Digestion and Absorption of Carbohydrates

Carbohydrates store energy and are used as a source of nutrition. Carbohydrates are present in numerous foods, such as bread, potatoes, bananas, and honey. Polysaccharides, mostly in the form of starch, are the main dietary source of carbohydrates for humans. To be used as energy by humans, most carbohydrates must be metabolized. Carbohydrate metabolism involves transforming complex starches into glucose, a monosaccharide that can subsequently be absorbed by the body. The digestion of carbohydrates begins in the mouth with the action of salivary amylases. The remaining starch is further broken down by pancreatic amylase in the intestines. The oxidation of 1 g of carbohydrate provides 4 kcal of energy.

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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

Mouth: 

  • Salivary amylase divides starches into maltose and polysaccharides
  • Amylases function at a high pH → no longer active in stomach

Stomach: 

The stomach has limited digestive function because of its highly acidic pH.

Small intestine: 

  • Pancreatic amylase (released from acinar cells) and other enzymes break down carbohydrates
  • Brush border has a microvillus membrane → digests oligosaccharides and disaccharides
Brush border digestion

Brush border digestion:
Diagram of the disaccharide lactose being hydrolyzed into constituent monosaccharides (galactose and glucose) in order to be absorbed by the enterocytes. The brush border is used in the digestion of many other disaccharides.
SGLT1: sodium–glucose-linked transporter 1

Image by Lecturio. License: CC BY-NC-SA 4.0
Table: Digestion overview
Carbohydrate Process required for digestion
Polysaccharides
  • Salivary and pancreatic amylases hydrolyze α-1,4 bonds of amylose and amylopectin, leaving oligosaccharides and other derivatives.
  • Isomaltase, in the small intestine, breaks down α-1,4 bonds, leaving monosaccharides
Disaccharides Brush border digestion:
  • Enzyme sucrase hydrolyzes sucrose into fructose and glucose.
  • Enzyme lactase hydrolyzes lactose into galactose and glucose.
  • Isomaltase, maltase, and sucrase breakdown maltose into 2 glucose molecules and maltotriose into 3 glucose molecules.

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.

  • SGLT1:
    • 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.
  • GLUT4 
    • Expressed mainly in adipose tissue 
    • Regulated by insulin
    • Stores glucose depending on cellular conditions
  • GLUT5 
    • 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).

Table: Review of transporters
Transporter Location Function
GLUT1 Most human cells: RBC, CNS, cornea, placenta, fetal tissue
  • Blood–brain barrier
  • High affinity for glucose
GLUT2
  • Liver: hepatocytes
  • Pancreatic β-islet cells
  • Kidney and small intestines
  • Transports monosaccharides from basolateral membrane of enterocytes into the blood
  • Low affinity for glucose
  • Allows hepatocytes to take up and release glucose for glycolysis and gluconeogenesis
GLUT3
  • Most human cells
  • CNS
  • Placenta
  • Blood–brain barrier
  • High affinity for glucose
GLUT4
  • Adipose tissue
  • Muscle tissue
  • Regulates glucose homeostasis
  • Stimulated by insulin: controlled glucose uptake and storage
  • Induced by physical exercise
GLUT5
  • Enterocytes
  • Spermatocytes
Transports fructose
Sodium–glucose-linked transporter 1 (SGLT1) 1st part of the proximal convoluted tubule
  • Low-affinity, high-capacity transport proteins
  • Responsible for > 90% of glucose reabsorption
SGLT2 Distal part of the proximal convoluted tubule
  • High-affinity, low-capacity transport proteins
  • Responsible for < 10% of glucose reabsorption

Clinical Relevance

  • 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.

References

  1. 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&sectionid=204296544
  2. Holesh, J.E., Aslam, S., Martin, A. (2021). Physiology, carbohydrates. StatPearls. Retrieved June 25, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK459280/
  3. 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/

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