Let's get back to Amy and her lunch mentioned in Chapter 1. She will eat her hamburger, fries, and chocolate shake, but how do these nutrients get to the tissues in the body that need them? Digestion is the process of preparing foods to enter the body. This may sound strange, but any foods inside the digestive system are not yet actually in the body. The digestive system is a long tube (about 30 feet when relaxed) with openings at both ends (Figure 4.1). This tube is contained within the body and anything that enters it must pass into the cells lining the tube in order to get into the body's tissues. As food passes through the digestive tube, it is processed and broken down gradually so that the nutrients (e.g., sugars, proteins, and fats) can be absorbed by microscopic cells. This process occurs through the steps of digestion (including ingestion and propulsion), and absorption.
The hamburger bun, the fries, and the shake contain sugars. Carbohydrates (types of sugars) must be broken down to individual units called monosaccharides. Some sugars, such as the starch in the bread and potatoes, have hundreds of monosaccharides. Other sugars, such as table sugar, the milk in the shake, or beer, have only two sugar units and are called disaccharides. Anything larger than a monosaccharide will not be absorbed through the
digestive tube and will be used by bacteria living in the intestines. As a result of this bacterial metabolism, some people experience abdominal cramping and diarrhea. This occurs when a person is lactose intolerant, which is discussed in Chapter 9.
The meat in the burger is a good source of protein. Proteins are composed of hundreds of amino acids and must be broken down into individual amino acids in order to be absorbed into the cells lining the digestive tube. The body will use these building blocks to make body proteins. Proteins must be broken down in order to be used by the body.
The beef of the hamburger also contains fats, as does the oil in which the fries are prepared. Fats, also called lipids, may or may not be broken down to get them into the lining cells of the digestive tube. Different types of fats were described in Chapter 2. Cholesterol is absorbed whole, while triglycerides are broken apart every time they must enter or leave a cell. Triglycerides cannot pass through any cell membrane intact, but cholesterol can. Triglycerides are composed of a single glycerol and three fatty acid chains. The fatty acid chains can be either saturated or unsaturated. Saturated fatty acids contain the maximum number of hydrogen atoms, or are saturated with them, while unsaturated fats are missing two or more hydrogen atoms. Because the fatty acid chains are absorbed through the digestive tube "as is," the body will build up a supply of triglycerides that contains whichever type of dietary fatty acids we ingest. If a person eats food high in saturated fatty acids, the fatty acids will be transported to the tissue of the body and stored there. Fats must be mixed with proteins in order to travel in the bloodstream. Otherwise, the combination of these fats and blood would look like Italian salad dressing, with vinegar (blood) on the bottom and oil (fats) on the top. Because these saturated fats separate from the proteins carrying them in the circulatory system more frequently than unsaturated fats, these fats tend to float separately and get stuck in small blood vessels. This may cause a blockage of blood in the heart or around the brain. If this blockage is severe enough, it might cause a heart attack or stroke. Cholesterol can also separate from its protein carrier, adding to the potential blockage of the blood vessels and increasing the risks of heart attack and stroke.
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