INBORN ERROR METABOLISM
PHENYLKETONURIA
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It is a group of autosomal recessive inherited disorders of phenylalanine metabolism. it is caused by impaired activity of liver enzyme, phenylalanine hydroxylase which oxidises phenylalanine to tyrosine.
PROGNOSIS
The accumulation of phenylalanine or its catabolic products, a deficiency of tyrosine or its products or all four circumstances will produce central nervous system damage. During this period, oligodendroglia migration may also be impaired, resulting in irreversible brain damage later in childhood. Protein synthesis is inhibited by high phenylalanine on blood brain barrier transport with consequent imbalance in intraneuronal amino acid concentrations. In the mature brain, neurodegeneration behavioural difficulties and prolonged performances times may result from depressed neurotransmitter synthesis. Impairment of these neuropsychologic returns towards normal concentrations in cells and blood.
SYMPTOMS
If not treated before 3 weeks of age, the metabolic imbalance produces irreversible mental retardation.
The central nervous system damage causes increased motor irritability, hyperactivity, convulsive seizures and bizarre behaviour.
DIAGNOSIS
Newborn with blood phenylalanine concentration greater than 2 mg/dl on the screening test is restricted. Guthrie (microbiological test) test is used worldwide.
NUTRITION AND DIETARY MANAGEMENT
The objective of nutrition support in the child with classic phenylketonuria is to maintain blood phenylalanine concentrations that will allow optimum growth and brain development by supplying adequate energy, protein and other nutrients while restricting phenylalanine and supplementing tyrosine intake.Phenylalanine restricted diet with isoleucine, leucine and valine have found improvement in behaviour and decreased plasma phenylalanine.
As soon as the diagnosis is made, breast feeding should be stopped and the infant bottle fed with a low phenylalanine milk substitute to prevent clinical manifestations. Within a mean of 4 days, the infant fed 67 kcal/dl low phenylalanine or free formula blood phenylalanine concentration should drop to treatment range. Recommendations for protein for nutrition support are greater than the RDA. Recommendation for energy and fluid intake are the same as those for normal infants and children.
Energy the first requirement of the body is necessary for growth. When energy is provided as carbohydrate and fat and if adequate nitrogen is available, non-essential amino acids may be synthesised from the keto acid metabolites. When energy intake is inadequate tissue catabolism occurs to meet energy needs. Provision of sufficient energy through generous use of non-protein and low protein foods is important to assure a normal growth rate.
Prescription for phenylalanine can be increased by addition of measured amounts of milk and/or solid foods. Concentrated chemically defined medical foods are frequently used without any untoward side effects.
Extra fluid must be offered between feedings to maintain appropriate water balance.
Assessment of nutrition support
Biweekly assessment of growth : height, weight, head circumference
Evaluation of development : developmental scales
Adequacy of phenylalanine and tyrosine : blood phenylalanine and tyrosine conc. twice weekly
A mother has to prepare a low phenylalanine diet for her child from five lists of foods.
She needs continuing help from a dietician. The lists are:
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Basic foods containing negligible phenylalanine which can be used freely- sugar, jams, sweets, solid vegetable oils and cooking oils.
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Fruits and vegetables which can be taken freely, since they provide negligible phenylalanine and protein in a normal helping. excess amounts may contribute some phenylalanine
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A basic list of 50 mg phenylalanine exchanges of foods.
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Manufacturers foods of negligible phenylalanine content.
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Exchanges of foods containing one gram of protein equivalent to 50mg of phenylalanine.
it is extremely important to maintain normal plasma phenylalanine concentrations in the reproductive female before and after conception.
GALACTOSEMIA
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Lactose is the principal carbohydrate and energy source for infants and young children. Galactose maintains a central metabolic role in human nutrition.
Galactosemia may occur because deficient functioning of any three of the enzymes. galactokinase, galactose-1-phosphate uridyl tranferase or UDP galactose-4-epimerase. Patients with galactokinase deficiency have cataracts.
By the time the diagnosis is made cataracts sets in. at least the younger siblings should be helped by avoiding galactose and lactose in the diet.
DIETARY MANAGEMENT
Galactose is present in some complex starches (stachyose, raffinose) in peas, soyabean and some vegetables. Hence formulas prepared from soya protein isolate can be given whereas whole soya flour preparations are avoided. Among fleshy foods, avoid organ meat like liver. Strict dietary restriction is necessary in early years which can be somewhat relaxed during school age.
GALACTOSEMIA-RESTRICTED DIET
| FOODS ALLOWED |
FOODS EXCLUDED |
| Beverages: Carbonated drinks |
All products containing milk |
| Fruit drinks free of apple, grape, pear and papaya |
Milk products |
| Breads without milk |
Bread with milk |
| Cheeses none |
All excluded |
| Desserts: water and fruit ices, gelatine |
Commercial cakes |
| Eggs all |
Omelettes and soufflés containing milk |
| Fats oil shortening not butter |
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| Fruits: canned, fresh, frozen that do not contain galactose |
Any fruits processed with lactose apple, apple sauce, banana, dates, figs, grapes, kiwi fruit, papaya, pears, water melon. |
Source
Srilakshmi .B 2003.Dietetics, New Age International (P) Publishers Ltd.Chennai.
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