver glycogen is to provide a reserve supply of glucose in order that blood glucose concentration can be kept at an adequate level to supply the brain (which does not use other fuels) during periods of fasting, or when glucose use is increased during physical work and exercise. Thus, after meals some of the carbohydrate consumed is stored as liver glycogen, and during fasting (even just overnight) this glycogen is broken down and the glucose is released into the blood. In a healthy adult, the liver glycogen store is usually between 50 and 100 g, containing enough glucose to satisfy the brain's requirements for up to 24 hours.

The main role of muscle glycogen is to provide fuel for the muscle's own contraction during exercise. In fact muscle glycogen cannot be broken down to glucose and so cannot be used to raise blood glucose concentration directly. However, in some circumstances, when their metabolism is partly anaerobic, skeletal muscles produce lactic acid from glycogen. When this lactic acid passes into the blood it is taken up by the liver, where it is converted into glucose; thus it can be used indirectly to raise blood glucose. The major stimulus causing the breakdown of muscle glycogen is contraction of the muscles. Thus, the onset of exercise is accompanied by the initiation of glycogen breakdown. The extent to which the muscles continue to use their glycogen store depends on the intensity of the exercise. With low intensity exercise (such as slow walking, cycling, or swimming) the muscles do not use much glycogen as they are able to take up fat from the blood as a source of energy for contraction. However, with higher intensity exercise (jogging, brisk uphill walking, running) the muscles need to use glycogen or glucose from the blood to support the higher rate of energy expenditure (see figure). A well-nourished person will have enough glycogen in their muscle to enable them to exercise for 1–2 hours at approximately two-thirds of their maximum capacity for aerobic exercise. However if people consume a very high carbohydrate diet, especially for at least three days after first depleting their muscle glycogen levels, it is possible to double this normal glycogen content, ensuring that a longer period of exercise can be sustained before it is used up. This is known as carbohydrate loading, or glycogen supercompensation, and is often used by distance — especially marathon — runners before an important race.

I. A. Macdonald

See also blood sugar; exercise; metabolism; muscle.