CONCEPTS IN BIOLOGY

PART VI. PHYSIOLOGICAL PROCESSES

25. Nutrition. Food and Diet

25.8. Nutrition for Fitness and Sports

Many people are very interested in the value of fitness and sports to a healthy lifestyle. Along with this, an interest has developed in the role nutrition plays in providing fuel for activities, controlling weight, and building muscle. The cellular respiration process described in chapter 6 is the source of the energy needed to take a leisurely walk or run a marathon. However, the specific molecules used to get energy depend on the length of the period of exercise, whether or not one warms up before exercise, and how much effort one exerts during exercise. The molecules respired by muscle cells to produce ATP may be glucose, fatty acids, or amino acids. Glucose is stored as glycogen in the muscles, liver, and some other organs. Fatty acids are stored as triglycerides in fat cells. Amino acids are found in small amounts in the blood. Which molecules are respired depends on the duration and intensity of exercise. Glucose from glycogen and fatty acids from triglycerides are typically the primary fuels. Amino acids provide 10% or less of a person’s energy needs, even in highly trained athletes.

Conditioning includes many interrelated body adjustments in addition to energy considerations. Training increases the strength of muscles, including the heart, and increases the efficiency of their operation. Practicing a movement allows for the development of a smooth action, which is more energy-efficient than a poorly trained motion. As the body is conditioned, the number of mitochondria per cell increases, the Krebs cycle and the ETS run more efficiently, the number of capillaries increases, fats are respired more efficiently and for longer periods, and weight control becomes easier.

Anaerobic and Aerobic Exercise

Anaerobic exercise involves bouts of exercise that are so intense that the muscles cannot get oxygen as fast as they need it; therefore, they must rely on the anaerobic respiration of glucose to provide the energy needed. Activities such as weight-lifting or running short sprints are almost entirely anaerobic. During anaerobic respiration lactic acid builds up in muscles. The lactic acid is eventually transferred to the blood and delivered to the liver where it is metabolized. Following a bout of anaerobic exercise, one breathes rapidly for a period until the lactic acid is metabolized.

Aerobic exercise occurs when the level of exertion allows the heart and lungs to keep up with the oxygen needs of the muscles (figure 25.14).

FIGURE 25.14. Anaerobic and Aerobic Exercise

Sprinters are involved in anaerobic exercise—the runners cannot get enough oxygen to their muscles during the short race. Exercise walkers and joggers are involved in aerobic exercise—they are exercising at a rate that allows oxygen to get to muscles as fast as it is used up.

Metabolic Changes. During Aerobic Exercise

Most exercise programs encourage participants to do warm-up activities before beginning strenuous exercise. These activities serve several purposes. A primary function is to increase heart rate, which has several benefits. Blood is pumped more rapidly, resulting in more blood reaching the muscles. In addition, the capillaries in the muscles dilate so that more blood is able to flow through muscles. Finally, the warm-up exercise actually increases the temperature of the muscles, which makes them less stiff and also reduces the viscosity (“thickness”) of the blood. All of these actions are important because they increase the flow of blood to muscles and allow the blood to supply oxygen efficiently to the muscles and increase the speed and power of muscular contraction. These activities also lead to an increased speed of nerve conduction and provide a psychological benefit to the athlete. Finally, the warm-up activities begin a metabolic shift toward breaking down glycogen in muscle to provide the energy for muscular activity.

The body goes through several metabolic changes during aerobic exercise. For a short time at the beginning of exercise, anaerobic respiration provides the energy muscles need. As exercise continues, the body shifts to aerobic respiration as the circulatory system and respiratory system make adjustments to be able to supply oxygen to the muscles as fast as it is used. This metabolic shift is often experienced as a “second wind.” The athlete experiences a period of “shortness of breath” at the beginning of exercise that disappears with the switch to aerobic respiration. This is particularly true if he or she did no warm-up activities.

There are shifts even after the body has switched to aerobic exercise. Initially the energy supplied to muscles comes from glycogen stored in the muscles. As the period of activity increases, there is another metabolic shift in which fats (triglycerides) begin to be metabolized. Fatty acids released into the blood from fat cells begin to be used to provide some of the energy. (A small amount of protein is also metabolized, particularly if the exercise is of long duration.) At this point both glycogen and fats are being used to supply energy. The balance between glycogen and fat metabolism shifts toward fat metabolism the longer the exercise continues. This is why moderate, longer periods of exercise are most beneficial in weight loss. Eventually, if the exercise continues for long enough, the body’s store of glycogen is exhausted and the athlete experiences a debilitating fatigue known as “hitting the wall.” This occurs because glucose is not available in high enough quantities.

After a period of exercise it is recommended that people engage in cool-down activities that allow the body to return slowly to a resting state. These activities generally involve 5 to 10 minutes of jogging or walking accompanied by stretching exercises. This allows lactic acid and other metabolic waste products to be removed from muscles and metabolized or eliminated. The muscles involved in exercise had been receiving large amounts of blood, so they also need to return to a resting state. This involves reducing blood flow to the muscles and actually using the muscle contractions of the cool-down activities to squeeze veins to assist in the return of blood from the muscles. There is also a metabolic shift back to one that is less demanding of glycogen.

Diet and Exercise

Diet is an important adjunct to any exercise program. During exercise a primary concern is to take in adequate amounts of water. Water is important for two reasons: (1) Evaporation of water is a primary mechanism for preventing overheating of the body during exercise; and (2) loss of water during exercise also causes the viscosity of the blood to increase and makes the heart work harder to pump the “thicker” blood. A water loss of only 5% of body weight can decrease muscular activity by as much as 30%. Drinking tap water is the best way for most casual athletes to replace the water they lose. Sports drinks that contain salts (electrolytes) and glucose may be helpful to those who lose a great deal of water during hot weather or prolonged exercise. The general rule is to replace the water you lose.

Individuals who are engaged in long bouts of exercise, such as long-distance running or cycling, need to take in additional Calories in the form of glucose. Because they are using up larger amounts of glycogen in their muscles, providing glucose during exercise can help them prolong their exercise. Some serious athletes practice carbohydrate loading. This practice involves consuming meals with large amounts of carbohydrates in the days before a competition. It has been demonstrated that when carbohydrate loading is coupled with training for endurance events, the amount of glycogen stored in the liver increases, allowing, the athlete to go longer before running out of glycogen.

The need for protein in an athlete’s diet has also been investigated. An increase in dietary protein does not automatically increase strength, endurance, or speed. In fact, most Americans eat the 10% additional protein that athletes require as a part of their normal diets. The body uses the additional protein for many things, including muscle growth; however, increasing protein intake does not automatically increase muscle size. Only when there is a need is the protein used to increase muscle mass. Exercise provides that need. The body will build the muscle it needs to meet the demands placed on it. Vitamins and minerals operate in much the same way. No supplements should be required as long as the diet is balanced and complex (Outlooks 25.4 and 25.5).

OUTLOOKS 25.4

Myths or Misunderstandings About Diet and Nutrition

OUTLOOKS 25.5

Nutritional Health Products and Health Claims

Drug stores, supermarkets, health-food stores, and websites sell a wide variety of products that are not drugs but claim to provide benefits to health. Drugs must be proven to work as described and be safe. Nutritional health products are considered to be foods and manufacturers are not required to prove they function as described. There are three common categories of these products.

Dietary supplements are materials that are consumed by mouth that the manufacturer claims have beneficial health effects. These materials can be vitamins, minerals, plant materials, amino acids, enzymes, hormones, tissues, extracts of tissues, or a metabolite (any substance involved in metabolism). Manufacturers of dietary supplements are not allowed to make specific health claims ("Vitamin A prevents blindness.") but are allowed to make three general kinds of claims:

1. Health claims ("Improves immune system function." or "Consumption of folate may reduce birth defects.")

2. Structure/function claims ("Promotes bone and joint health." or "Calcium builds strong bones.")

If the manufacturer makes a structure/function claim, it must provide the following disclaimer on the label. This statement has not been evaluated by the FDA. This product is not intended to diagnose. treat, cure, or prevent any disease.

3. Nutrient content claims ("High in fiber." or "Excellent source of lycopene.")

Manufacturers do not need permission to produce and sell their products, but must determine for themselves that the product is not harmful and that the claims they make are not false or misleading. They do not need to provide any government agency with proof of their claims.

Probiotics are live microorganisms thought to be beneficial to health when consumed. There are suggestions that these microorganisms may inhibit harmful microbes, and thus relieve several kinds of intestinal problems (diarrhea, colitis). Probiotics are typically consumed in fermented foods such as yogurt, sauerkraut, pickled vegetables, or in capsules as dietary supplements. A common use of certain lactic acid bacteria is to alleviate symptoms of lactose intolerance.

Prebiotics are nondigestible food substances that are thought to support the growth of "good" bacteria in the gut. The most commonly described prebiotic is dietary fiber. Soluble fiber is thought to be best for the growth of beneficial bacteria.

25.8. CONCEPT REVIEW

16. How do anaerobic and aerobic exercise differ?

17. Describe what metabolic changes take place during prolonged exercise.

18. What is carbohydrate loading?

19. Describe the importance of water, carbohydrate, and protein in the diet of a typical healthy active person.

Summary

To maintain good health, people must ingest nutrient molecules that can enter the cells and function in the metabolic processes. The proper quantity and quality of nutrients are essential to good health. Nutritionists have classified nutrients into six groups: carbohydrates, proteins, lipids, minerals, vitamins, and water. Energy for metabolic processes can be obtained from carbohydrates, lipids, and proteins and is measured in Calories. An important measure of the amount of energy required to sustain a human at rest is the basal metabolic rate. To meet this and all additional requirements, the U.S. Department of Agriculture (USDA) has established the Dietary Reference Intakes, recommended dietary allowances for each nutrient. The USDA publishes the Food Guide Pyramid, which places foods into the following categories: grains, fruits, vegetables, milk, oils, and meat and beans. It also provides recommendations about exercise. The goal of the Food Guide Pyramid is to provide easily understood guidelines that will help people develop a healthy diet and lifestyle.

Should there be metabolic or psychological problems associated with a person’s normal metabolism, a variety of disorders can occur, including obesity, anorexia nervosa, and bulimia. As people move through the life cycle, their nutritional needs change, requiring a reexamination of their eating habits to maintain good health. Exercise is an important part of a healthy lifestyle. A person’s diet may need to be adjusted to support the level of exercise he or she maintains. For most people, the most important nutritional need associated with exercise is water to replace that lost through sweating. Serious athletes often require diets that contain higher numbers of Calories, particularly in the form of carbohydrates.

Basic Review

1. _____ is the modification and incorporation of absorbed molecules into the structure of an organism.

a. Nutrition

b. Assimilation

c. Dieting

d. Absorption

2. The rate at which the body uses energy when at rest is called its

a. basal metabolic rate.

b. basal metabolic index.

c. specific dynamic action.

d. work.

3. A body mass index between 25 and 30 kg/m² indicates that a person is

a. within a normal range.

b. obese.

c. overweight.

d. extremely active.

4. Some complex carbohydrates are a source of _____, which slows the absorption of nutrients and stimulate peristalsis (rhythmic contractions) in the intestinal tract.

a. fiber

b. vitamins

c. minerals

d. l ipids

5. Linoleic acid and linolenic acid are called _____ because they cannot be synthesized by the human body and, therefore, must be a part of the diet.

a. amino acids

b. fatty acids

c. essential fatty acids

d. B-complex vitamins

6. Inorganic elements, found throughout nature, that cannot be synthesized by the body are called

a. vitamins.

b. essential amino acids.

c. electrolytes.

d. minerals.

7. When proteins are conserved and carbohydrates and fats are oxidized first as a source of ATP energy, the body is involved in

a. protein-sparing.

b. specific dynamic action.

c. absorption.

d. dieting.

8. Information on the amounts of certain nutrients various members of the public should receive is contained in

a. the Food Guide Pyramid.

b. the Protein-Sparing Guide.

c. kwashiorkor.

d. the Dietary Reference Intakes.

9. A person with _____ has a cycle of eating binges followed by purging of the body of the food by inducing vomiting or using laxatives.

a. bulimia

b. anorexia nervosa

c. obesity

d. kwashiorkor

10. The use of alcohol during pregnancy can result in decreased nutrient exchange between the mother and fetus, as well as other developmental abnormalities called

a. osteoporosis.

b. fetal alcohol syndrome (FAS).

c. bulimia.

d. fetal deficiency disease.

11. Eating large amounts of protein is required to build large muscles. (T/F)

12. Which of the following categories of people would be affected the most by a shortage of protein in their diet?

a. athletes

b. children

c. people in old age

d. adult men

13. Items in the grain food group are important because they supply

a. carbohydrates.

b. vitamins.

c. fiber.

d. All of the above are correct.

14. Which food group provides protein, calcium, and vitamin D in the diet?

a. meat and beans

b. fruit

c. vegetables

d. milk

15. For most persons who exercise, the most important nutrient is

a. carbohydrates.

b. protein.

c. fats.

d. water.

Answers

1. b 2. a 3. c 4. a 5. c 6. d 7. a 8. d 9. a 10. b 11. F 12. b 13. d 14. d 15. D

Thinking Critically

Getting Ready for Exercise

Imagine that you’re a 21-year-old woman who has never been involved in any kind of sport, but suddenly you have become interested in playing rugby, a very demanding contact sport. If you are to play well with only minor injuries, you must get into condition. Describe the changes you should make in your daily diet and exercise program to prepare for this new experience.