Diseases & Conditions


Carbohydrate Metabolism

Carbohydrates are sugars. Some sugars are simple, and others are more complex. Sucrose (table sugar) is made of two simpler sugars called glucose and fructose. Lactose (milk sugar) is made of glucose and galactose. Both sucrose and lactose must be broken down into their component sugars by enzymes before the body can absorb and make use of them. The carbohydrates in bread, pasta, rice, and other carbohydrate-containing foods are long chains of simple sugar molecules. These longer molecules must also be broken down by the body. If an enzyme needed to process a certain sugar is missing, the sugar can accumulate in the body, causing problems.

Glycogen Storage Diseases

Glycogen is made of many glucose molecules linked together. The sugar glucose is the body's main source of energy for the muscles (including the heart) and brain. Any glucose that is not immediately used for energy is held in reserve in the liver, muscles, and kidneys in the form of glycogen and released when needed by the body.

There are many different glycogen storage diseases (also called glycogenoses), each identified by a roman numeral. These diseases are caused by a hereditary lack of one of the enzymes that is essential to the process of forming glucose into glycogen and breaking down glycogen into glucose. About 1 in 20,000 infants has some form of glycogen storage disease.

Some of these diseases cause few symptoms; others are fatal. The specific symptoms, age at which symptoms start, and their severity vary considerably among these diseases. For types II, V, and VII, the main symptom is usually weakness. For types I, III, and VI, symptoms are low levels of sugar in the blood and protrusion of the abdomen (because excess or abnormal glycogen may enlarge the liver). Low levels of sugar in the blood cause weakness, sweating, confusion, and sometimes seizures and coma. Other consequences for children may include stunted growth, frequent infections, or sores in the mouth and intestines. Glycogen storage diseases tend to cause uric acid, a waste product, to accumulate in the joints (which can cause gout) and in the kidneys (which can cause kidney stones). In type I glycogen storage disease, kidney failure is common in the second decade of life or later.

The specific diagnosis is made when a chemical examination of a sample of tissue, usually muscle or liver, determines that a specific enzyme is missing.

Treatment depends on the type of glycogen storage disease. For many people, eating many small carbohydrate-rich meals every day helps prevent blood sugar levels from dropping. For people who have glycogen storage diseases that produce low blood sugar, glucose levels are maintained by giving uncooked cornstarch every 4 to 6 hours around the clock. Sometimes carbohydrate solutions are given through a stomach tube all night to prevent low blood sugar levels from occurring at night.

Types and Characteristics of Glycogen Storage Diseases


Affected Organs


Type O Liver, muscle Enlarged liver with accumulation of fat inside the liver cells (fatty liver); episodes of low blood sugar levels (hypoglycemia) when fasting

von Gierke's disease (Type IA) Liver, kidney Enlarged liver and kidney; slowed growth; very low blood sugar levels; abnormally high levels of acid, fats, and uric acid in blood

Type IB Liver, white blood cells Same as in von Gierke's disease but may be less severe; low white blood cell count; recurring mouth and intestinal infections or Crohn's disease

Pompe's disease (Type II) All organs Enlarged liver and heart, muscle weakness

Forbes' disease (Type III) Liver, muscle, heart, white blood cells Enlarged liver or cirrhosis; low blood sugar levels; muscle damage and heart damage in some people

Andersen's disease (Type IV) Liver, muscle, most tissues Cirrhosis in juvenile type; muscle damage and heart failure in adult (late-onset) type

McArdle's disease (Type V) Muscle Muscle cramps or weakness during physical activity

Hers' disease (Type VI) Liver Enlarged liver; episodes of low blood sugar when fasting; often no symptoms

Tarui's disease (Type VII) Skeletal muscle, red blood cells Muscle cramps during physical activity; red blood cell destruction (hemolysis)


Galactosemia (a high blood level of galactose) is caused by lack of one of the enzymes necessary for metabolizing galactose, a sugar present in lactose (milk sugar). A metabolite builds up that is toxic to the liver and kidneys and also damages the lens of the eye, causing cataracts.

A newborn with galactosemia seems normal at first but within a few days or weeks loses his appetite, vomits, becomes jaundiced, has diarrhea, and stops growing normally. White blood cell function is affected, and serious infections can develop. If treatment is delayed, affected children remain short and become mentally retarded or may die.

Galactosemia is detectable with a blood test. This test is performed as a routine screening test on newborns in nearly all states in the United States and particularly in those with a family member known to have the disorder.

Galactosemia is treated by completely eliminating milk and milk products—the source of galactose—from an affected child's diet. Galactose is also present in some fruits, vegetables, and sea products, such as seaweed. Doctors are not sure whether the small amounts in these foods cause problems in the long term. People who have the disorder must restrict galactose intake throughout life.

If galactosemia is recognized at birth and adequately treated, the liver and kidney problems do not develop, and initial mental development is normal. However, even with proper treatment, children with galactosemia often have a lower intelligence quotient (IQ) than their siblings, and they often have speech problems. Girls often have ovaries that do not function, and only a few are able to conceive naturally. Boys, however, have normal testicular function.

Hereditary Fructose Intolerance

In this disorder, the body is missing an enzyme that allows it to use fructose, a sugar present in table sugar (sucrose) and many fruits. As a result, a by-product of fructose accumulates in the body, blocking the formation of glycogen and its conversion to glucose for use as energy. Ingesting more than tiny amounts of fructose or sucrose causes low blood sugar levels (hypoglycemia), with sweating, confusion, and sometimes seizures and coma. Children who continue to eat foods containing fructose develop kidney and liver damage, resulting in jaundice, vomiting, mental deterioration, seizures, and death. Chronic symptoms include poor eating, failure to thrive, digestive symptoms, liver failure, and kidney damage.

The diagnosis is made when a chemical examination of a sample of liver tissue determines that the enzyme is missing. Treatment involves excluding fructose (generally found in sweet fruits), sucrose, and sorbitol (a sugar substitute) from the diet. Acute attacks respond to glucose given intravenously; milder attacks of hypoglycemia are treated with glucose tablets, which should be carried by anyone who has hereditary fructose intolerance.

Last full review/revision February 2003

Source: The Merck Manual Home Edition