What dietary instructions should be given to a patient diagnosed with Phenylketonuria (PKU)?

Dietary Management of Phenylketonuria

The correct instruction is B - avoid phenylalanine-containing foods. Patients with PKU require lifelong strict restriction of phenylalanine intake through a specialized low-phenylalanine diet supplemented with phenylalanine-free amino acid formulas to prevent intellectual disability and neurological damage. 1, 2

Core Dietary Principles

Phenylalanine restriction is the cornerstone of PKU management and must begin immediately upon diagnosis. 1, 3

• Restrict natural protein sources that contain phenylalanine, including meat, fish, eggs, dairy products, nuts, legumes, and regular bread/pasta 4, 5
• Provide phenylalanine-free amino acid formulas to meet protein requirements while avoiding phenylalanine accumulation 3, 4
• Use modified low-protein foods to meet energy requirements without excessive phenylalanine intake 3, 5
• Treatment must begin by 7-10 days of life when blood phenylalanine exceeds 10 mg/dL to prevent irreversible brain damage 2

Why Other Options Are Incorrect

Option A (no restrictions) is completely wrong - untreated PKU results in mental retardation, microcephaly, delayed speech, seizures, eczema, and behavioral abnormalities 1, 2

Option C (avoid high protein food and fructose and lactose) is partially correct but misleading - while high-protein foods are restricted because they contain phenylalanine, there is no specific need to avoid fructose, and lactose restriction is not a primary concern unless the patient has concurrent lactose intolerance 1, 3

Option D (gluten-free diet) is incorrect - PKU does not require gluten restriction; the diet focuses specifically on phenylalanine content, not gluten 1

Target Blood Phenylalanine Levels

Metabolic control requires maintaining specific phenylalanine levels based on age: 1, 3

• Children up to 12 years: 2-6 mg/dL (120-365 μmol/L) 1
• Adolescents and adults over 12 years: 2-15 mg/dL (120-910 μmol/L) 1, 3
• Women planning pregnancy: <6 mg/dL at least 3 months before conception 1, 3
• During pregnancy: 2-6 mg/dL 1, 3

Monitoring Schedule

Regular monitoring is essential to maintain metabolic control: 1, 2

• First year of life: Weekly 1, 2
• Ages 1-12 years: Twice monthly 1, 2
• After 12 years: Monthly 1, 3
• During pregnancy: Twice weekly 1, 2

Critical Pitfalls to Avoid

Dietary discontinuation leads to neuropsychiatric deterioration - even adults with stable IQ scores show worse performance on attention and processing speed measures when diet is discontinued 1, 3

Inadequate amino acid formula intake causes multiple problems - patients who stop taking phenylalanine-free amino acid mixtures have significantly higher plasma phenylalanine levels and develop deficiencies in thiamin, riboflavin, folate, calcium, and iron 6

Poor metabolic control affects cognition - high phenylalanine levels correlate with lower IQ scores, impaired attention, slower reaction times, and behavioral difficulties 1, 3

Unplanned pregnancies with poor control cause fetal damage - elevated maternal phenylalanine causes microcephaly, mental deficiency, and congenital heart disease in the fetus 1, 3

Nutritional Considerations

The PKU diet creates specific nutritional challenges that require monitoring: 4, 5

• The diet resembles a vegan diet with similar nutritional concerns 5
• Monitor vitamin D, B12, calcium, iron, and essential fatty acid status regularly 4, 5
• Ensure adequate intake of long-chain polyunsaturated fatty acids for neurodevelopment 5
• The diet is characteristically low in fiber, fat, and cholesterol 6

Breastfeeding can be incorporated - infants with PKU may be breastfed with supplementation of phenylalanine-free formula to maintain metabolic control 1, 5