Digestion of Carbohydrates
Carbohydrate digestion breaks down disaccharides, oligosaccharides, and polysaccharides into monosaccharides.
- Main dietary carbohydrates are hexoses (6-carbon sugars) such as glucose → readily absorbed by enterocytes
- Disaccharides and polysaccharides need to be broken down into smaller sugars for absorption
- Salivary amylase divides starches into maltose and polysaccharides
- Amylases function at a high pH → no longer active in stomach
The stomach has limited digestive function because of its highly acidic pH.
- Pancreatic amylase (released from acinar cells) and other enzymes break down carbohydrates
- Brush border has a microvillus membrane → digests oligosaccharides and disaccharides
|Carbohydrate||Process required for digestion|
|Disaccharides||Brush border digestion:|
Absorption of Carbohydrates
After digestion, carbohydrates are absorbed and transported via the portal circulation. Transport may be either an active, facilitated, or passive mechanism.
- Active transport involves the use of transporter enzymes, which move carbohydrates across the plasma membrane even against the concentration gradient.
- Facilitated diffusion occurs down concentration gradients with the additional aid of transmembrane enzymes that do not require energy.
- Passive absorption moves sugars down concentration gradients without enzymatic assistance or energy needed; it is the slowest mechanism.
Transporters have specific roles, and their functions may be active, facilitated, or passive.
- Found in the small intestine; functions to transport glucose
- Relies on an actively generated sodium gradient generated by an ATPase pump (Na+/K+-ATPase pump)
- Transports 2 sodiums, glucose or galactose, and water
- Glucose transporter 2 (GLUT2)
- Found in kidney, liver, and pancreas and on basolateral membrane of small intestine
- Transports glucose and fructose via facilitated diffusion
- Bidirectionality allows for a change in function depending on cellular conditions.
- Expressed mainly in adipose tissue
- Regulated by insulin
- Stores glucose depending on cellular conditions
- Transports fructose via facilitated diffusions
Passive glucose absorption represents a minority of absorptive capabilities. Most absorption occurs in the 1st part of the small intestine (duodenum, jejunum).
|GLUT1||Most human cells: RBC, CNS, cornea, placenta, fetal tissue|
|Sodium–glucose-linked transporter 1 (SGLT1)||1st part of the proximal convoluted tubule|
|SGLT2||Distal part of the proximal convoluted tubule|
- Lactose intolerance: malabsorptive GI disorder caused by a deficiency in lactase, a brush border enzyme involved in the digestion and absorption of lactose. Lactose is a disaccharide composed of glucose and galactose. Patients with lactose intolerance present with abdominal pain, diarrhea, and flatulence after consuming lactose products. Treatment is dietary modification or lactase supplementation in milder cases.
- Galactosemia: autosomal recessive condition that prevents galactose processing. Galactosemia is a serious condition that presents early in life in infants, who experience lethargy, nausea, vomiting, diarrhea, and jaundice. Patients may develop neurologic deficits such as ataxia. Treatment is dietary modification.
- Diabetes mellitus: metabolic condition caused by chronic hyperglycemia. Diabetes mellitus is due to deficiency in or resistance to insulin. GLUT4 transporters are insulin-sensitive and help store glucose under certain conditions. Patients with type 2 diabetes mellitus have a disruption in their response to insulin and thus glucose accumulates in the blood, causing the chronic hyperglycemia consistent with diabetes mellitus. Symptoms include urinary frequency, increased thirst, and increased appetite. Serious complications of diabetes mellitus include diabetic ketoacidosis, cardiovascular disease, neuropathy, and kidney disease.
- Barrett, K.E., Barman, S.M., Brooks, H.L., Yuan, J.X. (2019). Digestion & absorption of nutrients. Chapter 26 of Ganong’s Review of Medical Physiology, 26th ed. New York: McGraw-Hill Education. Retrieved June 25, 2021, from https://accessmedicine.mhmedical.com/content.aspx?bookid=2525§ionid=204296544
- Holesh, J.E., Aslam, S., Martin, A. (2021). Physiology, carbohydrates. StatPearls. Retrieved June 25, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK459280/
- Goodman, B.E. (2010). Insights into digestion and absorption of major nutrients in humans. Advances in Physiology Education 34(2):44–53. https://pubmed.ncbi.nlm.nih.gov/20522896/