ABSTRACT
Strictly speaking, digestion comprises only the final hydrolytic step (see Figure 2.2).
In the human diet, the most common carbohydrates are starch, sucrose, lactose, fructose, glucose, and dietary fibers. Most (±50-60% of daily intake) carbohydrates are starch, which is a mixture of linear (amylose) and branched (amylopectin) polymers of glucose with
a
-1, 4 and
a
-1, 4 +
a
-1, 6 linkages, respectively. Starch, as well as the disaccharides lactose and sucrose, is hydrolyzed in the upper part of the gastrointestinal system (Figure 4.1), essentially the oral cavity and the small intestine, whereas the dietary fibers are not. The monosaccharides that preexist in the diet (fructose and glucose) and that are produced by the hydrolysis of starch and disaccharides (lactose and sucrose) are absorbed and reach the systemic circulation via the portal vein. But the oligo-and monosaccharides that reach or are produced in the large bowel, essentially by bacterial hydrolysis of dietary fibers and, in some populations, lactose, are not absorbed but fermented. Strictly speaking, the digestion process concerns only starch, lactose, and sucrose, and the absorption process in the small intestine concerns fructose and galactose but mainly glucose.
