Carbohydrate Digestion: How are Sugars Digested by the Body

pasta with sauce in white ceramic bowl

Carbohydrates or sugars form a large part of the food we eat everyday and are an important source of energy for all activities of the body. The brain, especially, completely depends on carbohydrates for its energy. But the intestines cannot digest sugars just the way they are present in our food. This is where sugar digestion comes in.

Sugar or carbohydrate digestion is a process that breaks down dietary carbs into much smaller units that can be readily absorbed by cells of the small intestine. Learn more about the steps in sugar digestion and the digestive enzymes involved in the process.

Carbohydrate Foods and the Sugars they Contain

Carbohydrates may be monosaccharides, disaccharides or polysaccharides. Monosaccharides, like glucose, are the basic units of carbohydrates and are ring-shaped molecules that contain 5 or 6 carbon atoms with hydrogen and oxygen in the ratio of 2:1. Two of these link together to form disaccharides like sucrose and lactose. Many monosaccharides, from three to several thousand, join to form polysaccharides like starch. The aim of carbohydrate digestion is to reduce all these molecules to the easily absorbed monosaccharides.

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In our diet, carbohydrates are mainly represented by the following compounds:

  • sucrose – Or cane sugar, made up of one glucose and one fructose molecule
  • lactose – The disaccharide in milk, made of one glucose and one galactose
  • starch – A large polymer of glucose found in most plant food, especially cereals and grains
  • glycogen, alcohol, dextrins, pectins and amylose in smaller quantities

Our food also contains cellulose, a complex polysaccharide that cannot be digested by our intestines due to the lack of enzymes required for its breakdown.

How are Carbohydrates Digested – Hydrolysis

When monosaccharides combine to form disaccharides or other polymers, one hydrogen ion (–H) is removed from one monosaccharide molecule and a hydroxyl ion (–OH) is removed from the next so the two molecules can link at these sites of removal. Conversely, the breakdown of the disaccharide into its component monosaccharides requires the addition of a molecule of water (H2O) so that the previously removed hydrogen and hydroxyl ions can be replaced. This chemical reaction is called hydrolysis and can be summarized as follows:

Disaccharide + H2O –> Monosaccharide-H + Monosaccharide-OH

Hydrolysis requires the action of specific enzymes.

Digestive System Enzymes that Help Sugar Digestion

All digestive enzymes are proteins and are secreted by various parts of the digestive system. Each has its own very specific role in the process of digestion. Here is a summary of the enzymes required for carbohydrate breakdown:

  • ptyalin – an α-amylase secreted by the parotid glands
  • α-amylase – secreted by the pancreas
  • lactase, sucrase, maltase and α-dextrinase – present in the enterocytes or intestinal cells
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Carbohydrate Digestion Begins in the Mouth…

…and continues through the stomach and the small intestine. In the mouth, as food is chewed and mixed with saliva, it comes into contact with the enzyme ptyalin that has first go at breaking down carbohydrates. About 5% of them are broken down during the process of swallowing.

Salivary amylase continues its action even in the stomach until its activity is stopped by the acid in the gastric juices. This may be as long as an hour. Note that the stomach itself does not secrete any carbohydrate-digesting enzyme. Still, up to 30 to 40% of the starches may get digested before the food gets completely mixed with the acidic secretions.

Carbohydrate Digestion in the Small Intestine

The enzyme α-amylase secreted by the pancreas acts on the food as it is emptied into the duodenum from the stomach. This is a powerful enzyme and succeeds in digesting most of the carbohydrates in the diet.

By the time food reaches the small intestine, carbohydrates have mostly been reduced to disaccharides and other small chains of glucose. Enzymes lactase, sucrase, maltase and α-dextrinase present in the cells of the intestinal lining (called ‘brush border’ because of its microvilli) split these disaccharides and small chains into the individual monosaccharides that they are made of.

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While lactose yields one molecule of glucose and one galactose, sucrose gives one glucose molecule and one fructose and maltose splits into two molecules of glucose. More than 80% of the monosaccharides finally formed are glucose. These molecules are now ready to be absorbed by the intestinal cells.

As a result of carbohydrate digestion, sugars and starches present in the food get broken down to monosaccharides, mainly glucose, that are easily absorbed by the intestines. This is what happens to dietary carbs once they enter the gut and explains why blood glucose levels increase after a carbohydrate-rich meal.

Sources

Murray R.K., Granner D.K., et al, “Harper’s Illustrated Biochemistry” (McGraw Hill; 26th edition)

Guyton and Hall, “Textbook of Medical Physiology” (Saunders; 10th edition)