Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Carbohydrates
Carbohydrate digestion and absorption into the bloodstream

The breakdown of complex dietary CARBOHYDRATES begins in the Oral Cavity under the action of salivary amylase and maltase Enzymes (Fig. 60). The optimal activity of these enzymes is observed in an alkaline environment. Amylase breaks down starch and Glycogen, while maltase breaks down maltose. This process yields lower-molecular-weight carbohydrates—dextrins, along with partial amounts of maltose and glucose.

The breakdown of dietary carbohydrates does not occur in The Stomach, as there are no specific carbohydrate-hydrolyzing enzymes, and the acidic environment of gastric juice (pH 1.5–2.5) suppresses The activity of salivary enzymes. The primary Digestion of dietary carbohydrates takes place in the Small Intestine. In the duodenum, complex carbohydrates are gradually broken down into Disaccharides through the action of pancreatic amylase. Subsequently, disaccharides are split into Monosaccharides—primarily glucose, fructose, and galactose—by highly specific enzymes: maltase, sucrase, and lactase. These enzymes are located on the brush border of the intestinal mucosal epithelium; therefore, carbohydrate breakdown occurs not only within the intestinal lumen but also on the membranes of the mucosal Cells.

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Fig. 60 Diagram of dietary carbohydrate breakdown during digestion

The Human Body ingests a large amount of dietary fiber (Cellulose). It is not digested in the small intestine due to the absence of the enzymes required for its Hydrolysis. Partial breakdown of fiber into cellobiose and glucose occurs in the Large Intestine through the action of bacterial enzymes. The resulting carbohydrates are utilized by microorganisms to sustain their metabolic activity. Microorganisms also use fiber for The Biosynthesis of Vitamins K, B₁₂, and Folic acid. Undigested cellulose is excreted from the body.

The absorption of the resulting monosaccharides (glucose, fructose) by the walls of the small intestine and their entry into the bloodstream occur via Active Transport involving a carrier protein, a Na⁺ gradient, and ATP (see Chapter 5). Na⁺ ions activate ATPase, which accelerates ATP Cleavage and the release of energy required for The transport of these monosaccharides across the intestinal wall. The absorption of other monosaccharides occurs via passive diffusion, as their concentration in the Blood is low. The absorption rate varies among different monosaccharides. For instance, if the absorption rate of glucose is taken as 100%, the absorption rates for galactose are 110%, fructose 43%, mannose 19%, pentose 15%, and arabinose 9%.

The absorption of monosaccharides in the intestine is regulated by the nervous and hormonal systems. The Nervous system can alter the permeability of the intestinal epithelium, the degree of Blood supply to the intestinal mucosa, and the motility of the villi, thereby changing the rate at which monosaccharides enter the portal vein blood. Glucose Absorption is stimulated by Hormones from the adrenal cortex, Pituitary Gland, thyroid, and Pancreas, and is inhibited by adrenaline.

Intense muscular activity slows down Carbohydrate Absorption, whereas light and short-term work enhances glucose absorption. An increase in ambient Temperature to 35–40 °C inhibits carbohydrate absorption, while a decrease to 25 °C enhances it, which is likely associated with The stimulation of carbohydrate METABOLISM/26.html">Energy Metabolism.