Branched chain ketoaciduria - nutritional considerations

Alternative names
Galactosemia - nutritional considerations; Fructose intolerance - nutritional considerations; Maple sugar urine disease (MSUD) - nutritional considerations; Phenylketonuria (PKU) - nutritional considerations; Inborn errors of metabolism

Inborn errors of metabolism are rare genetic disorders in which the body cannot turn food into energy (metabolize food) normally. The disorders are usually caused by defects in the enzymes involved in the biochemical pathways that break down food components.

Side Effects
The food product that is unable to be metabolized (broken down into energy) can build up in your body and cause a wide array of symptoms. Several inborn errors of metabolism cause mental retardation if not controlled. Please see each specific disease for exact symptoms.

Inborn errors of metabolism often demand diet changes. The type and extent of the changes depends on the specific metabolic error. Registered dietitians and physicians can help with the diet modifications needed for each disease.

The following are some examples of inborn errors of metabolism.

Fructose intolerance:

  • A genetic disorder in the breakdown of the carbohydrate fructose. It is potentially life threatening but may be treated by diet changes.  
  • Food sources of fructose include fruits, fruit juices, sucrose (all sugars - cane, beet, white, brown, etc), corn syrups, honey, sorbitol, levulose, invert sugar, and some vegetable and starches.  
  • All fructose should be avoided in the diet. The severity of the restriction depends on individual tolerance. Sugar and fructose are found in many foods, making the diet difficult to follow.


  • A genetic disorder in the breakdown of the carbohydrate galactose to glucose. It can result in cataracts, enlarged liver, enlarged spleen and mental retardation. Typically, the disease is found in milk-fed infants shortly after birth. This is because milk contains large amounts of galactose.  
  • Food sources of galactose include mammalian milks, dairy products and foods containing them.  
  • It is recommended that milk and milk products should be avoided, including yogurt, cheese, ice cream. Galactose and lactose free milk substitutes and foods should be used. Other sources of galactose may include sugar beets, gums, seaweed, flaxseed, mucilage, whey, some vegetables, etc. Women who carry the genetic trait should also follow the diet since galactose may cause mental retardation to the fetus. Contact a Registered Dietitian for complete information on a galactose free diet.

Maple sugar urine disease (MSUD):

  • A rare genetic disorder in the breakdown of the branch chain amino acids valine, leucine, and isoleucine. Typically the disease is found shortly after birth and is characterized by maple syrup odor of the urine, vomiting, refusal to eat, and increased reflex actions. If left untreated, life-threatening neurological damage may result.  
  • Treatment includes a special diet. Strict compliance is necessary to prevent neurological damage. This requires close supervision by a registered dietitian or physician, and cooperation by parent(s).

Phenylketonuria (PKU):

  • A rare genetic disorder that can result in severe progressive mental retardation if untreated by diet. Most states require blood or urine testing for PKU is all newborns.  
  • A low phenylalanine diet is required. Strict compliance to the diet is necessary to reduce or prevent mental retardation. This requires close supervision by a registered dietitian or physician, and cooperation of the parent(s) and child.


Johns Hopkins patient information

Last revised: December 6, 2012
by Simon D. Mitin, M.D.

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