Intestinal Absorption of Monosaccharides

The upper jejunum absorbs the bulk of the monosaccharide products of digestion. However, different sugars have different mechanisms of absorption (Fig. 1). For example, galactose and glucose are taken into enterocytes by secondary active transport that requires a concurrent uptake (symport) of sodium (Na+) ions. The transport protein is the sodiumdependent glucose cotransporter 1 (SGLT-1). [Note: Sugar transport is driven by the Na+ gradient created by the Na+-potassium (K+) ATPase that moves Na+ out of the enterocyte and K+ in (see Fig. 1).]

Fructose absorption utilizes an energy- and Na+-independent monosaccharide transporter (GLUT-5). All three monosaccharides are transported from the enterocytes into the portal circulation by yet another transporter, GLUT-2. 

Figure 1 Absorption by enterocytes of the monosaccharide products of carbohydrate digestion. GLUT = glucose transporter; SGLT-1 = sodium (Na+)- dependent glucose cotransporter. K+ = potassium.