Nutritional Aspects of Carbohydrates PDF

Nutritional aspects of carbohydrates Us. Sulafa Bannaga  Digestion of carbohydrate The principal sites of dietary carbohydrate digestion are the mouth and intestinal lumen. There is little monosaccharide present in diets of mixed animal and plant origin. Therefore, the enzymes n...

Nutritional aspects of carbohydrates Us. Sulafa Bannaga  Digestion of carbohydrate The principal sites of dietary carbohydrate digestion are the mouth and intestinal lumen. There is little monosaccharide present in diets of mixed animal and plant origin. Therefore, the enzymes needed for degradation of most dietary carbohydrates are primarily disaccharidases and endoglycosidases (that break oligosaccharides and polysaccharides).  The major dietary polysaccharides are of animal (glycogen) and plant origin (starch, composed of amylose and amylopectin). During mastication, salivary α-amylase acts briefly on dietary starch in a random manner, breaking some bonds. the digest resulting from the action of α-amylase contains amixture of smaller, branched oligosaccharide molecules Further digestion of carbohydrates by pancreatic enzymes occurs in the small intestine When the acidic stomach contents reach the small intestine, they are neutralized by bicarbonate secreted by the pancreas, and pancreatic α-amylase continues the process of starch digestion. Final carbohydrate digestion by enzymes synthesized by the intestinal mucosal cells The final digestive processes occur at the mucosal lining of upper jejunum, declining as they proceed down the small intestine, and include the action of several disaccharidases and oligosaccharidases.For example, cleaves the bond in Isomaltose and cleaves maltose, both producing glucose, sucrase cleaves sucrose producing glucose and fructose.  cont…….. These enzymes are secreted through, and remain associated with, the luminal side of the brush border membranes of the intestinal mucosal cells. Absorption of monosaccharides by intestinal mucosal cells :- The duodenum and upper jejunum absorb the bulk of the dietary sugars. Insulin is not required for the uptake of glucose by intestinal cells.  cont… However, different sugars have different mechanisms of absorption. For example, galactose and glucose are transported into the mucosal cells by an active, energy-requiring process that involves a specific transport protein and requires a concurrent uptake of sodium ions.  Cont… Fructose uptake requires a sodium- independent monosaccharide transporter (GLUT-5) for its absorption. All three monosaccharides are transported from the intestinal mucosal cell into the portal circulation by yet another transporter, GLUT-2.  RDA The RDA for carbohydrate is set at 130 g/day for adults and children, based on the amount of glucose used by carbohydrate-dependent tissues, such as the brain and erythrocytes. However, this level of intake is usually exceeded to meet energy needs. Adults should consume 45 to 65 percent of their total calories from carbohydrates. is recommended that added sugar represent no more than 25 percent of total energy because of concerns that sugar may displace nutrient-rich foods from the diet, potentially leading to deficiencies of certain micronutrients.  Nutritional classification Monosaccharides: Glucose and fructose are the principal monosaccharides found in food. Glucose is abundant in fruits, sweet corn, corn syrup, and honey. Free fructose is found together with free glucose and sucrose in honey and fruits.  Cont…. Disaccharides: The most abundant disaccharides are sucrose(glucose + fructose), lactose (glucose + galactose), and maltose(glucose + glucose). Sucrose is ordinary "table sugar," and is abundant in molasses and maple syrup. Lactose is the principal sugar found in milk.  Polysaccharides: Complex carbohydrates are polysaccharides (most often polymers of glucose), which do not have a sweet taste. Starch is an example of a complex carbohydrate that is found in abundance in plants. Common sources include wheat and other grains, potatoes, dried peas and beans, and vegetables  cont.. Fiber: Dietary fiber is defined as the non digestible carbohydrates (a complex polymer of phenylpropanoid subunits) present in plants. Several different terms are used to described this complex group of compounds.For example, functional fiber etc….. Actions of Dietary fiber  Abnormal carbohydrates digestion The overall process of carbohydrate digestion and absorption is so efficient in healthy individuals that ordinarily all digestible dietary carbohydrate is absorbed by the time the ingested material reaches the lower jejunum. However, because predominantly monosaccharides are absorbed, any defect in a specific disaccharidase activity of the intestinal mucosa causes the passage of undigested carbohydrate into the large intestine.  Cont….. As a consequence of the presence of this osmotically active material, water is drawn from the mucosa into the large intestine, causing osmotic diarrhea. This is reinforced by the bacterial fermentation of the remaining carbohydrate to two- and three-carbon compounds (which are also osmotically active) plus large volumes of CO2 and H2 gas, causing abdominal cramps, diarrhea, and flatulence.  Digestive enzymes deficiency Hereditary deficiencies of the individual disaccharidases have been reported in infants and children with disaccharide intolerance. Alterations in disaccharide degradation can also be caused by a variety of intestinal diseases, malnutrition, or drugs that injure the mucosa of the small intestine. For example, brush border enzymes are rapidly lost in normal individuals with severe diarrhea, causing a temporary, acquired enzyme deficiency.  Cont…. Thus, patients suffering or recovering from such a disorder cannot drink or eat significant amounts of dairy products or sucrose without exacerbating the diarrhea. Lactose intolerance: More than one half of the world's adults are lactose intolerance. Treatment for this disorder is simply to remove lactose from the diet, or to take lactase in pill form prior to eating. THANK YOU