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Digestion of Monosaccharides

author image Dr. Tina M. St. John
Tina M. St. John runs a health communications and consulting firm. She is also an author and editor, and was formerly a senior medical officer with the U.S. Centers for Disease Control and Prevention. St. John holds an M.D. from Emory University School of Medicine.
Digestion of Monosaccharides
Woman biting into a green apple Photo Credit Medioimages/Photodisc/Photodisc/Getty Images

Your body is a biological machine, requiring fuel to generate the energy necessary to conduct the business of life. You "fuel up" each time you eat. Your body's favored fuel is sugar, the simplest form of carbohydrates. Your body can also use fats and proteins for fuel, if necessary. Single sugars, or monosaccharides, are absorbed into your bloodstream after your digestive system releases them from the food you consume.

Key Players

The monosaccharides in your diet include glucose, fructose and galactose. Honey and fruit contain significant amounts of fructose and glucose; galactose is less common in fruit. The food additive high-fructose corn syrup, or HFCS, also contains a large amounts of the monosaccharides glucose and fructose.

Pregastric Digestion

Pregastric digestion is the medical term for digestive processes that occur before food reaches your stomach. Digestion of monosaccharides and other nutrients begins with chewing. Grinding food into small pieces with your teeth makes it more accessible for the digestive processes that occur in your stomach and small intestine. Your saliva contains an enzyme called salivary amylase, which initiates the breakdown of complex sugars, releasing some monosaccharides from food before it passes into your stomach.

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Gastric Digestion

Your stomach is the food processor of your digestive system. Stomach lining cells secrete mucus, acid and enzymes, which help liquefy food as forceful muscular contractions churn your stomach contents. As a meal liquefies in your stomach, monosaccharides present in the food you have consumed are released into the thick fluid mixture known as chyme. Although physical digestion of monosaccharides and other carbohydrates takes place in your stomach, no chemical breakdown of carbohydrates occurs during the gastric phase of digestion. Your stomach slowly releases fully processed chyme into your small intestine for the next phase of digestion and absorption.

Intestinal Digestion and Absorption

Most chemical digestion of the food you consume occurs in your small intestine. Monosaccharides, however, are unique in that they are absorbed directly from chyme without further breakdown after arriving in your small intestine. Cells lining your small intestine absorb glucose and galactose via a transporter called SGLUT-1. The S in the name of this transporter refers to sodium, which is absorbed along with glucose and galactose. Fructose enters your intestinal lining cells via a different transporter known as GLUT5; sodium does not accompany fructose with this transporter.

Once inside your intestinal lining cells, monosaccharides transfer to another transporter called GLUT-2, which exports the sugars into your bloodstream. Large portions of the monosaccharides that enter your bloodstream are taken up by your liver or muscle cells for metabolic processing.

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