Essentially all the carbohydrates in food are absorbed in the form of monosaccharides; only a small fraction is absorbed as disaccharides and almost none is absorbed as larger carbohydrate compounds. By far the most abundant of the absorbed monosaccharides is glucose, which usually accounts for more than 80 percent of the carbo hydrate calories absorbed. The reason for this high per centage is that glucose is the final digestion product of our most abundant carbohydrate food, the starches. The remaining 20 percent of absorbed monosaccharides is composed almost entirely of galactose and fructose—the galactose derived from milk and the fructose as one of the monosaccharides digested from cane sugar.
Virtually all the monosaccharides are absorbed by a secondary active transport process. We will first discuss the absorption of glucose.
Glucose Is Transported by a Sodium CoTransport Mechanism. In the absence of sodium transport through the intestinal membrane, virtually no glucose can be absorbed because glucose absorption occurs in a co-transport mode with active transport of sodium (see Figure 1).
Fig1. Absorption of sodium, chloride, glucose, and amino acids through the intestinal epithelium. Note also osmotic absorption of water (i.e., water “follows” sodium through the epithelial membrane).
The transport of sodium through the intestinal mem brane occurs in two stages. First is active transport of sodium ions through the basolateral membranes of the intestinal epithelial cells into the interstitial fluid, thereby depleting sodium inside the epithelial cells. Second, a decrease of sodium inside the cells causes sodium from the intestinal lumen to move through the brush border of the epithelial cells to the cell interiors by a process of secondary active transport. That is, a sodium ion com bines with a transport protein, but the transport protein will not transport the sodium to the interior of the cell until the protein also combines with some other appropriate substance such as glucose. Intestinal glucose also combines simultaneously with the same transport protein and both the sodium ion and glucose molecule are then transported together to the interior of the cell. Thus, the low concentration of sodium inside the cell literally “drags” sodium to the interior of the cell, and glucose is dragged along with it. Once inside the epithelial cell, other transport proteins and enzymes cause facilitated diffusion of the glucose through the cell’s basolateral membrane into the paracellular space and from there into the blood.
To summarize, it is the initial active transport of sodium through the basolateral membranes of the intestinal epithelial cells that provides the eventual force for moving glucose through the membranes as well.
Absorption of Other Monosaccharides. Galactose is transported by almost exactly the same mechanism as glucose. Fructose transport does not occur by the sodium co-transport mechanism. Instead, fructose is transported by facilitated diffusion all the way through the intestinal epithelium and is not coupled with sodium transport.
Much of the fructose, upon entering the cell, becomes phosphorylated. It is then converted to glucose and finally transported in the form of glucose the rest of the way into the blood. Because fructose is not co-transported with sodium, its overall rate of transport is only about one half that of glucose or galactose.
