Fructose Metabolism

Hereditary Fructose Intolerance

INTRODUCTION

  • Fructose is a ketohexose present in fruits, honey and sucrose
  • Fructose is metabolized by fructokinase present in the liver.
  • Fructokinase: Independent of insulin (so in diabetic patients there is rapid turnover of fructose into its metabolites)

FRUCTOSE METABOLISM

  • The step involving PFK-1 is absent in fructose metabolism thus no bottleneck effect is encountered leading to rapid glycolysis thus increased lipogenesis
  • HYPERLIPIDEMIA: Thus in diabetic patients, these TAG in the tissues elevates cholesterol and LDL in blood, thus atherogenic.
  • HYPERURICEMIA: This rapid metabolism depletes the cell of all its ATP (which is the end product of denovo purine synthesis). This induces increased purine synthesis leading to increased URIC ACID (catabolic product of purine)
  • Free fructose in large quantities is present in seminal vesicles providing motility to spermatozoa.
  • High fructose levels are associated with myocardial infarction and aging

Fructose metabolism

HEREDITARY FRUCTOSE INTOLERANCE

  • Autosomal recessive
  • Inborn error of metabolism
  • Defective enzyme: ALDOLASE B
  • Age of onset: INFANTS (sucrose is introduced to diet around 6 months age)
Mechanism

Excess accumulation of FRUCTOSE-1-PHOSPHATE → Decreased inorganic Phosphate → Inhibition of glycogen phosphorylase and decreased ATP. It leads to –

    • Accumulation of glycogen in liver
    • Hypoglycemia
    • Toxic to various organs like liver, kidney, etc.
Clinical Features
  • Failure to thrive
  • Irritability
  • Feeding difficulty
  • Vomiting
  • Loss of appetite
  • Hepatomegaly and jaundice
  • Mental retardation
  • Convulsions
  • Hepatic failure → death
Lab Diagnosis
  • Urine BENEDICTS test- positive
  • Urine ketose sugar test-positive
  • Glucose oxidase test-positive
  • Enzyme test
  • Genetic mutation test
Treatment

Withdraw fructose containing food from diet

FRUCTOSURIA

  • Deficiency of fructokinase in liver
  • Accumulation of fructose → FRUCTOSURIA
  • Urine Benedicts test positive
  • No clinical symptoms
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