Maple syrup urine disease is a rare inborn error in the metabolism of the keto acids of the three branched-chain essential amino acids leucine, isoleucine, and valine. The name derives from the maple syrup-like odour of the urine. MSUD is transmitted through an autosomal recessive gene of unknown frequency. It is suspected that the diagnosis is often missed because of early infant deaths. The affected individual appears to lack the three decarboxylases required by the three keto acids derived from the transamination of branched-chain amino acids. It is important to understand maple syrup urine disease (MSUD) dietary treatment and types.
Biochemical Abnormalities in Infants
The defect is present at birth and the biochemical abnormalities occur as soon as the infant begins to take food protein (milk). The abnormalities are elevated blood levels of the branched-chain amino acids and three keto acids. The keto acids are also excreted in the urine, hence, another term for the disease is branched-chain ketoaciduria.
The infant is normal at birth but at the end of the first week of life, the first symptoms of poor feeding and apathy occur. As early as the fifth day of life the maple syrup-like odour of the urine is detectable. Neurological symptoms with convulsions appear early and, if the disease is untreated, death occurs during the first year of life, sometimes as early as the first week. To avoid neurological damage, diet therapy for maple syrup urine disease must be initiated in the first week of life and from present experience, it appears that dietary restrictions must be maintained throughout life.
Types of Maple Syrup Urine Disease
In addition to the classical form of MSUD described above, there are two variants forms: one is episodic in nature and the other is a milder form of the classical type. In the episodic form, the biochemical abnormalities are only present during major stress such as infection. These children do well on a normal diet without excessive protein. In the mild form, the infant responds to a protein intake limited to the requirement. One case of a third variant has been reported. The biochemical abnormalities in this child responded to massive doses of thiamine.
Dietary Treatment Maple Syrup Urine Disease (MSUD)
- Leucine, isoleucine and valine are essential amino acids. Therefore, in the treatment of MSUD, the diet must supply the infant’s requirement of these three amino acids without an excess for it to be catabolized through the branched-chain keto acid pathway.
- It has been observed that to support growth, the MSUD infant has a higher energy requirement than normal infants.
- All food proteins contain the branched-chain amino acids with a greater percentage in animal than vegetable protein. As in the treatment, special formula and low protein foods are used to feed these infants and children.
- Rice cereal, low-protein fruits, and vegetables are the foods used to supplement the infant’s formula. Low protein wheat starch products may be used as the child grows older as a source of energy.
- The first feedings for the infant with MSUD are made using a formula consisting of the dry base and synthetic amino acids containing approximately 20 Kcal. Per 30 ml. when the elevated BCAA blood level returns to normal, cow’s milk is added to the formula to supply the infant’s requirement for these essential amino acids.
- When the child is ready for infant foods, low protein cereals, fruits, and vegetables can be used, in addition to homogenized milk, to supply the daily requirements for BCAA. The formula of dry base and synthetic amino acids is continued to meet the infant’s needs for energy, protein, and all other nutrients.
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