Inborn Errors of Metabolism

Inborn errors of metabolism are inherited disorders in which the body cannot metabolize the components of food ( carbohydrates , proteins , and

fats). Metabolism is the biochemical process that changes food components into energy and other required molecules . These disorders may be caused by the altered activity of essential enzymes , deficiencies of the substances that activate the enzymes, or faulty transport compounds. Metabolic disorders can be devastating if appropriate treatment is not initiated promptly and monitored frequently.

Inborn errors of metabolism often require diet changes, with the type and extent of the changes dependant on the specific metabolic disorder. The particular enzyme absence or inactivity for each inborn error of metabolism dictates which components are restricted and which are supplemented. Registered dietitians and physicians can help an individual assess the diet changes needed for each disease. The goals of nutrition therapy are to correct the metabolic imbalance and promote growth and development by providing adequate nutrition, while also restricting (or supplementing) one or more nutrients or dietary components. Additional goals in some disorders include reducing the risk of brain damage, other organ damage, episodes of metabolic crisis and coma, and even death. These restrictions and supplementations are specific for each disorder, and they may include the restriction of total fats, simple sugars, or total carbohydrates.

Listed below are several of the metabolic disorders that respond to nutrition therapy. The appropriate dietary restrictions and modifications that are necessary for treatment are also listed.

Disorders of Amino Acid Metabolism

Phenylketonuria (PKU) is the most common disorder of amino acid metabolism. In this disorder the body cannot use the amino acid phenylalanine normally, and excess amounts build up in the blood. If untreated, PKU can cause mental retardation, seizures, behavior problems, and eczema . With treatment, persons with PKU have normal development and intelligence. The treatment for PKU consists of a special phenylalanine-restricted diet designed to maintain blood phenylalanine levels within an acceptable range. Medical formulas and foods, which do not contain phenylalanine, are used to provide the necessary intake of protein and other nutrients. Foods containing natural protein are prescribed in limited amounts to meet the body's requirement for phenylalanine, without providing too much.

Maple syrup urine disease (MSUD) is a disorder in which the body is unable to use the amino acids isoleucine, leucine, and valine in a normal way. Excessive amounts of these amino acids and their metabolites will build up in the blood and spill into the urine and perspiration, giving them the odor of maple syrup (which is how this disorder got its name). An untreated infant with MSUD may have some or all of the following symptoms: difficulty breathing, sleepiness, vomiting, irregular muscle movement, seizures, or coma, and the disease can cause death. Basic treatment involves restricting foods and infant formula that contain leucine, isoleucine, and valine. Medical formulas and foods, which contain very small amounts of leucine, isoleucine, and valine, are used to provide the necessary intake of protein and other nutrients.

Disorders of Carbohydrate Metabolism

Galactosemia is a disorder in which the body cannot break down the sugar called galactose. Galactose can be found in food, and the body can break down lactose (milk sugar) to galactose and glucose . The body uses glucose for energy. People with galactosemia lack the enzyme to break down galactose, so it builds up and becomes toxic. In reaction to this buildup of galactose the body makes some abnormal chemicals. The buildup of galactose and these chemicals can cause liver damage, kidney failure, stunted growth, mental retardation, and cataracts in the eyes.

If not treated, galactosemia can cause death. Over time, children and young adults with galactosemia can have problems with speech, language, hearing, stunted growth, and certain learning disabilities. Children who do not follow a strict diet have an increased risk of having one or more of the problems listed above. Even when a strict diet is followed, some children do not do as well as others. Most girls with galactosemia have ovarian failure. The treatment for galactosemia is to restrict galactose and lactose from the diet for life. Since galactose is a part of lactose, all milk and all foods that contain milk must be eliminated from the diet, including foods that contain small amounts of milk products such as whey and casein. In addition, organ meats should not be eaten because they contain stored galactose.

Glycogen storage diseases require different treatments depending on the specific enzyme alteration. The most common type of glycogen storage disease is classified as type 1A. In this disorder the body is missing the enzyme that coverts the storage form of sugar (glycogen) into energy (glucose). If food is not eaten for two to four hours, blood glucose levels drop to a low level, leading to serious health problems such as seizures, poor growth, enlarged liver, high levels of some fats circulating in the blood, and high levels of uric and lactic acids in the blood. Dietary management of GSD-1A eliminates table sugar (sucrose) and fruit sugar (fructose) and limits milk sugar (lactose), as the body cannot use some sugars in these foods. Frequent meals and snacks that are high in complex carbohydrates are recommended. In addition, often supplements of uncooked cornstarch are often eaten between meals to keep blood sugar levels stable. Eating a diet that prevents low blood sugar will promote normal growth, decrease liver enlargement, and the high blood levels of uric and lactic acids.

Disorders of fatty acid metabolism occur when the body is not able to break down fat to use as energy. The body's main source of energy is glucose, but when the body runs out of glucose, fats are used for energy. If untreated, these disorders can lead to serious complications affecting the liver, heart, eyes, and muscles. Treatment includes altering the kind and the amount of fat in the diet and frequent feedings of carbohydrate-containing foods.

Urea cycle disorders are inherited disorders of nitrogen metabolism. When protein is digested it breaks down into amino acids, and nitrogen is found in all the amino acids. Those who have these disorders cannot use nitrogen in a normal way. Dietary treatment for these disorders is to provide only the amount of protein that the body can safely use. The diet consists mostly of fruits, grains, and vegetables that contain low amounts of protein and, therefore, low amounts of nitrogen.

There are more than nineteen metabolic disorders that respond to nutrition therapy. The role of proper nutrition in the treatment of these disorders is crucial. Because these disorders are rare and require careful monitoring, affected individuals are best served by clinics specializing in metabolic disorders.

SEE ALSO Phenylketonuria (PKU) .

Patricia D. Thomas

Bibliography

Acosta, P. B., and Yannicelli, S. (1993). "Energy and Protein Requirements of Infants and Children with Inherited Metabolic Disorders." Metabolic Currents 6:1–8.

Acosta, P. B., and Yannicelli, S. (2001). Ross Metabolic Formula System: Nutrition Support Protocols, 4th edition. Columbus, OH: Ross Laboratories.

Scriver, C. R.; Beaudet, A. L.; Sly, W. S.; and Valle, D., eds. (2001). The Metabolic and Molecular Bases of Inherited Disease, 8th edition. New York: McGraw-Hill.