A young adult reports bloating, cramps and watery diarrhoea after drinking fruit juice, with normal results on coeliac serology and duodenal biopsy. Hydrogen breath testing confirms the diagnosis. The underlying defect involves:

• A Defective SGLT1 causing failure of glucose-galactose uptake
• B Bacterial overgrowth consuming dietary disaccharides before hydrolysis
• C Pancreatic lipase deficiency leading to undigested triglyceride fermentation
• D Limited capacity of GLUT5 transporters for fructose uptake in the small intestine, leaving unabsorbed fructose as an osmotic load ✓

Explanation

Fructose is absorbed solely by the facilitated transporter GLUT5 on the apical membrane of enterocytes, a process with limited capacity and no active component. When intake exceeds absorptive capacity, unabsorbed fructose remains in the lumen, draws in water and is fermented by colonic bacteria to gas and short-chain fatty acids, producing bloating and osmotic diarrhoea. Glucose promotes fructose uptake via GLUT2, which explains why juice with balanced glucose-fructose is better tolerated.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.