U01.01.106 Disorders of fructose metabolism

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Learning Objectives

  • Distinguish between the benign nature of Essential Fructosuria and the severity of Hereditary Fructose Intolerance.
  • Explain the biochemical mechanism of phosphate depletion in Aldolase B deficiency.
  • Identify the dietary triggers and necessary restrictions for affected patients.
  • Recognize the laboratory findings used to detect reducing sugars in the urine.

1. Comparison of Fructose Disorders

Fructose metabolism bypasses the rate-limiting step of glycolysis (PFK-1), allowing for rapid energy production. Deficiencies in the enzymes Fructokinase or Aldolase B lead to distinct clinical pathologies.

Feature Essential Fructosuria Hereditary Fructose Intolerance (HFI)
Enzyme Deficiency Fructokinase (Autosomal Recessive) Aldolase B (Autosomal Recessive)
Pathophysiology Fructose is not trapped in cells; Hexokinase becomes the primary pathway. Fructose-1-P accumulates, depleting intracellular phosphate.
Presentation Asymptomatic, benign. Hypoglycemia, jaundice, cirrhosis, vomiting.
Urine Findings Fructose in blood/urine. Reducing sugars in urine.

2. Clinical Manifestations and Triggers

In Hereditary Fructose Intolerance, the accumulation of Fructose-1-Phosphate “sequesters” inorganic phosphate. This lack of phosphate inhibits both glycogenolysis and gluconeogenesis, leading to profound hypoglycemia.

  • Timing: Symptoms typically appear when a baby is weaned from breast milk and introduced to fruit, juice, or honey.
  • Lab Detection: A standard urine dipstick will be negative (it only detects glucose), but a test for reducing sugars will be positive.
Memory Hook: Fructokinase deficiency is Kinder (benign). Aldolase B deficiency is Bad (liver failure and hypoglycemia).

3. Management and Treatment

Treatment focuses on strict dietary avoidance of all sources that can be metabolized into fructose.

  • Avoid Fructose: Found in fruits and honey.
  • Avoid Sucrose: A disaccharide composed of Glucose + Fructose.
  • Avoid Sorbitol: This sugar alcohol is metabolized into fructose by the polyol pathway.

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High-Yield Summary:

Essential Fructosuria is a laboratory “surprise” (fructose in urine) without clinical illness. HFI is a pediatric emergency triggered by the introduction of dietary sugars other than lactose.

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