Digestion and Absorption

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Since only monosaccharides can be absorbed, all carbohydrate polymers must be hydrolyzed during digestion (A).

This process is initiated in the mouth by salivary amylase and continues in the small intestine under the influence of pancreatic amylase. End products of this enzymatic reaction are partially glucose, but also large amounts of maltose and isomaltose. Just like sucrose or lactose, these disaccharides are split into monosaccharides by specific, membrane-bound disacchari-dases. Since mucosa cells are continuously sloughed off, small amounts of these hydrolases are also found in the intestinal lumen.

Further absorption into mucosal cells can occur in several ways (B). On the luminal side of the mucosa, glucose and galactose are transported actively against a concentration gradient. The carrier molecule binds Na+ and glucose in a 1:1 ratio and travels through the membrane. Na+ is subsequently removed from the cell by Na+/K+-ATPase— again against a concentration gradient. Glucose leaves the mucosa cells actively using the GLUT2 carrier protein. About 25 % of the glucose reaches the blood by passive diffusion; ~15% is transported back into the lumen by the carrier. Fructose is absorbed passively, with the GLUT5 transport system facilitating diffusion on the luminal side. Depending on the efficiency of these transport systems, the various mono-saccharides are absorbed at different rates. Glucose and galactose are absorbed fastest and compete for the same transport system. Fructose follows about 30% more slowly, while the absorption speed of all the other monosaccharides, including the sugar alcohols used as alternative sweeteners (e.g., sorbit, xylit), is as low as 10-20% of the speed of glucose.

If the individual capacity of one of the digestive/absorptive systems is exceeded, low molecular weight carbohydrates reach the colon. There, they bind water and can be fermented by intestinal bacteria, causing intestinal gas and diarrhea.

One example is the lactose/lactase system: large amounts of lactose always overload the system and cause diarrhea; this is why lactose can be used as a laxative. More important, however, is the occurrence of lactase deficiency. Overall, 30-50 million Americans are lactose intolerant, 90 % of Asian-Americans, and 75% of African- and Native Americans. Worldwide, 75% of adults have some degree of lactose intolerance. It is least common among Europeans and constitutes the most common form of intolerance to a disaccharide. Reduced lactase activity is a factor in many diseases of the intestine. Those afflicted ought to be aware that fermented milk products like hard cheeses contain only traces of lactose and rarely cause symptoms. Sour milk products tend to be well tolerated in spite of their lactose content. The increasing use of powdered milk in processed foods, as well as lactose in medical drug preparations and diet products, is problematic for those who suffer from lactose intolerance.

Digestion and Absorption 59

- A. Carbohydrate Digestion and Absorption

Food carbohydrates

Food carbohydrates

G Pancreatic amylase

hoCVOJOOH HOÖJ-.XH

Lactose

Lactase q

Maltose

Maltose q

Lactose

Maltose

Maltose q

Lactase q

Villus epithelial cell Galactose Glucose Glucose Glucose Glucose Fructose

Saccharose Disaccharides

Lumen

Villus epithelial cell Galactose Glucose Glucose Glucose Glucose Fructose

Galactose Na+ and energy Glucose Fructose Monosaccharides dependent Fadhtated Energy absorption diffusion dependent

Galactose Na+ and energy Glucose Fructose Monosaccharides dependent Fadhtated Energy absorption diffusion dependent

Absorption Monosaccharides

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Atkins Low Carb Diet Recipes

Atkins Low Carb Diet Recipes

The Atkins Diet is here. Dr Atkins is known for his great low carb diets. Excluding, Dr Atkins carb counter and Dr Atkins New Diet Revolution.

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