After completion of Lesson 2, the student should be able to:

Transcription

1 Content Display Unit 2 - Energy Metabolism : Lesson 2 KINE xxxx Exercise Physiology 3 Unit 2 - Energy Metabolism 3 Lesson 2 1 U2L2P1 - Introduction to Unit 2 - Lesson 2 Lesson 2 starts to apply the basics of energy metabolism specifically to exercise. The fundamental question addressed is: How is the energy for the work of exercise specifically provided during exercise of different types (intensities and durations)? Learning Objectives After completion of Lesson 2, the student should be able to: 1. Define the following terms, and be able to use each term appropriately in discussions: respiratory quotient, respiratory exchange ratio, cross-over concept, peptide bond, transamination, transaminase, nitrogen balance equation, nitrogen balance, positive nitrogen balance, negative nitrogen balance, anabolic state, catabolic state, carbohydrate loading. 2. Discuss the relationship between power output and power input during an acute bout of exercise; include one or more absolute principles that are applicable. 3. Discuss the roles of the ATP-formation methods in exercise of different types (intensities and durations). In each case, identify the most likely limitation of each acute bout of exercise if the exercise is limited by metabolism. 4. Compare and contrast carbohydrates and fats as substrates for ATP formation via aerobic metabolism. In doing this, point out the strengths and the weaknesses of these two "aerobic substrates." 5. Describe the effects of exercise intensity and duration on the relative amounts of carbohydrates and fats used as substrates for aerobic metabolism. 6. Discuss the role of proteins as substrate for ATP formation via aerobic metabolism in (a) a person with normal nutritional status and (b) a starving person. 7. Discuss some of the dietary regimens that can affect energy metabolism during acute exercise. Contents of Lesson 1: Description Page Introduction to Unit 2 - Lesson of 31 5/17/2001 3:17 PM

2 Introduction to Unit 2 - Lesson Formation of ATP During Exercise 3-17 Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism Review of Lesson 36 2 U2L2P2 - Introduction to Unit 2 - Lesson 2 (cont.) Outline of Content III. Formation of ATP during exercise (Lesson 2) A. Principles B. Application of principles Analysis of specific activities 1. Running a marathon in 3 hours 2. Running 100 meters in seconds C. Summary Potential metabolic limitations of exercise IV. Carbohydrates, fats and proteins as substrates for aerobic metabolism (Lesson 2) A. Carbohydrates and fats Contrasts; advantages and limitations B. Effect of exercise intensity on proportions of carbohydrates and fats used C. Effect of exercise duration on proportions of carbohydrates and fats used D. Protein use E. Effect of diet 3 U2L2P3 2 of 31 5/17/2001 3:17 PM

3 - Formation of ATP during Exercise In the last section of the previous lesson, I dealt with basic characteristics of the three methods for replenishing ATP as it is used. In this section I want to address the application to acute bouts of exercise. To maximize performance by an athlete, it is critical to analyze the metabolic requirements of the specific activity the athlete does in competition and to determine potential metabolic limitations of performance. Then this must be applied to performance itself and to training. There are two essential considerations: (a) Where does the energy come from to form the required ATP over the course of the entire performance? (b) Where does the energy come from to form the required ATP during transitions from low intensity to high intensity exercise? The most obvious examples of these transitions are the starts of running races, in which athletes change very quickly from essentially resting at the start to high or even maximal power. Other transitions occur during the races, but they involve smaller changes in power. 4 U2L2P4 - Formation of ATP during Exercise (cont.) There are several absolute principles that apply: (a) The major determinant of the rate of energy input needed at a given instant during a given activity is the power output (i.e., the intensity of the exercise). If the power output changes, so does the power input. (b) The power input required for a given activity has to be matched by the actual power input at every instant of the activity; it s a pay-as-you-go system. The instant actual power input fails to meet what is needed, the person must decrease the intensity of the exercise. Example: Assume that for a given athlete to run at 15.0 mph (i.e., a 4-minute-per-mile pace), energy is required at the rate of 0.5 kcal/sec (30.0 kcal/min), to replenish the ATP so that the active muscle fibers do not run out. If this runner is running at 15 mph, it is absolutely certain that his/her metabolic energy input is 0.5 kcal/sec. If that power input would fall to 0.4 (or even 0.49) kcal/sec, it is just as certain that this runner is NOT running at 15.0 mph. 5 U2L2P5 3 of 31 5/17/2001 3:17 PM

4 - Formation of ATP during Exercise (cont.) Absolute Principles (cont.) (c) At any instant in time, the total rate at which energy is turned over (i.e., total power input) is the sum of the individual rates of the three systems: Total Rate = Rate of CK + Rate of glycolysis + Rate of oxidative metabolism. EXAMPLE: Using the example of running at a 4-minute-per-mile pace with energy being required at the rate of 0.5 kcal/sec, the following table lists four (of an infinite number of) possible combinations of contributions of the three methods of ATP replenishment. Power Input (kcal/sec) CK Reaction Anaerobic Glycolysis Aerobic Metabolism Total Each of these is a realistic possibility for specific portions of the mile race. For the given point of time in the race, it doesn t matter how the total power input is divided up among the methods; what matters is the total. (How the total is divided up may matter a great deal, however, in terms of the entire exercise period, if the result is depletion of CP or accumulation of too much lactic acid.) 6 U2L2P6 4 of 31 5/17/2001 3:17 PM

5 - Formation of ATP during Exercise (cont.) Absolute Principles (cont.) (d) In general, especially over the long term, the body prefers aerobic metabolism as the method for replenishing ATP. This is because the body has such a large supply of energy that can be transferred this way and without producing end products that create problems. But the relatively slow rate of response and relatively slow maximal rate of ATP formation by aerobic metabolism necessitate involvement of other systems (i) during transitions from lower to higher exercise intensities and (ii) during exercise of high intensity. (e) Because of its relatively low energy capacity, the body tries to spare the CK reaction, using it only when the other two methods can t meet the requirement. (f) During transitions from lower to higher exercise intensities (and therefore from lower to higher power inputs), all three methods contribute to the total energy input (i.e., to formation of ATP), although not in equal amounts. NOTE: All three methods are switched on in response to the increased demand. It is NOT a matter of one method being turned on to provide the energy, working alone for awhile, and then switching off as another switches on. (g) In every bout of acute exercise, all three methods contribute to ATP formation during the activity, but the relative contribution of each method varies with the intensity and duration of the exercise. 7 U2L2P7 5 of 31 5/17/2001 3:17 PM

6 - Formation of ATP during Exercise (cont.) Before we apply these principles to specific exercise examples, I need to emphasize a couple of points about very-high-power activities that last less than about 3 seconds. Examples of these activities include a maximal vertical jump, putting the shot, and swinging a baseball bat with maximal force. These activities require the highest (or nearly highest) rates of power output and input the body or individual muscle groups can have. But these activities are so brief they can be done using only the ATP in the muscles at the start of the activity. Replenishment of the ATP during the activity is not a factor in performance. Replenishment becomes a factor if such activities have to be repeated in rapid succession, such as repeated jumps for rebounds in basketball or repeated swings of a bat during an at bat in baseball or softball. But during a single performance there is no need for replenishing ATP. In the discussion that follows, I will not include these very short activities. I will focus only on activities for which the muscle does not have enough stored ATP at the start, and therefore activities that must have replenishment of ATP during the exercise. Such activities can be limited by the rate at which ATP is formed. 8 U2L2P8 - Formation of ATP during Exercise (cont.) Now let s apply the principles listed on previous pages to specific exercise examples. Running races of different distances provide good examples. Let s analyze the same runner in different races. For simplicity, let s assume this runner has the following metabolic characteristics: CK Reaction Anaerobic Glycolysis Aerobic Metabolism of Carbohydrates Maximal Power (kcal/sec) (kcal/min) Capacity (total kcal for ATP formation) ,500 Note that these values are not identical to the summary textbook values given previously. Remember that those values were given as 6 of 31 5/17/2001 3:17 PM

7 values given previously. Remember that those values were given as typical guidelines, and actual values vary from one person to another. The runner we are dealing with here may represent a larger person and/or a more highly trained person, though not elite or even highly competitive. 9 U2L2P9 - Formation of ATP during Exercise (cont.) Let s start with our runner running a marathon in exactly 3 hours. (This is far from the world s best, but if you haven t tried it, don t knock it!) This is an average pace of about 8.7 mph (a little less than 7 minutes per mile; 233 m/min; 3.89 m/sec). For simplicity, let s assume that this runner maintains this pace exactly over the entire 26 miles 385 yards (42.2 km). This pace requires a total power input of 0.25 kcal/sec (15.0 kcal/min) and therefore a total of 2,700 kcal over the entire race. Let me remind you for emphasis that this runner absolutely must turn over energy at the rate of 0.25 kcal/sec every second that he runs at this speed. The required power input can be provided by aerobic metabolism, since the runner s maximal aerobic power = 0.33 kcal/sec (20 kcal/min). In fact, this pace requires 75% of the runner s maximal aerobic power. In the first 2 minutes of the race, however, aerobic metabolism cannot meet the demand, because it adjusts slowly to demand. Aerobic metabolism would start forming ATP at a faster rate the instant the race starts, but it would take a couple of minutes for it to adjust to the 15.0 kcal/min required. For example, the energy input from aerobic metabolism might be kcal/sec (2 kcal/min) during the first 15 seconds, 0.09 kcal/sec (5.4 kcal/min) from 0:15 to 0:30, 0.14 kcal/sec (8.4 kcal/min) from 0:30 to 0:45 and so on until leveling off at 0.25 kcal/sec (15 kcal/min) by about 2:00. During this time, the total power input of 0.25 kcal/sec (15 kcal/min) still has to be provided for this runner to run at 8.7 mph. And it is provided by adding contributions from the CK reaction and anaerobic glycolysis. In the first few seconds of the race, while aerobic metabolism and glycolysis are adjusting, almost all of the ATP is replenished by the CK reaction, because it adjusts instantaneously. By about 15 seconds, glycolysis has adjusted, so this system can provide what is needed beyond what aerobic metabolism can provide. At this point, the CK reaction can stop making ATP and thus spare the CP in case it is needed later. So, if aerobic metabolism is forming ATP at the equivalent rates given above, the input from glycolysis from 0:15 to 0:30 will be 0.16 kcal/sec (9.6 kcal/min), and from 0:30 to 0:45 will be 0.11 kcal/sec (6.6 kcal/min), providing for the 0.25 kcal/sec (15 kcal/min) required. The input from glycolysis will gradually decrease as the input from oxidative metabolism increases. At 2:00, when the total demand is being met by oxidative metabolism, glycolysis can stop making ATP. The graph depicts the contributions of the three methods of ATP 7 of 31 5/17/2001 3:17 PM

8 replenishment during the first 3 minutes of the runner s marathon, divided into 15-second segments. 10 U2L2P10 - Formation of ATP during Exercise (cont.) Note that in this analysis of this runner s energy metabolism during the marathon, I am oversimplifying to emphasize the concepts regarding the involvement of the three ATP replenishment systems during exercise. In actuality, there would still be some anaerobic glycolysis occurring in this runner throughout the race, but the net amount of ATP formed this way is very small. I will address this topic more later. For now, I hope it is obvious that a runner simply cannot have much lactic acid accumulation if he/she is going to run for 3 hours. Remember that the total capacity of anaerobic glycolysis is only 60 kcal in our runner, and a total of 2,700 kcal is needed over the entire race. Considering the entire 3-hour marathon, a few kilocalories of energy are turned over in the CK reaction and by glycolysis at the very start to supplement aerobic metabolism. But over the entire race, almost all of the 2,700 kcal would be turned over in aerobic metabolism. Over 99% of the energy input for this marathon race is provided by aerobic metabolism and less than 1% from anaerobic metabolism (CK + glycolysis). Note, however, that it is not 100% from aerobic metabolism. Anaerobic mechanisms played an absolutely essential role at the start of the race. Also, if this runner had made any sudden increases in pace at other times during the race, small inputs from the anaerobic mechanisms would have been needed until oxidative metabolism adjusted. Let s consider possible metabolic limitations confronting this 8 of 31 5/17/2001 3:17 PM

9 Let s consider possible metabolic limitations confronting this runner during the 3-hour marathon. Depletion of CP is not a factor since the CK reaction contributes only a small amount to power input, at the start of the race (and a little at other times if the runner had accelerated during the race). Similarly, lactic acidosis is not a factor since anaerobic glycolysis contributes only a small amount to power input. The major concern is depletion of glycogen stored in the fibers of the muscles of the legs involved in running. This glycogen is the major substrate for aerobic metabolism in this 3-hour race, although some fat would also be used. If the glycogen would be totally depleted at some point before finishing the marathon, the runner would be dependent on fat as the substrate for aerobic metabolism from that point on. As a consequence, he would have to slow his running speed greatly, because the potential power input from fat is no more than 50% of the maximal power from carbohydrates. The fact that our runner maintained his power input at 75% of maximum throughout the 3-hour race verifies that he did not run out of muscle glycogen. 11 U2L2P11 - Formation of ATP during Exercise (cont.) Now let s analyze the energy turnover of the same runner during a sprint of 100 meters in 12 seconds. (No world record here either, but our runner is doing the best he/she can!) This is an average speed of 8.33 m/sec (18.6 mph). As with our analysis of the marathon run, for simplicity, let s assume that this runner is able to maintain this constant speed throughout the race (although we know this is impossible, because of the acceleration required from a standing start. In reality this runner would be running slower than the average speed early in the race and faster than the average speed at the end). This running speed is more than twice the speed of running a marathon in 3 hours, and the required power input is more than twice as much also. Let s assume this 100-meter pace requires 0.6 kcal/sec (36 kcal/min; 7.2 kcal total over the 12 seconds). In other words, if ever the runner s power input falls below 0.6 kcal/sec, he will no longer be running at 8.33 m/sec. First, let's ask wheather aerobic metabolism can meet this demand? The answer is no for two reasons: (a) This runner s maximal aerobic power is 0.33 kcal/sec, so even if it were at maximum it could not meet the demand. (b) The rate of energy turnover from aerobic metabolism increases slowly at the start of exercise, and it may take 2 minutes for aerobic power input to adjust completely. The 12 seconds of this race is not nearly enough time for complete adjustment. Does this mean that aerobic metabolism does not contribute at all to this bout of exercise? No, aerobic metabolism does 9 of 31 5/17/2001 3:17 PM

10 all to this bout of exercise? No, aerobic metabolism does contribute. Let s assume that aerobic power input has increased to 33% of its maximal value during the 12 th second, that is, 0.11 kcal/sec. This would be only one-sixth of the total requirement during the 12 th second, and of course the contribution from aerobic metabolism would be less than that in the previous 11 seconds. So, the contribution of aerobic metabolism to the total required over the 12 seconds is small, but it is not zero. 12 U2L2P12 - Formation of ATP during Exercise (cont.) What about the contribution of anaerobic glycolysis? The runner s maximal glycolytic power is 0.67 kcal/sec, so glycolysis is capable of meeting the total power input required. But, anaerobic glycolysis may take seconds after the start of exercise to adjust. So, it is possible that in the last couple of seconds of the 100-m race, glycolysis is providing most of the energy for ATP replenishment (remember that aerobic metabolism is providing a little). But while the rate of glycolysis is increasing during the first 10 seconds or so, the power input from glycolysis and aerobic metabolism combined is less than the required 0.6 kcal/sec. The additional power needed must come from the CK reaction. What is the contribution of the CK reaction? Immediately after the runner starts, nearly all of the required 0.6 kcal/sec must be provided by the CK reaction while aerobic metabolism and especially anaerobic glycolysis are adjusting. Remember that the CK reaction can adjust instantaneously to its maximal power, if needed. And in this case, the required power is well below the maximal power of the CK reaction (i.e., 1.0 kcal/sec). So the CK reaction can easily meet the required power at the start. In fact, it could meet the entire power input over the entire 12-second period. It doesn t need to, however, since the combined contribution of aerobic metabolism and anaerobic glycolysis increases with each second of the race. Therefore, the energy turnover in the CK reaction is highest immediately after the start of the race and then gradually decreases the rest of the time. In the last couple of seconds, if glycolysis and aerobic metabolism are meeting the entire demand, the contribution from the CK reaction would be zero. The figure presents a second-by-second analysis of the runner s power input during the 12-second 100-meter sprint. 10 of 31 5/17/2001 3:17 PM

11 13 U2L2P13 - Formation of ATP during Exercise (cont.) Was there a metabolic limitation for this runner in this 100-meter race? Based on our simplified analysis, probably not. Certainly 12 seconds is not nearly long enough for muscle glycogen to be depleted via glycolysis (anaerobic and aerobic). What about lactic acidosis? There would certainly be some acidosis because of the high rate of glycolysis, especially near the end of the run. But more than 12 seconds of high glycolytic activity is needed to have maximal or near-maximal levels of lactic acid accumulation. That leaves the possibility of a limitation related to the CK reaction. Was muscle CP depleted? No. Even if the CK reaction had provided all of the energy for the entire run (i.e., 7.2 kcal), this would not have depleted the CP. This runner had the equivalent of 15 kcal of CP stored in his muscles at the start of the race, available for transfer of energy to ADP via the CK reaction. So, at the end of this race, the runner s muscle CP concentration was reduced, but it was not totally depleted. REMEMBER: The very fact that this runner did this exercise (i.e., ran at 8.33 m/sec for 12 seconds) verifies that there was no metabolic limitation. He absolutely had to be providing the required energy via metabolism. Is it possible that this runner could have a metabolic limitation to running 100 meters in 10 seconds? Absolutely. 14 U2L2P14 11 of 31 5/17/2001 3:17 PM

12 - Formation of ATP during Exercise (cont.) Let me highlight key concepts we have been dealing with in this section. During every bout of acute exercise, some of the energy input for making ATP comes from each of the three methods. But the contribution of each method varies from a fraction of a percent to a very high percentage, depending especially on the intensity of the exercise. The following table presents examples of the contributions of the ATP replenishment methods to maximal-effort exercise of various durations. Obviously, a person can exercise at very high intensities for very brief periods, and the intensity must be decreased if exercise is to be prolonged. Therefore, in the table, the absolute intensity of the exercise goes down as the duration increases. Contribution to Total Power Input Exercise Duration CK Reaction Anaerobic Glycolysis Aerobic Metabolism 5 seconds 95% 4% 1% 10 seconds 65% 25% 10% 1 minute 20% 50% 30% 2 minutes 10% 40% 50% 4 minutes 5% 25% 70% 10 minutes 2% 8% 90% 30 minutes 1% 4% 95% 60 minutes <1% <2% 98% 15 U2L2P15 12 of 31 5/17/2001 3:17 PM

13 - Formation of ATP during Exercise (cont.) A coach and athlete should use this table and the concepts it summarizes to analyze specific activities, in order to estimate the contributions of the different metabolic methods of ATP replenishment. This does two things. First, it tells what is most likely to be limiting, from the point of view of muscular ability to replenish ATP. (NOTE: I am not saying here that it is always a limitation of metabolism in active muscles that limits performance. Such metabolic factors are only one category of factors that can limit performance. Many other things can limit performance too.) In general: Very high power activities of about seconds in duration are most likely to be limited by depletion of CP in muscles. High power activities of about 1-10 minutes are most susceptible to limitation by lactic acid build-up in active muscles (activities of longer duration can be too, but the longer the duration the less likely it is for acidosis per se to be limiting). Activities of 1-3 hours are often limited by availability of glycogen in active muscles. Long-duration activities may also be limited by availability of glycogen in the liver. Liver glycogen supplies glucose to the blood. When liver glycogen gets too low, blood glucose concentration falls (hypoglycemia), and central nervous system symptoms such as dizziness, light-headedness, disorientation and even fainting may occur. (More about this later.) A second use of analyzing activities in terms of contributions of the methods of ATP replenishment is the basis for making decisions about proportion of training time to spend on different types of training. For example, if an athlete competes in an activity in which 10% of the energy is provided by the CP method, 40% by glycolysis, and 50% by aerobic metabolism, most training should be spent with exercise that stress glycolysis and aerobic metabolism. HOWEVER, exercise that stresses the CP method cannot be totally ignored. Furthermore, if the athlete is already strong in terms of aerobic metabolism and relatively weak in terms of glycolysis, the training may be adjusted to improve the weaker area (but obviously without weakening the strong area). 16 U2L2P16 13 of 31 5/17/2001 3:17 PM

14 - Formation of ATP during Exercise (cont.) The graph below shows the analysis of maximal-effort activities of different durations based on contributions of aerobic metabolism and anaerobic metabolism to ATP formation during the exercise. Note that the CK reaction and glycolysis are combined into a single anaerobic metabolism category. You can see from this that, as a general guideline, maximal-effort exercise that is about 2 minutes in duration is about 50:50; that is, about 50% of the ATP is provided by aerobic metabolism and about 50% by anaerobic metabolism. Almost 100% of ATP replenishment during maximal-effort exercise of 10 seconds or so comes from anaerobic methods, and almost 100% of ATP replenishment during maximal-effort exercise of 60 minutes and longer comes from aerobic metabolism. 17 U2L2P17 14 of 31 5/17/2001 3:17 PM

15 - Formation of ATP During Exercise (cont.) I want to emphasize several points about the graph on the previous page and about the breakdown of activities based on the source of ATP replenishment. The percentages presented are general guidelines; they are not absolute in all cases. For example, it is very useful to remember that with maximal-effort exercise, about half of the energy for ATP replenishment during the exercise comes from aerobic metabolism and about half from anaerobic metabolism when the exercise is about 2 minutes in duration. But this could occur with exercise that is less than 2:00 in some individuals and more than 2:00 in others. This is an approximation. These breakdowns are dealing with energy that is provided during the exercise, and they do not include the recovery period. We will deal with metabolism in recovery in more detail later. For now you should realize that energy during recovery is essentially completely provided by aerobic metabolism. It is the contributions of the three ATP replenishment methods during exercise that determines whether they limit performance. Recovery cannot limit prior performance (though it may be a factor in subsequent performance)! These breakdowns have referred to maximal-effort exercise. If the same exercise is done submaximally, the percent contributions of the ATP replenishment methods will be shifted towards a relatively higher value for aerobic metabolism and proportionately lower values for the anaerobic mechanisms. For example, let s assume that a given person s PR (personal record) for running 800 m is 2:00. When this runner runs 800 m in 2:00, it is maximal effort, and about 50% of the energy for ATP formation during the exercise comes from aerobic metabolism and about 50% from anaerobic metabolism. When this same runner jogs 800 m in 3:30, this is far below a maximal effort, and much more than 50% of the energy for ATP formation during this 800-m jog comes from aerobic metabolism and much less than 50% from anaerobic metabolism. 18 U2L2P18 15 of 31 5/17/2001 3:17 PM

16 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism In this section we will study the contributions of the various foodstuffs as substrates or starting fuels for aerobic metabolism. With the anaerobic methods there are no options for substrates for ATP replenishment; the CK reaction must use CP, and glycolysis must use carbohydrates. But aerobic metabolism can use carbohydrates, fats, or proteins. In normal persons in normal nutritional states, proteins provide less than 10% of the energy for aerobic ATP replenishment during exercise. In other words, carbohydrates and fats account for over 90% of aerobic ATP replenishment. Let s study the contributions of these two predominant substrates first. Please remember that we are dealing only with aerobic metabolism in this section. 19 U2L2P19 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Let s contrast carbohydrates and fats as substrates for aerobic metabolism, to see the advantages and disadvantages of each: 1) Maximal rates of aerobic ATP synthesis ATP can be formed at least twice as fast with carbohydrates as the substrate than with fats as substrate. This has implications for the relative use of carbohydrates and fats depending on exercise intensity (explained more below). 2) Energy densities Nine kilocalories of energy is stored in 1 gram of fat, and 4 kcal in 1 gram of carbohydrate. Thus, fats are far superior to carbohydrates as a compact storage form of energy. Each of us would have to weigh a lot more if we had no storage fat and had the same amount of energy stored as glycogen. 3) Total body stores (i.e., total energy potentially available) Adults typically have about 1,000-3,000 kcal of energy stored as carbohydrates, mostly as glycogen in skeletal muscles. (This is not including periods of glycogen depletion after prolonged exercise.) The exact amount depends on body size and amount of muscle mass, diet, and training program. In contrast, adults will have tens of thousands (some even more than 100,000) of kilocalories of energy stored as fat, primarily in adipose tissue under the skin and around internal organs. Even a lean athlete will have much more energy stored as fat than as carbohydrate. For example, a 100-pound (45.4 kg) gymnast who is 6% fat has 2.72 kg (2,720 grams) of fat. This is equivalent to 24,480 kcal of energy (2,720 g x 9 kcal/g). 16 of 31 5/17/2001 3:17 PM

17 20 U2L2P20 21 U2L2P21 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Contrast between carbohydrates and fats (cont.): 4) Amount of ATP formed per amount of oxygen consumed When carbohydrates are completely broken down (oxidized), 5.05 kcal of energy are made available for every liter of oxygen consumed. In contrast, fat yields 4.69 kcal/l oxygen. Thus, carbohydrates make more effective use of oxygen consumption in transferring energy to ATP. This also has implications regarding use of these substrates at different exercise intensities (see below). When we are nearing the limits of our capacity for transport and use of oxygen (i.e., VO2max), it is metabolically advantageous to use carbohydrates as substrate to get the most energy per unit of oxygen consumed. 22 U2L2P22 23 U2L2P23 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) 17 of 31 5/17/2001 3:17 PM

18 Contrast between carbohydrates and fats (cont.): 5) Amount of carbon dioxide formed per amount of oxygen consumed When carbohydrates are completely oxidized, exactly the same volume of carbon dioxide is produced as the volume of oxygen consumed. When fats are completely oxidized, the volume of carbon dioxide produced is only 70% of the volume of oxygen consumed. This ratio of VCO2 to VO2 is referred to as the respiratory quotient (RQ) when analyzing cellular metabolism. In other words, the RQ of carbohydrates is 1.00 and the RQ of fats is (Actually, the value for fats varies from 0.68 to 0.71, depending on the exact form of fat, but on average it is 0.70). This information can be used in the laboratory to determine the percentages of carbohydrates and fats used as fuel by the whole body during certain resting and exercise conditions. When we calculate the VCO2/VO2 ratio of the whole body, we use the term respiratory exchange ratio (abbreviated RER or R). Briefly, when steady-state conditions exist, an RER value of 0.85 indicates that carbohydrates and fats are contributing equally as substrates for aerobic metabolism. As the RER gets closer to 1.00, the percent contribution of carbohydrates increases and percent contribution of fats decreases; as the RER gets closer to 0.70, the percent contribution of fats increases and percent contribution of carbohydrates decreases. This is a powerful tool in the laboratory for assessing carbohydrate and fat metabolism. One might question whether the extra carbon dioxide produced (per volume of oxygen consumed) with carbohydrate metabolism compared with fat metabolism presents a problem. It doesn t for people with normal lung function; this extra carbon dioxide is easily excreted via the lungs. Individuals with certain lung diseases, however, have difficulty excreting carbon dioxide. In extreme cases, high fat diets would actually benefit such persons by reducing the amount of carbon dioxide produced in aerobic metabolism. 18 of 31 5/17/2001 3:17 PM

19 24 U2L2P24 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Effect of Exercise Intensity on Relative Proportions of Carbohydrates and Fats Used as Substrate for Aerobic Metabolism In principle, the ratio of carbohydrate utilization to fat utilization is directly related to the intensity of exercise. There is a rule of thumb : When exercise intensity is 50% of VO2max, 50% of the substrate for aerobic metabolism is carbohydrate and 50% is fat. (NOTE: As with all rules of thumb, these numbers are not absolutes; they are typical values. Actual values vary from person to person and in different situations, depending on a number of factors.) This relationship is depicted in the graph. The term cross-over concept has been applied to this relationship. At low exercise intensities (less than 50% of VO2max), more fat is used in aerobic metabolism than carbohydrate. That is, the ratio of carbohydrate to fat is less than 1.0. As exercise intensity is increased, this ratio increases. At about 50% of VO2max there is a cross-over. The proportion changes from more fat than carbohydrate to more carbohydrate than fat; the ratio of carbohydrate to fat utilized becomes greater than U2L2P25 19 of 31 5/17/2001 3:17 PM

20 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Effect of Exercise Intensity on Relative Proportions of Carbohydrates and Fats Used as Substrate for Aerobic Metabolism (cont.) There is probably no situation in which either fat or carbohydrate makes up 100% of the substrate. In principle, however, at rest and during low-intensity exercise, much more fat is used than carbohydrate; and at very high exercise intensities, much more carbohydrate is used than fat. As with everything in physiology (even if we don t yet recognize it), there is logic to this. When power input needs to be high, carbohydrates are preferred because the maximal rate of ATP formation is much higher with carbohydrate as substrate than with fat as substrate. And more energy is turned over per volume of oxygen consumed with carbohydrate as substrate (oxygen transport has a limit that is stressed during high intensity exercise). When the demand for power input is low, fat metabolism can meet the demand, even with its slower rate of ATP formation, and the concern about effective use of the oxygen consumed is less because the overall rate of oxygen consumption is well below the limits of the oxygen transport system. The big advantage of using fats rather than carbohydrates whenever possible is that there is, for all practical purposes, an unlimited supply of energy from fats. This is not true for carbohydrates. This use of fats at lower exercise intensities is one example of what is often referred to as glycogen sparing. The body does many things to try to spare glycogen, because it is in rather low supply and because its use is critical in higher intensity exercise. 26 U2L2P26 20 of 31 5/17/2001 3:17 PM

21 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) As we have discussed, the relative proportion of fats used as substrate for aerobic metabolism is inversely related to exercise intensity (i.e., higher percentage of fats used at lower intensities and lower percentage of fats used at higher intensities). Because of this, it is common to hear fitness and weight-management experts advise that the best exercise for burning fat is very low intensity exercise. This is an oversimplification, and in many cases it is not true. Two factors determine the number of calories derived from fats during exercise: (a) The percentage of the total power input derived from fats and (b) the total power input. So, as the exercise intensity increases, it is true that the percentage of energy derived from fat decreases, but the total energy input per minute increases. Therefore, energy from fat is a smaller percentage of a larger value. So, often the rate at which energy is derived from fat is about the same or may even be greater at higher exercise intensities. What s the best advice? IF a person is going to exercise for a given distance (e.g., walk or run 1 mile) or total caloric expenditure (e.g., 200 or 300 kcal), then more total energy is derived from fats at lower intensities than at higher intensities. But IF a person is going to exercise for a given period of time (e.g., 30 minutes), more total energy is derived from fats at higher intensities than at lower intensities. Most people in fitness and weight-loss programs have a given period of time for exercise. In such cases, a person burns more total fat exercising at the highest intensity he/she can tolerate for the given time period. Please note that this is only considering the calories derived from fat. Other factors must be considered when prescribing exercise intensity (e.g., cardiovascular limitations, risk of injury, motivation of the exerciser). 27 U2L2P27 21 of 31 5/17/2001 3:17 PM

22 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Effect of Exercise Duration on Relative Proportions of Carbohydrates and Fats Used as Substrate for Aerobic Metabolism The intensity of exercise (power output) is the primary determinant of the relative contributions of fats and carbohydrates as substrates for aerobic power input. But the duration of exercise affects this too. For one thing, no matter what the intensity of exercise, there is typically a relatively high use of carbohydrates during the first few minutes of exercise, during the period of transition from low energy demand at rest to the higher energy demand of the exercise. Of greater significance, during prolonged exercise, after at least 30 minutes, the ratio of carbohydrate use to fat use gradually decreases as exercise continues. For example, if 60% of the energy from aerobic metabolism was derived from carbohydrates and 40% from fats during the first minutes of exercise (a carbohydrate-to-fat ratio set by the exercise intensity), these values may be 55% carbohydrate and 45% fat after 1 hour, and 50% carbohydrate and 50% fat after 2 hours. You can see that this is another example of glycogen sparing. As exercise goes on for a long period of time, there is more and more risk of depleting glycogen stores. To decrease this risk, the body shifts to use of relatively more fat and relatively less carbohydrate, compared to the earlier values (not necessarily more fat than carbohydrate). The effect of exercise duration on the relative use of carbohydrates and fats may be thought of as being superimposed on the major determinant of substrate use, the intensity of the exercise. 28 U2l2P28 22 of 31 5/17/2001 3:17 PM

23 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Protein Use Proteins are in general very large molecules, made up of many amino acids bonded together by peptide bonds. There are approximately 20 amino acids in the body, and each has a unique chemical structure. But there is one characteristic feature of all amino acids: one carbon atom has both an amino group (-NH2) and an organic acid group (-COOH) attached to it. A peptide bond is a specific type of chemical bond connecting the carbon of the acid group of one amino acid to the nitrogen of the amino group of another amino acid. There are very many proteins in the body, and they fall into various functional categories: enzymes (e.g., CK, ATPase), hormones (e.g., insulin), structural material (e.g., collagen of connective tissue), mechanics (e.g., the muscle proteins, actin and myosin), transport (e.g., hemoglobin and albumin in the blood), defense (e.g., antibodies, blood coagulation factors), and substrate for energy metabolism. The focus of this brief section is on the use of proteins as substrate for energy metabolism. 29 U2L2P29 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Protein Use (cont.) The many carbon-carbon bonds that exist in amino acids have energy similar to the carbon-carbon bonds that are in fatty acids and sugars. Therefore, there is potentially a huge supply of energy available in the form of proteins in the body, as much as or more than the energy stored in fat, depending on a person s nutritional state and body composition. Because most proteins serve important functions in the body, use of proteins as substrate for energy metabolism could compromise critical functions. In spite of this, and even though the body normally has a huge supply of energy stored as fat, some proteins (amino acids) are catabolized in exercise. Amino acids can be substrates for aerobic metabolism, but not anaerobic metabolism. During low-intensity exercise, perhaps 2-3% of the ATP formed in aerobic metabolism is derived from amino acid breakdown. During high-intensity exercise, or during the later stages of prolonged exercise, this percentage may be 5-10%. So the relative contribution of amino acids to aerobic energy metabolism is small. But remember that a given power input is required for every exercise intensity, and every contribution to the total is essential. 23 of 31 5/17/2001 3:17 PM

24 30 U2L2P30 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Protein Use (cont.) How are amino acids used for ATP formation? Amino acids that are parts of proteins must first be freed. This is accomplished by enzymes called proteases, which break the peptide bonds. Then the nitrogen-containing amino groups must be removed. In skeletal muscle, this is done by the process called transamination, catalyzed by enzymes known as transaminases. In transaminase reactions, the amino group is transferred to another molecule and replaced with a keto group (C=O). In the most common transaminase reaction, an amino acid reacts with pyruvic acid. The keto group from the pyruvic acid is transferred to the amino acid, and the amino group from the amino acid is transferred to pyruvic acid in exchange. In the process, pyruvic acid becomes the amino acid alanine. This may appear to accomplish nothing, since we have simply exchanged one amino acid for another. But this reaction collects amino groups from many different amino acids into a single form, alanine, and this alanine can be processed further. For example, alanine can be put into the blood and carried to the liver, which can use it to make glucose (which is discussed further later in this unit). After the amino acids are freed of their amino groups (and become keto acids), they are converted to various substances that are in the normal pathways of aerobic metabolism. These include pyruvic acid, acetyl CoA, and various substances in the Krebs Cycle, depending on the specific amino acids. Skeletal muscles have fairly high levels of transaminase activity. They seem to prefer using branched-chain amino acids as substrate for aerobic metabolism, but they can use any amino acid as substrate. 31 U2L2P31 24 of 31 5/17/2001 3:17 PM

25 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Protein Use (cont.) Proteins in the body are constantly turning over. That is, "old" ones are broken down and replaced by "new" proteins. Various proteins differ in rates of turnover, and many turn over very slowly. Nevertheless, there is constant breaking down and building up of proteins in the body. Nutritional status with reference to protein intake and breakdown is described by the nitrogen balance equation. Nearly all of the nitrogen ingested in the diet, "stored" in the body, and excreted by the body is or was part of amino acids attached to proteins. Therefore, protein nutritional status can be described by the relationship between nitrogen taken in (NIN) and nitrogen put out (NOUT). When NIN = NOUT, exactly the same amount of nitrogen is being excreted as the amount of nitrogen ingested. In this state the person is in nitrogen balance. On the whole, this indicates that there is no net build-up of tissue protein nor net breakdown of proteins. When NIN > NOUT, more nitrogen is being taken in than is being excreted. This is referred to as positive nitrogen balance. This is evidence of an anabolic state with more tissue protein being formed than is being broken down. This is the normal state during developmental growth spurts. It is also the desired state of athletes who are trying to build muscle mass. When NIN < NOUT, more nitrogen is being excreted than is being taken in the diet. This is referred to as negative nitrogen balance. This is evidence of a catabolic state with more tissue protein being broken down than is being formed. This state is not normally desirable, especially in athletes. A general catabolic state occurs with muscle-wasting diseases, such as muscular dystrophy, and with aging after a certain age (especially in sedentary people). 32 U2L2P32 25 of 31 5/17/2001 3:17 PM

26 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Protein Use (cont.) What are guidelines related to dietary protein requirements? Several decades ago, the general rule in terms of dietary intake was: 1 gram of protein per kilogram of body weight per day. Based on that guideline, a 60-kg person should take in 60 g of protein a day, a 70-kg person should take in 70 g per day, etc. Most nutritionists agreed that this was a liberal guideline, and that most adults are in nitrogen balance with only about g of protein per kilogram of body weight per day. This guideline pertained to average adults. What about very active persons, especially athletes during periods of intense training? During periods of intense exercise training, whole-body protein turnover is increased. This suggests a need for greater nitrogen intake. Furthermore, many athletes want to be in positive nitrogen balance, to gain mass (especially muscle). What are the dietary protein requirements in these cases? The earliest research studies on this topic dealt with athletes training to increase muscle mass. This is not surprising since the aim of these athletes is to increase masses of tissues made up largely of proteins. In general, most of these athletes will ingest enough protein to be in positive nitrogen balance if they follow the old guideline of 1 g protein/kg body weight/day. To be on the safe side, however, many sports nutritionists recommend that athletes who are just starting intense weight training (e.g., novice body builders), in whom gains in mass are relatively fast, should take in g protein/kg/day. They recommend that experienced lifters, in whom gains in mass are slower, should ingest g/kg/day. More recently, studies have indicated that endurance athletes may need more dietary protein than the average person. The recommendation is that during intense endurance training, athletes ingest g protein/kg/day. 33 U2L2P33 26 of 31 5/17/2001 3:17 PM

27 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) Protein Use (cont.) The next practical question is: Are athletes taking in enough protein? Most are. Studies have shown that most athletes in the United States take in g protein/kg/day in their diets. Weight-training athletes often take in more. This suggests that dietary protein intake is not limiting for most athletes. A possible exception to this is the athlete who religiously avoids dietary fat. Dietary fat is often associated with proteins, so limiting fats may limit protein intake. Endurance athletes may be most susceptible, since they consciously try to keep weight down and they typically place great emphasis on carbohydrates in the diet. Are dietary protein supplements beneficial for athletes? Psychologically, perhaps. If an athlete is convinced that he/she is doing better with the supplement, or conversely, is convinced he/she is doing worse without the supplement, then it may be worthwhile. But protein supplements probably have no physiological benefit. As discussed in the previous paragraph, few athletes need more protein than they ingest in the diet. If dietary protein intake is inadequate, of course supplements will help. But excess proteins beyond nutritional needs are converted to other substances, such as fat, or are simply metabolized for energy. The nitrogen excreted will increase to match the increased nitrogen taken in, to keep the person in nitrogen balance. Gaining muscle mass isn't as simple as taking in extra protein! There are interesting questions about potential benefits of ingesting specific amino acids, since it is conceivable that diets could be deficient in certain amino acids. Furthermore, certain amino acids may play key regulatory roles in terms of anabolism, delaying fatigue, or the like. I am not aware of research that supports beneficial effects of ingesting larger amounts of specific amino acids (assuming one has a balanced protein intake and is not dietarily deficient). Perhaps future research will show this. In the meantime, I would remind you that marketing people do not have to back up claims with scientific evidence. Makers of supplements in general are more interested in athletes' money than in their physiology! 34 U2L2P34 - Carbohydrates, Fats and Proteins as Substrates for Aerobic Metabolism (cont.) 27 of 31 5/17/2001 3:17 PM

28 Effect of Diet One of the major challenges for the body in terms of aerobic metabolism is to use just the right mix of fats and carbohydrates. Using relatively more carbohydrates is advantageous because of the much higher power that can be generated using carbohydrates, and because more energy can be derived per unit of oxygen consumed. On the other hand, if exercise is prolonged and endurance is a factor, using carbohydrates at too high a rate could lead to depletion of carbohydrate stores before the end of the event. As we have seen, one of the most important considerations in this regard is the exercise intensity. Another is diet, both diet prior to competition and ingestion of fuel during competition. Precompetition Diet. There is abundant research evidence to support benefits of a high-carbohydrate diet for endurance athletes. Total body stores of carbohydrates are increased when on a high-carbohydrate diet (60-70% of total caloric intake from carbohydrates) compared with a low-carbohydrate (30-40% of calories from carbohydrates) or even a mixed diet (50-60% of calories from carbohydrates). And most of this increase is in the form of glycogen stored in skeletal muscles. Therefore, the athlete who is on a high-carbohydrate diet has an advantage during endurance performance; since he/she is starting the event with more stored glycogen, glycogen depletion is less likely. Either separately or in addition to a regular diet that is high in carbohydrates, many athletes use a regimen of carbohydrate-loading for several days before competition. Carbohydrate-loading is also known as glycogen loading, carbohydrate or glycogen packing, and glycogen supercompensation. In one form, this involves simply eating a diet very high in carbohydrates for 2-3 days prior to competition. This increases muscle glycogen stores. A variation of this involves intense endurance exercise to lower muscle glycogen stores during the 4-6 days before competition, sometimes while eating a relatively low-carbohydrate diet. Then a very high-carbohydrate diet is eaten during the 2-3 days before competition. There is some evidence that the combination of initial depletion of muscle glycogen followed by a high-carbohydrate diet results in even higher muscle glycogen levels than the dietary maneuver by itself. A word of caution is in order here. Changing diets and manipulating carbohydrate stores in the body can affect a lot of things, including how a person feels (and therefore psychological variables), body weight, and performance during training bouts. Regimens aimed at increasing muscle glycogen stores should be practiced before major competitive events. The athlete should be comfortable with and confident in the routine prior to competition. 35 U2L2P35 28 of 31 5/17/2001 3:17 PM