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FLUID REPLACEMENT AND HEAT STRESS Fluid Replacement and Heat Stress, 1993 Pp. 99-110. Washington, D.C. National Academy Press 8 Timing of Carbohydrate Supplementation During Prolonged Strenuous Exercise Edward F. Coyle1 and Andrew R. Coggan INTRODUCTION Both muscle glycogen and plasma glucose are oxidized by skeletal muscle to supply energy during prolonged exercise (Ahlborg and Felig, 1982; Ahlborg et al., 1974; Bergstrom and Hultman, 1966, 1967; Gollnick et al., 1981; Hermansen et al., 1967; Ivy et al., 1983; Pallikarakis et al., 1986; Pirnay et al., 1982; Wahren, 1970). Although the underlying mechanisms are uncertain, there appears to be a gradual shift from intramuscular glycogen toward blood-borne glucose as the predominant carbohydrate energy source as exercise proceeds and as muscle glycogen is depleted (Coggan and Coyle, 1987; Coyle et al., 1986; Gollnick et al., 1981; Ivy et al., 1983; Wahren, 1970). The contribution of glucose to oxidative metabolism may be limited, however, by a decline in the plasma glucose concentration late in exercise as liver glycogen stores diminish. Therefore, it may be necessary to ingest carbohydrate to maintain or elevate the blood glucose concentration. We 1 Edward F. Coyle, The Human Performance Laboratory, The University of Texas at Austin, Austin, TX 78712
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FLUID REPLACEMENT AND HEAT STRESS have previously demonstrated that feeding carbohydrate throughout exercise at 70%-74% of maximal O2 uptake (i.e., ) can delay fatigue by 30 to 60 min (e.g., from 3 h to 4 h) (Coyle et al., 1983, 1986). A major finding was that carbohydrate feedings did not spare muscle glycogen utilization and that trained cyclists were able to exercise for the additional hour when fed carbohydrate without relying upon muscle glycogen for a fuel (Coyle et al., 1986). Instead, it appears that when the blood glucose concentration is maintained at 5 mM by carbohydrate feeding, highly trained cyclists are capable of relying upon blood glucose for almost all of their carbohydrate energy during the later stages of prolonged strenuous exercise. When exercising without feedings, the blood glucose concentration declines progressively after the first hour and reaches hypoglycemic levels (i.e., <2.5 mM) after 3 h of exercise (Coyle et al., 1983, 1986). Figure 8-1 describes our theory that the source of carbohydrate energy shifts from muscle glycogen to blood glucose as the duration of exercise progresses. Thus, blood glucose appears to be the most important source of energy after 3 h of strenuous cycling. It is therefore important that people have adequate glucose in their blood during the later stages of exercise in order to delay fatigue. FIGURE 8-1 Theoretical representation of the sources of energy during prolonged cycling at 70% of maximal oxygen uptake. Source: Redrawn from Coyle et al. (1986).
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FLUID REPLACEMENT AND HEAT STRESS This paper addresses the question of whether there is a critical time when carbohydrate must be administered during exercise in order to delay fatigue. Two recent studies from our laboratory are summarized in an attempt to answer this question (Coggan and Coyle, 1987; 1989). REVERSAL OF FATIGUE BY CARBOHYDRATE INFUSION OR INGESTION We first determined whether it is possible to reverse fatigue late in exercise through carbohydrate supplementation (Figure 8-2) (Coggan and Coyle, 1987). Instead of providing carbohydrate feedings throughout exercise, the cyclists received only water and exercised at 70% of until fatigued (i.e., exercise bout 1). After they were fatigued, they rested for 20 min and received one of the following treatments: FIGURE 8-2 Respiratory exchange ratio (R) and plasma glucose responses during exercise bouts 1 and 2. The asterisks denote a significant (P<0.05) decline during bout 1; the daggers denote values during exercise bout 2 that were significantly higher (P<0.05) than at the point of fatigue during bout 1. Source: Coggan and Coyle (1987).
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FLUID REPLACEMENT AND HEAT STRESS Intravenous glucose infusion at a rate that elevated and maintained blood glucose concentrations at normally high levels (5.0-5.5 mM; euglycemic clamp procedure). Oral ingestion of 200 g of a carbohydrate solution at the beginning of the rest period [400 ml of 50% (w/v) solution of glucose polymers with sucrose (Exceed, Ross Laboratories)]. Oral ingestion of a placebo solution (i.e., aspartame sweetened, colored, and flavored) that contained no energy. The placebo was used to determine the extent to which the 20-min rest period alone restored work tolerance. After receiving one of these treatments, the cyclists then attempted to continue exercising at the original work rate (i.e., exercise bout 2). It should be noted that the exercise tests were conducted in the laboratory by using a stationary cycle ergometer (Quinton model 845). Exercise bout 1 was begun after an overnight fast. Exercise Bout 1 Figure 8-2 summarizes the results. As we have shown previously (Coyle et al., 1983, 1986), the cyclists exercised for 168 to 172 min before fatiguing during exercise bout 1. The purpose of exercise bout 1 was to produce fatigue. As was shown previously, fatigue during exercise bout 1 was preceded by a decline in the subjects' respiratory exchange ratio (R), which reflected a proportional decline in the rate at which carbohydrate was used for energy. During the second hour of exercise, the plasma glucose concentration began to decline, and it continued to decline to relatively low values at the point of fatigue (i.e., 3.0-3.2 mM), which occurred after 168 to 172 min. No experimental treatment was provided during exercise bout 1, and each of the three trials elicited identical responses. Exercise Bout 2 After a 20-min rest period and application of one of the experimental treatments (i.e., placebo, intravenous glucose infusion, or carbohydrate ingestion), the cyclists attempted to continue exercise at the original work rate (i.e., 70% ) as long as possible (exercise bout 2). Placebo Ingestion. Plasma glucose increased from 3.1 ± 0.2 to 3.8 ± 0.3 mM because of the 20-min rest period during the placebo trial. As shown in Figure 8-2, however, the plasma glucose concentration declined rapidly to
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FLUID REPLACEMENT AND HEAT STRESS 3.1 mM during exercise bout 2. The subjects were able to tolerate only an average of 10 ± 1 min of exercise before they became fatigued (range, 6-12 min). The cyclists' ability to oxidize carbohydrate did not increase above fatigued levels, as reflected by the R value (Figure 8-2). These findings during the placebo trial agreed with the concept that carbohydrate depletion (e.g., muscle glycogen depletion and low blood glucose concentration) caused fatigue. The 20-min rest period allowed blood glucose to increase slightly, but it quickly declined and fatigue resulted after only an additional 10 min of exercise. Glucose Infusion. To ensure that the blood glucose concentration was restored to normally high levels and that it was maintained during exercise bout 2, glucose was infused intravenously using a Harvard syringe pump. Infusion of a priming dose (4.1 ± 0.8 g) elevated plasma glucose to 4.6 ± 0.4 mM at the start of exercise bout 2. Blood samples were obtained every 5 min during exercise, and the rate of glucose infusion was adjusted in order to maintain blood glucose in the range of 4.5-5.5 mM. In this way it was possible to ensure that the supply of glucose presented to the exercising musculature was adequate. Another important aspect of this infusion trial was that it provided an accurate estimate of the rate at which the exercising musculature was relying upon blood glucose for energy. The rates of glucose removal from blood and oxidation are approximately equal to the rate of glucose infusion when the glucose concentration in the blood remains stable, as shown in Figure 8-2. Stated another way, when glucose infusion does not change the blood glucose concentration, this indicates that the rate of entry equals the rate of removal. Almost all of the infused glucose was probably taken up and oxidized by the exercising musculature because insulin concentrations remained low (9-11 µU/ml) and the muscle glycogen concentration in the vastus lateralis remained low (40-47 mmol of glycosyl units per kilogram of muscle) during exercise bout 2. In order to maintain the plasma glucose concentration at 4.5-5.5 mM, glucose had to be infused at an average rate of 1.08 ± 0.06 g/min during exercise bout 2. This suggests that the infused glucose was being taken up and oxidized by the exercising musculature and other tissues at a similar rate. During this period, the rate of total carbohydrate oxidation was 1.6 g/min. It appears that approximately 70% (i.e., 1.1/1.6) of the carbohydrate energy was provided by the infused glucose. It should be realized that endogenous glucose, from gluconeogenesis and liver glycogenolysis, was probably oxidized in addition to the infused glucose and therefore it is possible that even more than 70% of the carbohydrate energy was provided by total glucose oxidation.
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FLUID REPLACEMENT AND HEAT STRESS As shown in Figure 8-2, glucose infusion increased the rate of carbohydrate oxidation, as reflected by R, above the levels associated with fatigue (i.e., R of 0.81). Fatigue was also reversed with glucose infusion. The cyclists exercised for an additional 43 ± 5 min (range 27-60 min; P < 0.05 versus placebo) during exercise bout 2 before they again became fatigued. Fatigue during exercise bout 1 therefore appears to be due primarily to an inadequate supply of carbohydrate for the exercising musculature, which can be reversed for 43 min with a high rate of glucose infusion (Coggan and Coyle, 1987). Although it is not practical to infuse glucose intravenously in field situations, these data are important because they indicate that the exercising musculature relies heavily upon blood glucose for fuel during the later stages of prolonged exercise. Therefore, it is not critical that glucose supplementation occur throughout exercise, since it does not spare muscle glycogen utilization and liver glycogen can adequately maintain blood glucose during the early stages of exercise. It is the availability of blood glucose late in exercise that is critical, as demonstrated by the fact that exercise could be maintained for an additional 43 min after fatiguing when glucose was readily availabel through infusion. More importantly, for fatigue to be delayed and for the blood glucose concentration to be maintained, glucose must enter the blood at the rate of more than 1 g/min in trained cyclists. Therefore, feeding schedules and drink composition (e.g., carbohydrate concentration) should be designed with the aim of providing the exercising musculature with 1 g of glucose per min late in exercise. As shown below, in order to provide the exercising musculature with glucose at these high rates late in exercise, carbohydrate ingestion must begin a given length of time before fatigue. Carbohydrate Ingestion. Ingestion of approximately 200 g of carbohydrate [50% solution containing glucose polymers, sucrose (Exceed, Ross Laboratories)] during the 20-min rest period restored the blood glucose concentration to 5.1 ± 0.4 mM at the beginning of exercise bout 2 (Figure 8-1). Carbohydrate oxidation was also restored during the first 10 to 15 min of exercise. However, the plasma glucose oxidation declined progressively to 3.9 ± 0.3 mM during exercise bout 2. Fatigue occurred after an average of 26 ± 4 min of further exercise (range, 11-44 min). This was significantly (P < 0.05) longer than that when the placebo solution was ingested (10 ± 1 min) but significantly less than that when glucose was infused (43 ± 5 min). Fatigue was preceded not only by declining blood glucose but also by a reduction in the rate of carbohydrate oxidation or R (Figure 8-2). The progressive reduction in blood glucose during exercise following carbohydrate ingestion and earlier fatigue compared with that after infusion
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FLUID REPLACEMENT AND HEAT STRESS with glucose suggests that ingested carbohydrate cannot enter the blood quickly enough to maintain the blood glucose concentration and meet the energy requirements of exercise. Therefore, a person generally should not wait until he or she is fatigued to ingest carbohydrate. This agrees with the empirical observation of competitive cyclists that one should feed before becoming hungry. Interpretation. These findings indicate that people should not wait until they are fatigued before they ingest carbohydrate, because it is likely that the rate of entry of glucose into the blood is too slow to match the rate of removal. We directly demonstrated through glucose infusion that the rate of glucose removal from blood can be well in excess of 1 g/min. These findings were somewhat anticipated based upon the experience of cyclists in competition. We therefore thought it practically important to determine also how long before the point of fatigue a cyclist should begin ingesting carbohydrate in order to restore and maintain blood glucose throughout exercise while improving performance ability. FEED BEFORE FATIGUE During a fourth trial the cyclists ingested approximately 200 g of carbohydrate (i.e., 3 g/kg of body weight) in a 50% solution (Exceed, Ross Laboratories) prior to the point of fatigue. The feeding was given after 135 min of exercise, which was on average approximately 35 min prior to the point of fatigue when a placebo solution was ingested. Based upon previous gastric emptying experiments (Foster et al., 1980), we reasoned that 35 min might be long enough to allow a sufficient amount of glucose to enter the blood. Performance Figure 8-3 compares the responses to exercise with placebo as opposed to the responses when carbohydrate was ingested after 135 min. Fatigue occurred after 170 ± 10 min when placebo was ingested and was delayed 21% and occurred after 205 ± 14 min when carbohydrate was ingested (P < 0.05). All subjects but one demonstrated an improvement in performance, with fatigue delayed by 25 to 58 min. The one individual who did not improve when fed before he became fatigued appeared to become depleted of carbohydrate prematurely during this trial in comparison with his other
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FLUID REPLACEMENT AND HEAT STRESS FIGURE 8-3 Respiratory exchange ratio (R) and plasma glucose responses to exercise bout 1 when a placebo was ingested compared with that when a glucose polymer solution (Feed) was ingested 35 min prior to the estimated point of fatigue. The asterisks indicate that Feed was significantly higher than Placebo (p<0.05). Source: Coggan and Coyle, 1989. trials, with declines in R and in plasma glucose after 105 min of exercise to values similar to those observed at fatigue (145-167 min) in his other trials. Because fatigue seemed imminent, the carbohydrate drink was provided at 105 min. As a result, plasma glucose and R increased, and he was able to continue exercising for an additional 45 min. Plasma Glucose Response As shown in Figure 8-3, carbohydrate feeding after 135 min reversed the decline in plasma glucose and successfully restored and maintained euglycemia throughout the remainder of exercise. The decline in R was also halted. These findings indicate that fatigue can be effectively delayed by 35 min in cyclists when carbohydrate feeding is not begun until 35 min prior to
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FLUID REPLACEMENT AND HEAT STRESS the time that fatigue would occur without feeding. The 35-min delay of fatigue when subjects were fed prior to fatigue was of similar magnitude to the 43 min of further exercise that was permitted by intravenous glucose infusion. Interpretation These findings suggest that ingested carbohydrate can provide energy at sufficient rates during the later stages of prolonged exercise when it is given at least 35 min prior to the point at which blood glucose supplementation becomes critical. As discussed previously, a person should not wait until he or she is fatigued before ingesting carbohydrate. We expect that the effectiveness of carbohydrate feeding in delaying fatigue, when first provided at a time less than 30 min prior to anticipated fatigue, varies among individuals. Large differences in individual response were observed when individuals were fed at fatigue, as previously discussed. We therefore recommend that carbohydrate feeding should begin, at the latest, 30 min prior to the anticipated fatigue, unless there is reason to believe that given individual can wait longer but still fully benefit from the feeding (i.e., get it into the blood at high rates). It should be emphasized that the present study was designed to determine how long a cyclist can wait before beginning to feed during prolonged exercise. Although we have determined that he or she can, if need be, wait until approximately 30 min prior to fatigue before beginning to feed, we are not suggesting that this is the ideal feeding schedule. It only indicates that if a person must wait this long before feeding, for whatever reason, the potential for performance enhancement by carbohydrate feeding is not appreciably compromised. Carbohydrate feedings appear to be an important energy source late in exercise. An obvious question is whether there is any advantage to ingesting carbohydrate throughout exercise or whether an individual should wait until the later stages of exercise before beginning to feed. In separate studies we have shown that fatigue can be delayed 30-60 min both when feedings are taken throughout exercise (Coyle et al., 1983, 1986) and when they are initiated 35 min prior to the point of fatigue when fasted (this study; Coggan and Coyle, 1989). Therefore, we have no reason to recommend that one feeding schedule is superior to the other. The important aspect is that the ingested carbohydrate be capable of supplementing blood glucose stores at a rate of over 1 g/min late in exercise (Coggan and Coyle, 1987).
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FLUID REPLACEMENT AND HEAT STRESS Carbohydrate will be availabel to supplement blood glucose late in exercise if feedings are taken throughout exercise. In our previous studies (Coyle et al., 1983, 1986), cyclists began feedings after 20 min of exercise (approximately 70 g) and continued feeding every 20 min thereafter (20-28 g). A person should determine whether this type of regimented feeding schedule is possible and tolerable during prolonged activity. If it is not possible to ingest carbohydrate throughout prolonged intense exercise, a feeding schedule should be adopted that allows the ingested glucose to enter the blood at a rate of approximately 1 g/min late in exercise. It should be realized that these recommendations are specific to intense exercise (i.e., cycling at 70%-75% ) performed for prolonged periods (i.e., 3-4 h). METABOLIC AND PERFORMANCE EFFECTS OF CARBOHYDRATE FEEDING DURING MILD- TO MODERATE-INTENSITY EXERCISE Although the benefits of carbohydrate feeding during prolonged exercise at approximately 70% are clear, it has not been firmly established that carbohydrate feeding delays fatigue during exercise of mild to moderate intensities (i.e., 30%-50% ). The energy for mild exercise is derived largely from the oxidation of blood-borne substrates such as glucose and fatty acids with less reliance upon muscle glycogen (Ahlborg and Felig, 1982; Ahlborg et al., 1974; Gollnick et al., 1981; Pallikarakis et al., 1986; Pirnay et al., 1982). Carbohydrate feeding during exercise at 30%-50% results in an increase in carbohydrate oxidation and a sparing of endogenous carbohydrate stores (Ahlborg and Felig, 1982; Ahlborg et al., 1974; Ivy et al., 1983; Pallikarakis et al., 1986; Pirnay et al., 1982). It appears that the exogenous ingested glucose is oxidized in place of endogenous glucose (i.e., liver glycogen) and free fatty acids. It is not clear whether carbohydrate feeding during prolonged mild-intensity exercise alters muscle glycogen degradation or whether it stimulates muscle glycogen resynthesis in humans (see the paper by J. L. Ivy, this volume). It does appear that carbohydrate feeding has the potential to delay fatigue during some types of exercise that elicit low percentages of . For example, Ivy et al. (1983) observed that carbohydrate feeding during prolonged uphill walking allowed subjects to exercise 31 min longer than when they received a placebo (4 h 59 min versus 4 h 28 min; P < 0.05). Although the exercise intensity was only 45% , subjects relied heavily upon carbohydrate for energy and fatigued during the fifth hour, primarily because of localized fatigue in the lower leg.
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FLUID REPLACEMENT AND HEAT STRESS Our concept is that carbohydrate feedings have the potential to delay fatigue because they maintain the blood glucose concentration late in exercise and become an important source of carbohydrate energy without which exercise cannot be tolerated. The salient point is that exercise should be of sufficiently high intensity to demand a prerequisite rate of carbohydrate oxidation. Exercise intensity cannot simply be judged according to the percentage of whole-body maximal oxygen uptake that is elicited. It is possible that exercise performed with a relatively small muscle mass may not elicit a large percentage of but that it can be relatively stressful for a particular muscle group, thus stimulating reliance upon carbohydrate in certain muscle groups or fibers. Stated another way, exercise that is mild regarding the percentage of can be strenuous enough to cause localized fatigue if a relatively small amount of muscle performs a disproportionate amount of work. It is quite likely that field soldiers who performed repetitive motions of lifting, climbing, or walking uphill experience localized muscle fatigue resulting from carbohydrate depletion, and it is possible that they would benefit from carbohydrate feeding. To our knowledge, however, this has yet to be carefully investigated. REFERENCES Ahlborg, G., and P. Felig. 1982 Lactate and glucose exchange across the forearm, legs, and splanchnic bed during and after prolonged leg exercise. J. Clin. Invest. 69:45-54. Ahlborg, G., P. Felig, L. Hagenfeldt, R. Hendler, and J. Wahren. 1974 Substrate turnover during prolonged exercise in man: splanchnic and leg metabolism of glucose, free fatty acids, and amino acids. J. Clin. Invest. 53:1080-1090. Bergstrom, J., and E. Hultman. 1966 The effect of exercise on muscle glycogen and electrolytes in normals Scand. J. Clin. Invest. 18:16-20. Bergstrom, J., and E. Hultman. 1967 A study of the glycogen metabolism during exercise in man. Scand J. Clin. Invest. 19:218-228. Coggan, A.R., and E.F. Coyle. 1987 Reversal of fatigue during prolonged exercise by carbohydrate infusion oringestion. J. Appl. Physiol. 63:2388-2395. Coggan, A.R., and E.F. Coyle. 1989 Metabolism and performance following carbohydrate ingestion late in exercise. Med. Sci. Sports Exercise 21:59-65. Coyle, E.F., J.M. Hagberg, B.F. Hurley, W.H. Martin, A.A. Ehsani, and J.O. Holloszy. 1983 Carbohydrate feeding during prolonged strenuous exercise can delay fatigue. J. Appl. Physiol. 55:230-235.
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FLUID REPLACEMENT AND HEAT STRESS Coyle, E.F., A.R. Coggan, M.K. Hemmert, and J.L. Ivy. 1986 Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. J. Appl. Physiol. 61:165-172. Foster, C., D.L. Costill, and W.J. Fink. 1980 Gastric emptying characteristics of glucose and glucose polymer solutions Res. Q. Exercise Sport 51:299-305. Gollnick, P.D., B. Pernow, B. Essen, E. Jansson, and B. Saltin. 1981 Availability of glycogen and plasma FFA for substrate utilization in leg muscle of man during exercise. Clin. Physiol. 1:27-42. Hermansen, L., E. Hultman, and B. Saltin. 1967 Muscle glycogen during prolonged severe exercise. Acta Physiol. Scand. 71:129-139. Ivy, J.L., W. Miller, V. Dover, L.G. Goodyear, W.M. Sherman, S. Farrel, and H. Williams. 1983 Endurance improved by ingestion of a glucose polymer supplement. Med. Sci. Sports Exercise 15:466-471. Pallikarakis, N., B. Jandrain, F. Pirnay, F. Mosora, M. Lacroix, A.S. Luyckx, and P.J. Lefevre. 1986 Remarkable metabolic availability of oral glucose during long-duration exercise in humans. J. Appl. Physiol. 60:1035-1042. Pirnay, F., J.M. Crielaard, N. Pallikarakis, M. Lacroix, F. Mosora, G. Krzentowski, A.S. Luyckx, 1982 and P.J. Lefebvre. Fate of exogenous glucose during exercise of different intensities in humans. J. Appl. Physiol. 53:1620-1624. Wahren, J. 1970 Human forearm muscle metabolism during exercise. IV. Glucose uptake at different work intensities. Scand. J. Clin. Lab. Invest. 25:129-135.
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