How are dietary carbohydrates digested?
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Normally, just about all non-fiber dietary carbohydrate will be absorbed across the wall of our small intestine. Monosaccharides are the absorbed form of carbohydrate, therefore disaccharides and starch must be digested into monosaccharides. Carbohydrate digestion begins in the mouth as chewing breaks up food and mixes it with saliva. The tongue helps position the food for chewing and mix it with saliva. Saliva contains salivary amylase, which is an enzyme that begins to break down starch. The activity of salivary amylase is short lived due to the rather brief period of time that food stays in the mouth. As the swallowed food/saliva mixture reaches the stomach the acidic juice reduces the activity of salivary amylase, which halts carbohydrate digestion. |
How are carbohydrates absorbed from the digestive tract?
Carbohydrates are primarily absorbed as monosaccharides, thus disaccharides and starch must be digested. Chemical digestion of carbohydrates picks up again in the small intestine as the pancreas delivers pancreatic amylase along with a battery of other digestive enzymes. Pancreatic amylase resumes the assault upon starch molecules, breaking them into smaller links of glucose. The cells that line the small intestine will play the final role in carbohydrate digestion as they produce enzymes that digest the smaller carbohydrates, such as disaccharides and the remaining branch points on what was once starch. The enzymes that split sucrose, maltose, and lactose into monosaccharides are called sucrase, maltase, and lactase, respectively.
Once monosaccharides are liberated they can move into the cells lining the wall of the small intestine. They can then move out the back end of these cells and then into tiny blood vessels (capillaries) in the wall of the small intestine. These capillaries drain into a larger blood vessel that leaves the intestines and travels to the liver (see Carbohydrate Absorption Figure). It should be mentioned that the absorption of glucose and galactose requires energy (ATP) but fructose does not.
What is lactose intolerance?
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By early childhood much of the world population, especially people of African, Asian, and Greek descent, loses the ability to produce sufficient amounts of the digestive enzyme lactase. In fact, lactase production decreases an average of 90% by age 5 resulting in poor lactose digestion. This is believed to be the natural course for humans and a similar situation can be seen in many other mammal species after they wean from their “mother’s milk”. In some populations such as Swedes, Finns and Caucasians in United States, the incidence of lactose intolerance is low (<12%). It is believed that this is the result of genetic change that occurred long ago that minimized the reduction in lactose production has resulting in this trait today. |
Undigested lactose is not absorbed and continues to move through the small intestine into the colon where it becomes available to bacteria. Bacteria easily break down lactose for energy and produce gases such as hydrogen gas and carbon dioxide and other substances in the process. Lactose intolerance can be diagnosed by the Hydrogen Breath Test during which 50 grams of lactose is provided and the amount of hydrogen in breath (derived from production in the intestines) is measured.
The gases produced in lactose intolerant people can lead to bloating, cramping, and flatulence. Also, as lactose moves through the digestive tract it will hold onto water, which soften feces and possibly produce diarrhea. These discomforts are collectively referred to as lactose intolerance. To deal with lactose intolerance many people add a product called Lactaid (lactase enzyme) to their milk to predigest the lactose. Lactaid milk containing pre-digested lactose is also available. This appears to be an effective method of adapting to lactose intolerance.
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