Absorption is the movement of molecules across the gastrointestinal (GI) tract into the circulatory system. Most of the end-products of digestion, along with vitamins, minerals, and water, are absorbed in the small intestinal lumen by four mechanisms for absorption: (1) active transport, (2) passive diffusion, (3) endocytosis, and (4) facilitative diffusion. Active transport requires energy.

Nutrient absorption is efficient because the GI tract is folded with several surfaces for absorption and these surfaces are lined with villi (hairlike projections) and microvilli cells. As one nutrition textbook puts it, each person has a surface area "equivalent to the surface of a tennis court" packed into his or her gut (Insel et al., p. 81). Efficient absorption can be compromised due to lactose intolerance. Lactose intolerance is not uncommon in the world, affecting about 25 percent of the U.S. population and 75 percent of the worldwide population. It is usually due to the lack or absence of the enzyme lactase, which breaks down milk sugar.

Lactose intolerance is not a food allergy. Food allergies are serious, even life threatening, but most people with lactose intolerance can digest small amounts of milk, especially in yogurt and cheese.

Protein, carbohydrate, lipid, and most vitamin absorption occur in the small intestine. Once proteins are broken down by proteases they are absorbed as dipeptides, tripeptides, and individual amino acids. Carbohydrates, including both sugar and starch molecules, are broken down by enzymes in the intestine to disaccharides called sucrose, lactose, and maltose, and then finally into the end-products known as glucose, fructose, and galactose, which are absorbed mostly by active transport. Lipase, an enzyme in the pancreas and the small intestine, and bile from the liver, break down lipids into fatty acids and monglycerides; these end-products then are absorbed through villi cells as triglycerides.

Alcohol is not a nutrient, but 80 percent of consumed alcohol is absorbed in the small intestine. The other 20 percent is absorbed into the stomach. Alcohol is absorbed by simple diffusion, which explains why gastric ulcers are not uncommon in people who drink excessively.

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Food Allergies

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