Metabolic Disorders: Hereditary Fructose Intolerance and Phenylketonuria

Clinical Management

Because the disorder is rooted in the inability to process specific sugars, the primary treatment is dietary intervention.

• Low Fructose Diet: Patients must strictly limit their intake of fructose to prevent the buildup of toxic metabolites.
• Glucose Monitoring: Because HFI directly impacts glucose metabolism, clinicians must closely monitor blood glucose levels to ensure stability.

Exam Tip: If a question asks why a patient with HFI can still undergo glycolysis from glucose but cannot process fruit, remember that Aldolase A and C remain functional; only Aldolase B is deficient.

Phenylketonuria (PKU)

Phenylketonuria (PKU) is another recessive genetic disorder, but unlike HFI, it involves the metabolism of an amino acid rather than a sugar. The disorder is caused by a mutation in the enzyme phenylalanine hydroxylase (PAH).

The Biochemical Failure of PAH

In a healthy metabolic pathway, the enzyme PAH facilitates the conversion of the essential amino acid phenylalanine into tyrosine. This conversion is vital because tyrosine serves as a precursor for several critical substances, including:

• Melanin (pigmentation)
• Dopamine (a key neurotransmitter)
• Catecholamines (such as norepinephrine).

When the PAH enzyme is mutated or non-functional, two catastrophic biochemical failures occur:

1. Toxic Accumulation: Phenylalanine levels rise significantly, becoming neurotoxic.
2. Downstream Deficiency: The lack of tyrosine leads to a shortage of essential neurotransmitters, specifically dopamine and norepinephrine.

Clinical Manifestations and Risks

The neurological impact of PKU is severe. The accumulation of phenylalanine and the lack of neurotransmitters can lead to a wide range of symptoms and long-term developmental issues.

Common Symptoms:

• Vomiting and nausea.
• An unusual odor in the breath or urine.
• Changes in skin and hair.
• Growth delays.

Statistical Risks of Untreated PKU: If left untreated, the neurological damage can be profound:

• Developmental delay: ~92%
• Mental deficits: 75–90%
• Microcephaly (dysmorphism): ~73%
• Heart disease: ~12%.

Clinical Note: A classic diagnostic clue for PKU is a "mouse-like" or musty odor in the urine, which is a hallmark of the phenylalanine accumulation.

Management of PKU

To prevent severe neurological complications and intellectual disability, patients must adhere to a strict low-phenylalanine diet. This management strategy focuses on limiting the intake of protein, as amino acids are the building blocks of protein.

Dietary Restrictions and Support

• Limiting High-Protein Foods: Patients must avoid or strictly limit meat, eggs, nuts, and milk.
• Avoiding Artificial Sweeteners: Many artificial sweeteners contain phenylalanine and must be avoided.
• Nutritional Substitution: To prevent malnutrition, patients use specialized protein substitutes or medical formulas. This ensures they receive all essential nutrients and amino acids without the excess phenylalanine that would trigger toxicity.

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