a. Phenylketonuria: Issue Statement
Phenylketonuria (PKU) is an inborn disorder affecting the phenylalanine hydroxylase gene that immediately results to the inability of the body to metabolize a compound called tyrosine. Considering PKU’s fundamental disease process, pediatric patients with PKU are the most at-risk age group due to the nutritional impact of the disease. Hence, PKU diet control and nutritional plan are essential tools for promoting health status and preventing potential complications among these PKU patients.
Escott-Stump (2008) states that PKU is a rare, genetic disorder occurring in one out of 10,000 births in United States (p.189). Pediatric age groups from infants to school-aged children are greatly vulnerable to the nutritional impact of PKU, especially with the restricted but vital milk consumption among infants. According to Klossner and Hatfield (2006), early diagnosis is the most efficient prevention of PKU complications, especially among infants, since defects can begin as soon as the newborn take milk, which generally contains phenylalanine (p.543). Due to the incapacity of the body to metabolize phenylalanine, the substance accumulates in the bloodstream initiating damage to the brain tissues and eventually resulting to mental problems (McPhee, Lingappa and Ganong, 2006 p.14). Therefore, well-planned nutritional intake and diet control are essential interventions for the survival of children with PKU disorder.
b. Scope and Limitations
Nutrition and diet control are important considerations in the nursing care of children with PKU. The following review of literature compiles the different nutrition- and diet-based studies among PKU patients in relation to the establishment and formulation of a nutritional program suited and applicable to the needs of children with PKU disorder. The sections of the study consist (a) an overview of the disease, (b) epidemiologic statistics, (c) pathophysiology with emphasis on diet and nutrition, (d) disease manifestations associated with PKU, and (e) health promotion and prevention interventions applicable for pediatric PKU patients.
II. Discussion: Related Literature
PKU is referred to as recessive, inborn errors of metabolism affecting the processing of phenylalanine compounds taken and absorbed by the body. According to McPhee, Lingappa and Ganong (2006), PKU is diagnosed through laboratory findings indicating elevated levels of urinary phenylpyruvate and phenylacetate, which occur when circulating phenylalanine levels exceed the normal 0.6 and 0.1 mmol/L (p.14). Shaw and Lawson () emphasize the importance of newborn screening in the prevention of PKU complications, such as mental retardation, hyperactive behavior, seizures, eczema and hyperpigmentations (p.311).
Once the child is diagnosed with PKU disorder, the patient is immediately placed under strict diet regimen comprising of synthetic formula milk low in phenylalanine. Newborn feeding needs to be adjusted according to the maintenance level of phenylalanine serum concentration, which is below 1 mmol/L (McPhee, Lingappa and Ganong, 2006 p.14). Patients with PKU require long-term care interventions that are best facilitated by supporting family members; hence, family education with emphasis on PKU family nursing is crucial to the long-term care of chronically ill patient (Klossner and Hatfield, 2006 p.543).
In addition, family education must emphasize the child’s dietary requirement for phenylalanine, protein and energy according to age and development. According to Escott-Stump (2008), recommended phenylalanine intake according to age categories are as follows: (a) infants (0-3 months): 60-90 mg/kg, (b) infants: 4-6 months: 40 mg/kg, (c) infants (7-9 months): 35 mg/kg, (d) infants (10-12 months): 30mg/kg, (e) children (1-2 years): 25 mg/kg, and (f) children (2 years and above): 20 mg/kg (p.189).
According to Klossner and Hatfield (2006), dietary treatment consisting of (a) provision of low-phenylalanine formula milk (e.g. Lofenalac and Phenyl-Free, etc.) and (b) food restriction on selected products (e.g. fish, bread, meat, dairy products, nuts and legumes, etc.) are crucial to the prevention of phenylalanine buildup (p.543). However, the study of Przyrembel and Bremer (2000) has discovered significant linkage between the strict diets of PKU children and growth retardation of these children due to insufficiencies of essential nutrients. Added by the study of Dobbelaere, Michaud and Debrabander (2003), growth retardation among 20 samples aged 8 to 7 months is due to inadequate nutritional supplementation, particularly in zinc (data after measurement: Daily allowances for energy: 91%±18%, proteins: 146%±26%, zinc: 2.4±2.2 mg/day).
PKU is rare genetic condition affecting a very small population of newborns annually. Based from the past statistical reference obtained from the 10-year study of Hofman, Kazazian and Valle (1991), PKU has been found to be more prevalent among whites compared to blacks (1 out of 50,000 black live births; 1 out 10,000 white live births). According to Biller (2007), the prevalence of PKU in U.S is approximately 1 out of 14,000 to 20,000 live births annually, while most studies (McPhee, Lingappa and Ganong, 2006; Shaw and Lawson, 2007) state the prevalence rate of approximately 1 per 12,000 live births. Variations of epidemiological statistics can be due to the genetic origin of the samples assessed.
Furthermore, various genetic-toxicology studies, such as Kabesch (2006) and Oh, Lee and Park (2005), point out gene-environment interactions as the cause of epidemiology variations since genetic malformations largely depend on the biological mutations induced by external elements found in the environment. For example, the results of Hardelid, Cortina-Borja and Munro’s (2007) study implemented in South East England, compared to the latter statistics of other studies done in United States, reveal almost different epidemiological statistics of PKU among whites (0.96–1.33 per 10,000 live births), blacks (0.02-0.37) and Asian ethnic groups (0.10 – 0.63). Meanwhile, Ollendick and Schroeder (2003) assert that these epidemiological statistics are dependent according to country of origin, ethnicity and racial background (p.476). This explains the variations in epidemiologic statistics among diversified samples from different countries, cultures and races.
c. Pathophysiology: Diet and Nutritional Implications
Phenotypic and genotypic relationships have always been attributed to the pathophysiological nature of phenylketonuria wherein environmental variables play significant roles in the genetic alterations leading to the disease process (McPhee, Lingappa and Ganong, 2006 p.14). The principal component responsible to condition of PKU is the accumulation of phenylalanine and absence or insufficient serum level of the enzyme phenylalanine hydroxylase. “Phenylalanine is a toxic substance that inhibits the transport of large amino acids into the brain causing inhibition of protein and neurotransmitter synthesis (dopamine and serotonin)” (Ollendick and Schroeder, 2003 p.476).
Phenylalanine hydroxylase is responsible for the breaking down or metabolism of excess phenylalanine in the bloodstream (McPhee, Lingappa and Ganong, 2006 p.14). According to Ollendick and Schroeder (2003), the mental retardation effect of phenylalanine toxicity can be attributed into two hypothetical etiologies, namely (a) “the high levels of phenylalanine that competes with tyrosine at the blood-brain barrier preventing the metabolism of dopamine”, and (b) “high levels of phenylalanine that inhibits oligodendrocytes, which sheaths the neuron and affects its conductivity” (p.476). According to Klossner and Hatfield (2006), phenylalanine buildup in the serum bloodstream of an undiagnosed child can occur between 1 to 2 days with milk consumption. As the accumulation peaks, phenylpyruvic acid appears in the urine of the child between the 2nd and 6th week of life indicating severe and progressive mental damage brought by the phenylalanine intoxication of the brain tissues (p.543).
d. Disease Manifestations: Phenylketonuria
McPhee, Lingappa and Ganong (2006) state that the structure of PKU disease manifestations is distinctly associated with the condition called hyperphenylalanemia, which is caused by substrate accumulation when normal intermediary metabolite fails to be eliminated properly, while concentrations reach the level of toxicity (p.14). According to Shaw and Lawson (2007), irreversible mental retardation is usually prevented if the PKU newborn has started the treatment regimen within the first three weeks of life (p.311-312). Newborn screening for PKU utilizes a small amount of dried blood sample obtained between 24 to 72 hours of age. After the initial screening, a second test is done prior to the child’s two weeks of age (McPhee, Lingappa and Ganong, 2006 p.14). Once the diagnosis is established, the child immediately undergoes the lifetime phenylalanine treatment by setting standard phenylalanine consumption achieved through combinations of synthetic and commercial products designed for PKU patients.
Determining the appropriate dietary consumption and nutritional intake of a PKU patient is considered the simplest phase of the treatment. The most difficult part in treating children with PKU is the establishment of adherence and understanding of their diet restrictions and requirements (Shaw and Lawson, 2007 p.315). If PKU diet is not provided between the recommended two to three weeks of age, the child may suffer mental deficiency brought by the disease. Meanwhile, complications during the process of PKU diet may also arise due to the lowered nutritional consumption (Ollendick and Schroeder, 2003 p.447).
In case, serum levels of iron, vitamin b12, folic acid, selenium and other nutrients are found low, it is important to provide diet enhancing supplements or multivitamin-mineral supplements as needed (Escott-Stump, 2007 p.189). According to Gilbert (2000), low protein contents of PKU diet leads to insufficient protein absorption in the body, which consequently gives rise to various PKU complications, such as (a) learning disability and (b) vomiting and convulsions, brought by alterations in neurologic conductions and deficits in production of neurotransmitters (p.212).
The study of Giovanni, Verduci and Salvatici (2007) emphasize the need for absorption of long-chain polyunsaturated fatty acids levels (LC-PUFA) to correct neurological deficits and central nervous system development. Other manifestations of PKU are related to deficiencies in iron, calcium, vitamin B12, selenium and zinc. Gilbert (2000) adds that PKU children with high phenylalanine intake may produce phenylacetic acid excrete via urine or sweat giving off mousy smell (p.212).
Meanwhile, according to the study of Modan-Moses, Vered and Schwartz (2007), PKU patients also confront compromised bone mass due to the long-term dietary deficiency in protein, calcium, vitamin D and other minerals; although, recommendations of the study suggest further studies between the correlations of bone mass decline and calcium absorption among PKU patients. Lastly, according to Edelstein and Sharlin (2008), insufficiencies in various electrolytes (e.g. calcium, potassium, etc.) and vitamins (e.g. vitamin D, B12, etc.) due to dramatic food restrictions trigger (a) hypertonic and hyperactive deep tendon reflex, (b) microencephaly, (c) growth retardation and (d) dermatologic rashes (p.147).
e. Diet and Nutrition Promotion
Nursing interventions provided among new born PKU patients are relatively simple since food intake and diet control largely depend on the child’s caretaker. However, as the child grows, food and diet compliance becomes difficult due to the child’s building independency confronted by various diet temptations. According to MacDonald (2000), failure in diet compliance is affected by their acceptability, format and timing of administration, such as the dry, hard and insipid low protein products that commonly lead to under usage or meal constraints brought by changing developmental tasks of the child. Gilbert (2000) adds that PKU diet regimens are usually non-palatable, which commonly becomes the primary reason for the child’s compromised diet adherence (p.212). According to Klossner and Hatfield (2006), nursing care for pediatric patients with PKU includes (a) prompt diagnosis, (b) early initiation of treatment and (c) health teaching (p.543). Gilbert (2000) emphasizes strict adherence to diet restrictions especially for children age 10 years and below (p.212).
PKU diet control and planned food intake are necessary components of PKU treatment. As supported by Shaw and Lawson (2007), PKU diet regimen comprises of comprehensive, synthetic combinations of vitamins and minerals supplements, and protein substitutes (e.g. minaphlex, PKU Gel, etc.) (p.323). Meanwhile, Edelstein and Sharlin (2000) comment on these synthetic commercial medical foods stating “…these products do not guarantee adequate nutritional intake since absorption of nutritional contents depend largely to the extent of phenylalanine hydroxylase deficiency…” (p.148).
Omission and prevention of food consumption of high phenylalanine-containing foods, such as meat, fish, poultry, bread, milk, cheese and legumes, are crucial to PKU diet. Meanwhile, nutritionists usually resort to food alternatives low in phenylalanine content, such as low-protein bread, pasta, crackers, cookies and muffin mixes (Escott-Stump, 2007 p.189). Aside from adequate and appropriate food intake, health education of the patient (if applicable) and supporting individuals, such as family, peers, etc., is crucial to the survival of the child, prevention of complications and improvement in the child’s nutritional status.
According to Escott-Stump (2007), teachings related to nutritional intake of PKU patient must include (a) diet aiding growth and development, (b) encouraging self-feeding when possible, (c) strategies in establishing positive attitude towards diet, and (d) monitoring for deficiencies associated with insufficient intake of vitamin B12, folic acid, selenium, iron and zinc (p.189). Health education must emphasize the need for lowered protein and phenylalanine consumption with strict adherence to PKU medical food diet regimen. However, PKU commercial diets are expensive ranging from $200 to $500 per monthly consumption and most patients cannot afford to have regular intakes of these products (Shaw and Lawson, 2007 p.323).
In order to compensate, nutritionists recommend low phenylalanine food components that still provide the natural nutrient and mineral sources. Alternative sources of protein are commercially prepared low-protein bread, pasta and cereals, while alternative energy sources include pure starches, sugar and oil (Edelstein and Sharlin, 2008 p.148). Meanwhile, diet components that should be prevented include (a) aspartame, (b) high protein substances (e.g. egg, meat, poultry, fish, dairy products, etc.) and (c) under-recommended phenylalanine diet regiments ((Shaw and Lawson, 2007 p.324). Nutrition of pediatric patients with PKU is important to their survival, capacity to perform daily tasks and prevention of various nutritional deficiencies associated with PKU diet regimen.
In synthesis of the study, the principal conflict in administering PKU diet regimen is the compliance of the child. Pediatric patients are usually confronted by compromised diet adherence due to lack of family support, structure and taste of the food, and the temptation brought by their restricted foods. Despite the problems in food compliance, the caregiver must be able to impose strict and religious provision of the child’s PKU diet regimen in order to prevent various nutritional deficiencies resulted by the low nutritional contents of PKU diet. Nutritional deficiencies linked to PKU are mostly due to poor intake and absorption of calcium, vitamin D, B12 and C, selenium, zinc and iron. Nutritionists commonly suggest commercially prepared low-phenylalanine medical foods to supplement the patient’s nutritional needs; however, various studies cited in the discussion indicate compromised nutritional state associated with the long-term consumption of PKU diet. The following gaps have been determined and recommended for future studies related to this subject:
a. Determine the quantitative correlation between nutritional deficiencies and long-term PKU diet regimen
b. Determine the qualitative perceptions of the child samples in their PKU diet regimen in relation to their diet incompliance
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