Digestion and absorption of carbohydrate and protein
How starch, disaccharides and protein are broken down by luminal and brush border enzymes, which transporters carry sugars, amino acids and peptides across the enterocyte, and how defects in each step present, with worked numbers for lactose malabsorption, breath testing and the D-xylose test.
Tier 2Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- Carbohydrate and protein digestion each run in three stages: luminal enzymes, brush border enzymes, then transporters. Only monosaccharides, free amino acids and small peptides cross the enterocyte.
- SGLT1 couples glucose and galactose uptake to sodium. This survives cholera toxin, which is why oral rehydration solution works. See Intestinal fluid transport.
- Unabsorbed carbohydrate causes osmotic diarrhoea, gas and acidic stool. Unabsorbed protein rarely causes symptoms; it shows up as weight loss, oedema and low albumin.
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Sources
- Sleisenger and Fordtran's Gastrointestinal and Liver Disease (11th ed.)Elsevier (Feldman, Friedman, Brandt, 2021), 2021
- Guyton and Hall Textbook of Medical Physiology (14th ed.)Elsevier (Hall, Hall, 2021), 2021
- Rezaie A et al. Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. Am J Gastroenterol 2017;112(5):775-84American Journal of Gastroenterology, 2017
- Misselwitz B et al. Update on lactose malabsorption and intolerance: pathogenesis, diagnosis and clinical management. Gut 2019;68(11):2080-91Gut, 2019
- Savaiano DA, Boushey CJ, McCabe GP. Lactose intolerance symptoms assessed by meta-analysis: a grain of truth that leads to exaggeration. J Nutr 2006;136(4):1107-13Journal of Nutrition, 2006
- Last updated
- 14 November 2026
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