Lesson 1, Topic 1
In Progress


March 28, 2021

Cholesterol is a type of lipid that has many uses in the body (see Chapter 2). The body derives steroid hormones from cholesterol (see Chapter 12) and uses cholesterol to stabilize the phospholipid bilayer that forms the plasma membrane and membranous organelles of all cells (see Chapter 3).
So why does such a useful substance have such a bad reputation? The reason lies in the fact that an excess of cholesterol in the blood, a condition called hypercholesterolemia, increases the risk of developing atherosclerosis (see arrow in figure). You may recall from Chapter 14 that atherosclerosis develops into a type of arteriosclerosis, or “hardening of the arteries,” that can lead to heart disease, stroke, and other problems.

Hypercholesterolemia occurs most often in people with a genetic predisposition but is certainly also affected by other factors such as diet and exercise. People with hypercholesterolemia are encouraged to switch to diets low in cholesterol and saturated fats and to participate in aerobic exercise, both of which tend to lower blood cholesterol levels. Drugs such as statins are often used to control blood cholesterol when exercising and diet are not sufficient.
Chapter 2 discusses different types of cholesterol and their roles in health and disease.

Protein metabolism
In a healthy person, proteins are catabolized to release energy only to a very small extent. When fat reserves are low, as they are in the starvation that accompanies certain eating disorders such as anorexia nervosa, the body can start to use more of its protein molecules as an energy source. Excess proteins in the diet can also be used for energy.

Specifically, the amino acids that make up proteins are each broken apart. The nitrogen group amine is removed by the liver and converted to the waste product urea. The rest of the molecule is converted by gluconeogenesis to a form that can enter the citric acid cycle—and thus release energy to “charge up” ATP (see Figure 19-5).

After a shift to reliance on protein catabolism as a major energy source occurs, death may quickly follow because vital proteins in the muscles and nerves are catabolized.

A more common situation in normal bodies is protein anabolism, the process by which the body builds amino acids into complex protein compounds (for example, enzymes and proteins that form the structure of the cell). Proteins are assembled from a pool of at least 20 different kinds of amino acids. If any one type of amino acid is deficient, vital proteins cannot be synthesized—a serious health threat.

One way your body maintains a constant supply of amino acids is by making them from other compounds already present in the body. Only about half of the required 20 types of amino acids can be made by the body, however. The remaining types of amino acids must be supplied in the diet. Essential amino acids are those that must be in the diet. Nonessential amino acids can be missing from the diet because they can be made by the body (Table 19-2).

TABLE 19-2
Amino Acids

*Essential in infants and, perhaps, adult males.

†Can be synthesized from phenylalanine; therefore, is nonessential as long as phenylalanine is in the diet.





George Washington Carver (1864–1943)


At the dawn of the twentieth century, one figure loomed large in the world of food science—George Washington Carver. Born a slave on a Missouri plantation during the Civil War, Carver overcame great obstacles to become one of the most admired American scientists of his era. Although talented in music and art, it was his knack for agriculture that led him to a long and successful career as a professor, researcher, and inventor in the agriculture department of Alabama’s Tuskegee Institute.

At Tuskegee, his work resulted in the creation of 325 products made from peanuts, nearly 200 products from yams (sweet potatoes), and hundreds more from other plants native to the southern United States. Development of these new products helped poor farmers survive by allowing them to make money from a variety of crops that thrived on their land.

Today, breakthroughs continue to be made in the world of agriculture and food science. Farmers and ranchers work closely with agricultural scientists and technicians to improve food crops and to improve methods of raising livestock. As did Carver, they strive to work in ways that benefit the land and people. Of course, nutritionists, dietitians, chefs, and food preparers all play a role in getting these crops to our table in a healthy and appetizing way.
Food scientists and other industrial scientists work to develop technologies and methods for preparing, preserving, storing, and packaging foods.

Overview of vitamins
One glance at the label of any packaged food product reveals the importance we place on vitamins and minerals. We know that carbohydrates, fats, and proteins are used by our bodies to build important molecules and to provide energy. So why do we need vitamins and minerals?
First, let’s discuss the importance of vitamins. Vitamins are organic molecules needed in small quantities for normal metabolism throughout the body.

Most vitamin molecules attach to enzymes or coenzymes (molecules that assist enzymes) and help them work properly. Many enzymes are totally useless without the appropriate vitamins to activate them.
Some vitamins play other important roles in the body. For example, a form of vitamin A plays an important role in detecting light in the sensory cells of the retina. Vitamin D can be converted to a hormone that helps regulate calcium homeostasis in the body, and vitamin E acts as an antioxidant that prevents highly reactive oxygen 539molecules called free radicals from damaging DNA and molecules in cell membranes.

Most vitamins cannot be made by the body, so we must eat them in our food. The body can store fat-soluble vitamins—A, D, E, and K—in the liver for later use. Because the body cannot store water-soluble vitamins such as B vitamins and vitamin C, they must be continually supplied in the diet. Vitamin deficiencies can lead to severe metabolic problems. Table 19-3 lists some of the more well-known vitamins, their sources, functions, and symptoms of deficiency.

TABLE 19-3
Major Vitamins

Vitamin imbalances
Vitamin deficiency, or avitaminosis, can lead to severe metabolic problems. For example, avitaminosis C (vitamin C deficiency) can lead to scurvy (Figure 19-6). Scurvy results from the inability of the body to manufacture and maintain collagen fibers. As you may have gathered from your studies thus far, collagen fibers are critical in many of the connective tissues that hold the body together. In scurvy, the body falls apart in the same way that a neglected house eventually falls apart.

FIGURE 19-6 ​Scurvy. ​In scurvy, lack of vitamin C impairs the normal maintenance of collagen-containing connective tissues, causing bleeding and ulceration of the skin, gums, and other tissues, as these lesions on the skin show.

More details about scurvy and other types of avitaminosis are given in Appendix A at evolve.elsevier.com.
Some forms of hypervitaminosis—or vitamin excess—can be just as serious as a deficiency of vitamins. For example, chronic hypervitaminosis A can occur if very large amounts of vitamin A are consumed daily over a period of 3 months or more. This condition first manifests itself with dry skin, hair loss, anorexia (appetite loss), and vomiting, but may progress to severe headaches and mental disturbances, liver enlargement, and occasionally cirrhosis. Acute hypervitaminosis A, characterized by vomiting, abdominal pain, and headache, can occur if a massive overdose is ingested.
Excesses of the fat-soluble vitamins (A, D, E, and K) are generally more serious than excesses of the water-soluble 540vitamins (B complex and C) because fat-soluble vitamins are stored, whereas excess water-soluble vitamins can be excreted.

Minerals are just as essential for health as vitamins. Minerals are inorganic elements or salts found naturally in the earth and in many foods. As with vitamins, mineral ions can bind to enzymes and help them work effectively.
Minerals also function in a variety of other vital chemical reactions. For example, sodium, calcium, and other minerals are required for nerve conduction and for contraction in muscle fibers. Without these minerals, the brain, heart, and respiratory muscles would cease to function.
Information about some of the more important minerals is summarized in Table 19-4.

TABLE 19-4
Major Minerals

Like vitamins, minerals are beneficial only when taken in the proper amounts. Many of the minerals listed in Table 19-4 are required only in trace amounts. Any intake of such minerals beyond the recommended trace amount may become toxic—perhaps even life threatening.

Foods that go beyond simply providing the nutrients needed for health and wellness because they have specific characteristics that prevent disease are often called functional foods. To learn more about this concept, review the article Functional Foods at Connect It! at evolve.elsevier.com.