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The breakdown and absorption of carbohydrates

Learning the amazing journey a piece of pizza takes through our body.




We love carbs and no matter how many "diet experts" out there will tell you that you need to go on low-card diets - our bodies are designed to consume and use carbohydrates as fuel.


The ultimate aim of the process of breaking down carbohydrate is to get to the simplest molecule in the body called glucose (C6H12O6). This molecule is a very important ingredient to make ATP (Adenosine triphosphate) - the currency our body uses to power our metabolism and every cell in our body.

Fun fact: the brain accounts for ~2% of the body weight, but it consumes ~20% of glucose-derived energy making it the main consumer of glucose.


Everything from the sugar we put in the tea, to the bread we have for lunch and the delicious sweet potatoes we have for dinner constitutes carbohydrates.

Carbohydrates are made out of biomolecules consisting of carbon, hydrogen and oxygen atoms. The complexity and amount of these atoms we ingest are dependent on what food our diets are made of.


Carbohydrates can be categorised based on their chemical structure into the following:

1. Monosaccharides (one sugar molecule): Glucose, Fructose and Galactose

2. Disaccharides (two sugar molecules): Maltose ( digested in 2 molecules of Glucose), Sucrose (digested into Fructose and Glucose) and Lactose (digested in Glucose and Galactose)

3. Polysaccharides: Starch (digested into Glucose), Glycogen (digested into Glucose) and Cellulose (non-digestable, fibre)

 

Breakdown


This journey of breaking down our food into simple formulas begin in the Oral cavity and make its way into the GIT tract and ultimately is absorbed into the blood to be transported to all the cells in need.


  1. Oral cavity 

First we start with our oral cavity. As we masticate the delicious lunch we are having, salivary glands (intrinsic and extrinsic) produce saliva that contain this enzyme called salivary (α) amylase or ptyalin which will start breaking down the carbohydrates.

Carbohydrate units bond between themselves through 2 types of bonds:  α 1-4 glycosidic bond and α 1-6 glycosidic bond.


Starch carbohydrate is made out of polysaccharide chains: Amylopectin ( α 1-4 glycosidic bonds and α 1-6 glycosidic bonds) and amylose (only made out of α 1-4 glycosidic bonds) 

The salivary amylase will only target α 1-4 glycosidic bonds. The enzyme is able to break down these bonds into 2 types of molecules: Maltose, Maltotriose and we are also left with α Limit dextrin which are the remaining 1-6 glycosidic bonds.


In addition to the above, depending on the food being ingested there will be unbroken molecules of lactose and sucrose which will have their own chemical process later on in the journey.

Digestion in the oral cavity is limited as we rest out food in our mouth for very little time - but we can see here the importance of slow eating to aid digestion. The more we masticate and allow for the digestion to start in our oral cavity the easier it will be for the next organs to break this food further.  


From the oral cavity through swallowing the broken down food reaches the stomach. Some of the amylase enzyme makes to the stomach when swallowing but because of the difference in acidity very little survives.. The stomach grinds and churns the food to break it down into small particles. It then pushes the small particles of food into the first part of the small intestine, called the duodenum.


2. The Pancreas


In the duodenum, pancreatic juices act further on the breakdown of the food. The digestive enzyme pancreatic amylase is released from the cells of the acini and flow into the pancreatic duct. The pancreatic duct joins the common bile duct at the sphincter of Oddi, where both flow into the duodenum.


Pancreatic amylase will further breakdown only the α 1-4 glycosidic bonds in Maltose, Maltotiose and α limit dextrin (which are the remaining 1-6 glycosidic bonds)


3. Brush border enzymes in your small intestines


The breakdown continues down the Small intestine where is aided by a third and last 4 types of brush border enzymes: Lactase, Maltase, Sucrase and Isomaltase.


These brush border enzymes are found on the enterocytes (cells of intentional linings) called microvilli which help increase the surface for digestion and absorption.


Maltase (Glucoamylase) actions on both Maltose and Maltotriose being able to break it down into two glucose units.

Sucrase can action on Maltose, Maltotriose and Sucorse to break it down into glucose units.

Isomaltase is a special enzyme that targets the α 1-6 glycosidic bonds (the bonds that haven't been able to be broken down by either the salivary amylase or pancreatic amylase) for the remaining α-Limit Dextrin. This will also be broken down into Glucose. 

Lactase actions on Lactose being able to break it down into Galactose and Glucose.

At the end of this process we are left with the three main monosaccharides: Glucose, Fructose and Galactose. The latter two will further be broken down into Glucose in the liver.


So here we are, after several chemical processes the body has managed to break down complex food into small sugar molecules.


Absorption

Subsequent to the breakdown in the small intestine, the glucose and galactose molecules get absorbed into the epithelial cells lining the gut via Sodium Potassium pump (Na+/K+ Pump). They use Na+ molecules to enter the cell by taking advantage of the opening created by Na+ molecules that are against their concentration levels. This is a primary active transportation as it uses ATP. This process is called the Sodium Glucose Transporter (SGLT).


The absorption of fructose happens through a special channel called GULT-5 which will facilitate the diffusion to the enterocytes.


These epithelial cells are very special cells (like pretty much all cells): they have one side designed to ingest the portion of the plasma membrane facing the intestine, the apical surface, is specialised for absorption; the rest of the plasma membrane, the lateral and basal surfaces, often referred to as the basolateral surface, send these nutrients in the surrounding fluids which lead to the blood. These three monosaccharides will make their way to the blood through the basolateral surface through GLUT-2 transporter.


Glucose is transported across erythrocyte membranes by a uniporter GULT-1, a type of facilitated diffusion protein to follow its course to the Hepatic Portal vein to the liver where it will undergo further processes that will ensure we get glucose to every cell to power our metabolism.

This amazing process allows our body to transform carbohydrates into tiny molecules that can used to power our every day the activities that keeps us alive like: breathing or thinking.



Sources:


1. Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 15.7, Transport across Epithelia. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21502/

2. Mergenthaler P, Lindauer U, Dienel GA, Meisel A. (2013) Sugar for the brain: the role of glucose in physiological and pathological brain function.Trends Neurosci. 2013;36(10):587‐597. doi:10.1016/j.tins.2013.07.001 Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3900881/


3. Navale, A. M., & Paranjape, A. N. (2016). Glucose transporters: physiological and pathological roles.Biophysical reviews,8(1), 5–9. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5425736/


4. College of Naturopathic Medicine lecture on Digestive system - https://www.naturopathy-uk.com/


5. Gastrointestinal | Digestion & Absorption of Carbohydrates from the Ninja Nerd Science Available at: https://www.youtube.com/watch?v=BXXxtEW5v3o


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