Introduction

The three basic monosaccharides important in human nutrition are glucose, fructose, and galactose. Glucose is the product of the digestion of starch. In human metabolism, all simple sugars are converted into glucose. Glucose is the circulating form of carbohydrate in the bloodstream. Fructose is the sweetest of the simple sugars, and it is found in fruits and naturally occurring substances such as honey. Fructose consumption has increased greatly in the USA since the 1970s, when high-fructose corn syrup started to be widely used in food processing. High-fructose corn syrup is the major sweetening agent used by the food industry. Galactose is produced by the digestion of lactose, the major carbohydrate in milk.

Digestion

Most carbohydrates have to be converted to glucose in order to be used for energy production. The digestion of carbohydrates starts in the mouth, with mastication and the enzymatic action of salivary amylase, which converts starch to dextrins and maltose. Successive contractions of the stomach (peristalsis) move the food to the lower part of the stomach, while 20-30% of the carbohydrate is already converted to maltose. Peristalsis facilitates digestion in the small intestine, while the chemical digestion of carbohydrates is completed by pancreatic amylase (which continues the breakdown of starch to maltose) and intestinal disac-charidases (sucrase, lactase, and maltase for the breakdown of fructose, lactose, and maltose, respectively). The monosaccharide products of carbohydrate digestion are then absorbed into the portal circulation.

Absorption

Glucose accounts for the largest quantity of absorbed carbohydrate (80%), and galactose and fructose account for only a small amount (20%). The body quickly absorbs and transports the simple sugars, which enter the portal circulation via the capillaries of the intestinal villi and are transported to the liver. In the liver, fructose and galactose are converted to glucose, which is either used immediately for energy or stored in the form of glycogen. The liver can store approximately 5% of its mass in the form of glycogen, which can be readily converted to glucose for the production of energy.

Transport

Monosaccharides traverse the epithelial lining of the intestine by simple or facilitated diffusion or by active transport. The transport system for the passage of glucose and galactose through the apical membrane of the intestinal villi is called the Na+-dependent glucose transporter (Na+-dependent GLUT). Fructose uses a different transporter, called GLUT5, for the same passage. All monosaccharides are then transported from the enterocyte to the bloodstream by another sugar transporter known as GLUT2. The passage of glucose and galactose across both membranes of the intestine requires the presence of Na+, while the passage of fructose is dependent on fructose concentration not Na+ concentration.

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