Optimization of Glycogen Stores
Chapter 7 Optimization of Glycogen Stores JOHN L. IVY Introduction The importance of carbohydrates as a fuel source during physical activity has been recognized for many years (Krogh & Lindhard 1920; Levine et al. 1924; Dill et al. 1932). Krogh and Lindhard (1920) reported that subjects placed on a high fat diet complained of feeling tired and had difﬁculty performing a standardized 2-h exercise protocol on a cycle ergometer. However, 3 days on a high carbohydrate diet relieved their symptoms of tiredness and the subjects were able to complete the 2-h exercise task without undue stress. Similarly, Christensen and Hansen (1939a, 1939b) found that the capacity for prolonged exercise was three times greater after 3–7 days on a high carbohydrate diet as opposed to a high fatprotein diet. They also reported that exhaustion was accompanied by hypoglycaemia, and that ingestion of a carbohydrate supplement during recovery rapidly returned the blood glucose concentration back to normal and allowed considerable additional exercise to be performed. These results were in agreement with the earlier observations of Levine et al. (1924), who found that blood glucose levels of runners fell to very low levels during a marathon. They also noted that in conjunction with this hypoglycaemic state, the participants were physically fatigued and displayed neuroglucopenia symptoms such as muscular twitching and disorientation. Based on these observations, it was generally accepted that hypoglycaemia resulting from liver glycogen depletion was responsible for fatigue dur- ing prolonged strenuous exercise. However, this view would be modiﬁed as technical advances allowed the direct investigation of muscle metabolism during and following prolonged strenuous exercise. Based on the ﬁndings of several Scandinavian research groups, it became apparent that there is an increased reliance on muscle glycogen as a fuel source as exercise intensity increases, and that perception of fatigue during prolonged strenuous exercise parallels the declining muscle glycogen stores. It was also found that aerobic endurance is directly related to the initial muscle glycogen stores, and that strenuous exercise could not be maintained once these stores are depleted (Hermansen et al. 1965; Ahlborg et al. 1967a; Bergström et al. 1967; Hultman 1967). The amount of glycogen stored in skeletal muscle, however, is limited. If muscle glycogen was the only fuel source available, it could be completely depleted within 90 min of moderate intensity exercise. Therefore, because of the limited availability of muscle glycogen and its importance during prolonged strenuous exercise, methods for increasing its concentration above normal prior to exercise and for its rapid restoration after exercise have been extensively investigated. This chapter discusses our current understanding of the regulation of muscle glycogen synthesis, the effect of diet and exercise on the muscle glycogen concentration prior to exercise, and methods for muscle glycogen restoration immediately after exercise. The chapter concludes with recommendations for increasing 97 98 nutrition and exercise and maintaining muscle glycogen stores for competition and training. Regulation of muscle glycogen synthesis Figure 7.1 illustrates the metabolic reactions controlling glycogen synthesis. Upon crossing the sarcolemma, glucose is rapidly converted to glucose-6-phosphate (G6P) by the enzyme hexokinase. The G6P is then converted to glucose-1phosphate (G1P) by phosphoglucomutase. Next, uridine triphosphate and G1P are combined to form uridine diphosphate (UDP)-glucose, which serves as a glycosyl carrier. The glucose attached to the UDP-glucose is then transferred to the terminal glucose residue at the non-reducing end of a glucan chain to form an a(1 Æ 4) glycosidic linkage. This reaction is catalysed by the enzyme glycogen synthase. The initial formation of the glucan chain, however, is controlled by the protein glycogenin, which is a UDP-glucoserequiring glucosyltransferase. The ﬁrst step involves the covalent attachment of glucose to a single tyrosine residue on glycogenin. This reaction is brought about autocatalytically by glycogenin itself. The next step is the extension of the glucan chain which involves the sequential addition of up to seven further glucosyl residues. The glucan primer is elongated by glycogen synthase, but only when glycogenin and glycogen synthase are complexed together. Finally, amylo (1,4 Æ 1,6) transglycosylase catalyses the transfer of a terminal oligosaccharide fragment of six or seven glucosyl residues from the end of the glucan chain to the 6-hydroxyl group of a glucose residue of the same or another chain. This occurs in such a manner as to form an a(1 Æ 6) linkage and thus create a compact molecular structure. The synthesis of the glycogen molecule is terminated by the dissociation of glycogen synthase from glycogenin (Smythe & Cohen 1991; Alonso et al. 1995). Following its depletion by exercise, muscle glycogen synthesis occurs in a biphasic manner (Bergström & Hultman 1967b; Piehl 1974; Ivy 1977; Maehlum et al. 1977; Price et al. 1994). Initially, there is a rapid synthesis of muscle glycogen that does not require the presence of insulin (Ivy 1977; Maehlum et al. 1977; Price et al. 1994). In normal humans, the rate of synthesis during this insulin-independent phase has been found to range between 12 and 30 mmol · g–1 wet weight · h–1 and to last for about 45–60 min. The second phase is insulin dependent (Ivy 1977; Maehlum et al. 1977; Price et al. 1994) and in the absence of carbohydrate supplementation occurs at a rate that is approximately seven- to 10-fold slower than that of the rapid phase (Price et al. 1994). If supplemented immediately after exercise with carbohydrates, however, the rate of synthesis during the slow phase can be increased several-fold, and if supplementation persists, the muscle glycogen level can be increased above – Hexokinase Glucose Glucose-6-phosphate Phosphoglucomutase + Glucose transporter Glycogen synthase D + glycogenin Glucose-1-phosphate UDP-glucose I pyrophosphorylase UDP-glucose Glycogen Fig. 7.1 The metabolic reactions and the enzymes controlling the reactions that are responsible for the synthesis of muscle glycogen. Enzymes are in italic. See text for details. optimization of glycogen stores normal (Bergström et al. 1967). This elevation in muscle glycogen above normal is referred to as glycogen supercompensation. Interestingly, the effectiveness of the carbohydrate supplement to speed muscle glycogen recovery during the slow phase is directly related to the plasma insulin response to the supplement (Ivy 1991). Rapid phase of glycogen synthesis after exercise After exercise that is of sufﬁcient intensity and duration to deplete the muscle glycogen stores, the activity of glycogen synthase is increased. Glycogen synthase is the rate-limiting enzyme in the glycogen synthesis pathway. Its activity is strongly inﬂuenced by the muscle glycogen concentration (Danforth 1965; Bergström et al. 1972; Adolfsson 1973). Generally, the percentage of glycogen synthase in its active I-form is inversely related to the muscle glycogen concentration. That is, as the muscle glycogen concentration declines the percentage of glycogen synthase in the I-form increases. Conversely, as the glycogen concentration increases the percentage of glycogen synthase in its inactive D-form increases. An exercise-induced increase in glycogen synthase activity can catalyse the rapid restoration of glycogen only if adequate substrate is available. Thus, an additional factor that makes possible the rapid increase in muscle glycogen after exercise is a protracted increase in the permeability of the muscle cell membrane to glucose (Holloszy & Narahara 1965; Ivy & Holloszy 1981; Richter et al. 1982). The increase in glucose transport induced by muscle contractile activity, however, reverses rapidly in the absence of insulin, with most of the effect lost within several hours (Cartee et al. 1989). This rapid decline in muscle glucose transport appears to be inversely related to the muscle glycogen concentration (Cartee et al. 1989; Richter et al. 1984). Thus, the increase in membrane permeability to glucose, together with the activation of glycogen synthase, allows for an initial rapid insulin-independent resynthesis of muscle glycogen following exercise. 99 Slow phase of muscle glycogen synthesis after exercise After the direct, insulin-independent effect of exercise on glucose transport subsides, it is rapidly replaced by a marked increase in the sensitivity of muscle glucose transport and glycogen synthesis to insulin (Garetto et al. 1984; Richter et al. 1984; Cartee et al. 1989). The magnitude of the increased insulin sensitivity induced by exercise can be extremely high, and result in muscle glucose uptake and glycogen synthesis with insulin concentrations that normally have no detectable effect on either process (Cartee et al. 1989; Price et al. 1994). Furthermore, this increase in sensitivity can be sustained for a very long period of time, and does not appear to reverse completely until glycogen supercompensation has occurred (Garetto et al. 1984; Cartee et al. 1989). These ﬁndings therefore suggest that an increase in muscle insulin sensitivity is a primary component of the slow phase of glycogen synthesis. Although the percentage of glycogen synthase in the I-form may be as high as 80% immediately after exercise-induced glycogen depletion, as glycogen levels are normalized the percentage of glycogen synthase I decreases back to the preexercise level or lower in a negative feedback manner (Danforth 1965; Bergström et al. 1972; Adolfsson 1973; Terjung et al. 1974). However, during the slow synthesis phase of glycogen, glycogen synthase appears to be transformed into an intermediate form which has a depressed activity ratio, but enhanced sensitivity to activation by G6P (Kochan et al. 1981). Thus, a second important component of the slow phase of glycogen synthase appears to be an increase in the sensitivity of glycogen synthase to G6P. Another possible mechanism that might contribute to an increase in glycogen synthesis during the slow insulin-dependent phase of recovery is an increase in GLUT-4 concentration. Recently, Ren et al. (1994) reported an increase in GLUT-4 protein in skeletal muscle of rats after a single prolonged exercise session. The increase in GLUT-4 protein was accompanied by a propor- 100 nutrition and exercise tional increase in insulin-stimulated glucose transport and glycogen synthesis. Ren et al. (1994) concluded that a rapid increase in GLUT-4 expression is an early adaptation of muscle to exercise to enhance replenishment of muscle glycogen stores. Effect of carbohydrate supplementation It is evident that the exercise-induced increase in muscle permeability to glucose, insulin sensitivity, and glycogen synthase activity are, together, not sufﬁcient to result in the rapid repletion and supercompensation of the glycogen stores, since only a small increase in muscle glycogen occurs following exercise in the absence of carbohydrate feeding (Bergström et al. 1967; Bergström & Hultman 1967b; Maehlum & Hermansen 1978; Ivy et al. 1988b). Probable factors that prevent the rapid repletion of muscle glycogen in the fasted state are a depressed circulating insulin concentration, and an increase in plasma free fatty acids and fatty acid oxidation by muscle (Ivy et al. 1988a). These conditions are actually advantageous during fasting as they serve to slow muscle glucose uptake and conserve blood glucose for use by the nervous system until sufﬁcient carbohydrate is available. Carbohydrate feeding after exercise probably stimulates glycogen synthesis by increasing arterial plasma insulin and glucose concentrations. The increase in circulating insulin not only functions to increase muscle glucose uptake, but also functions to keep glycogen synthase activity elevated. It may also play a role in increasing GLUT-4 expression which would facilitate insulin-stimulated glucose transport (Ren et al. 1993; Hansen et al. 1995; Brozinick et al. 1996). With increasing plasma glucose concentration, glucose transport increases regardless of the level of muscle permeability to glucose (Nesher et al. 1985). Therefore, the increase in arterial plasma glucose concentration functions to increase the rate of glucose transport, further increasing substrate availability, and providing sufﬁcient G6P for activation of glycogen synthase. It has also been demonstrated that the insulin response to carbohydrate supplementation increases over subsequent days while glucose tolerance remains the same or actually improves (Ivy et al. 1985). This increase in insulin response to carbohydrate loading is thought to be the result of an increase in the pancreatic response to glucose (Szanto & Yudkin 1969). Since insulin is required for normal glycogen repletion and supercompensation, it is possible that the hyperinsulinaemic response following several days of high carbohydrate consumption is responsible for the increased sensitivity of glycogen synthase to G6P. The elevated plasma insulin may also serve to increase the rate of muscle glucose transport, thus increasing the availability of glucose to glycogen synthase, as well as possibly increasing the intracellular concentration of G6P. Hexokinase activity in muscle is also increased during subsequent days of carbohydrate loading (Ivy et al. 1983). This too could be of functional signiﬁcance since an increase in hexokinase activity would prevent the rate-limiting step in glucose uptake from shifting from transport to glucose phosphorylation as the G6P concentration increased. Glycogen supercompensation regimens The discovery by Bergström and Hultman (1967b) that a high carbohydrate diet following the depletion of muscle glycogen by exercise would result in an above normal muscle glycogen concentration led to a series of studies to identify the regimen of exercise and diet that would best supercompensate the muscle glycogen stores. Bergström et al. (1967) had subjects exercise to exhaustion to deplete their muscle glycogen stores. Six of the subjects then received a high fat–protein diet for 3 days. This was followed by another exhaustive exercise bout and 3 days of a high carbohydrate diet. The remaining three subjects followed the same protocol as above except the order of administration of the diets was reversed. When the high carbohydrate diet followed the high fat–protein optimization of glycogen stores diet, the muscle glycogen concentration was 205.5 mmol · g–1 wet weight. This represented a 100% increase above the initial muscle glycogen concentration. When the high carbohydrate diet preceded the high fat–protein diet, the muscle glycogen concentration was 183.9 mmol · g–1 wet weight following the high carbohydrate diet. It was suggested that a period of carbohydrate-free diet further stimulated glycogen synthesis when carbohydrates were given following exercise. Based on this study and several similar studies (Ahlborg et al. 1967b; Bergström & Hultman 1967a), it was recommended that the best way to glycogen supercompensate was, ﬁrst, to deplete the muscle glycogen stores with an exhaustive exercise bout; second, to eat a carbohydrate-free diet for 3 days; third, to deplete the glycogen stores once more with an exhaustive exercise bout; and fourth, to consume a high carbohydrate diet for 3 days. Because of the strenuous nature of this regimen, many athletes have found it impractical, even though it has been used successfully by very elite performers. The 3 days of low carbohydrate diet may cause hypoglycaemia, irritability and chronic fatigue. The two bouts of exhaustive exercise prior to competition may result in injury, soreness and fatigue, and prevents a proper taper before competition. To address this problem, Sherman et al. (1981) studied three types of muscle glycogen supercompensation regimens on six trained runners. Over a 6-day period, . the subjects ran at approximately 75% of Vo2max. for 90 min, 40 min, 40 min, 20 min, 20 min, and rested, respectively. During each taper, the subjects received one of three dietary treatments: 1 a mixed diet composed of 50% carbohydrate (control diet), 2 a low carbohydrate diet (25% carbohydrate) for the ﬁrst 3 days and a high carbohydrate diet (70% carbohydrate) for the last 3 days (classic diet), and 3 a mixed diet (50% carbohydrate) for the ﬁrst 3 days and a high carbohydrate diet (70% carbohydrate) the last 3 days (modiﬁed diet). Muscle biopsies were obtained on the morning of the 4th and 7th days of each trial. During the 101 control treatment, muscle glycogen concentrations of the gastrocnemius were 135 and 163 mmol · g–1 wet weight on days 4 and 7, respectively. During the classic treatment, the corresponding muscle glycogen concentrations were 80 and 210 mmol · g–1 wet weight, and during the modiﬁed treatment they were 135 and 204 mmol · g–1 wet weight (Fig. 7.2). These results suggest that a normal training taper in conjunction with a moderate carbohydrate–high carbohydrate diet sequence is as effective as the classic glycogen supercompensation regimen for highly trained endurance athletes. Enhancement of glycogen synthesis after exercise Long-term recovery The amount of carbohydrate consumed has a signiﬁcant effect on the rate of glycogen storage after exercise. Unless sufﬁcient carbohydrate is ingested, muscle glycogen will not be normalized on a day-to-day basis between training bouts, nor will efforts to supercompensate muscle glycogen stores be successful. In general, with an increase in carbohydrate ingestion there is an increase in muscle glycogen storage. Costill et al. (1981) reported that consuming 150–650 g carbohydrate · day–1 resulted in a proportionately greater muscle glycogen synthesis during the initial 24 h after exercise, and that consumption of more than 600 g carbohydrate · day–1 was of no additional beneﬁt (Fig. 7.3). It has also been demonstrated that when the carbohydrate concentration of the diet was inadequate, successive days of intense, prolonged exercise resulted in a gradual reduction in the muscle glycogen stores and a deterioration in performance (Costill et al. 1971; Sherman et al. 1993). For example, Sherman et al. (1993) fed endurance athletes either 5 or 10 g carbohydrate · kg–1 · day–1 over 7 days of controlled training. The lower carbohydrate diet contained 42% of energy from carbohydrate and the higher carbohydrate diet contained 84% of energy from carbohydrate. Both diet and exercise were controlled during the 7 days prior to the 102 nutrition and exercise 220 Glycogen (µmol.g–1 wet wt) Exercise 165 110 Exercise Exercise 55 0 0 1 2 3 4 5 6 Regimen (days) Fig. 7.2 A comparison of the classic Bergström et al. (1967) glycogen supercompensation method and a modiﬁcation of that method by Sherman et al. (1983). The classic method (—) consisted of depleting the glycogen stores with an exhaustive exercise bout, followed by 3 days on a low carbohydrate diet. This was followed with another glycogen-depleting exercise and 3 days on a high carbohydrate diet. The modiﬁcations by Sherman (- - - -) included a hard exercise bout that was followed by 6 days of exercise tapering. During the ﬁrst 3 days of the taper, a mixed diet consisting of 50% carbohydrates was consumed. During the last 3 days, a high carbohydrate diet was consumed. The two values for the classic regimen on day 3 represent before and after an exhaustive exercise bout. 䊐, low carbohydrate diet; 䉬, mixed diet; 䊉, high carbohydrate diet. From Sherman et al. (1983), with permission. Glycogen storage (µmol.g –1 wet wt.h –1) 80 60 40 20 0 –20 0 200 400 600 Consumed carbohydrate (g.day–1) Fig. 7.3 The relationship between the amount of carbohydrate consumed and the rate of muscle glycogen storage during a 24-h period after glycogen depletion by exercise. From Costill et al. (1981), with permission. experimental period to ensure that subjects started with similar muscle glycogen levels. The lower carbohydrate diet produced a signiﬁcant 30% decline in muscle glycogen by day 5 of training, which was then maintained through day 7. However, there was no decline in muscle glycogen during the 7 days of training when the athletes consumed the higher carbohydrate diet. The type of carbohydrate consumed also appears to have an effect on the rate of glycogen resynthesis following exercise. Costill et al. (1971) fed glycogen-depleted runners a starch or glucose diet (650 g carbohydrate · day–1) during the 2 days following depletion. During the ﬁrst 24 h there was no difference in the synthesis of muscle glycogen between the two diets, but after the 2nd day, the starch diet resulted in a signiﬁcantly greater glycogen synthesis than the glucose diet. A difference in glycogen storage between simple and complex carbohydrates, however, was not demonstrated by Roberts et al. (1988). Following glycogen-depleting exercise, their subjects were fed diets consisting of either 88% simple and 12% complex carbohydrates or 15% simple and 85% complex carbohydrates. After 3 days of recovery, it was found that the two diets had produced equivalent increases in muscle glycogen storage. The difference between studies is not immediately clear, but may be due to differences in the glycaemic indexes of the different carbohydrates provided. The only study that appears to have investigated the impact of the glycaemic index of carbohydrate on muscle glycogen storage after optimization of glycogen stores Short-term recovery While procedures for increasing muscle glycogen above normal levels in preparation for competition and maintaining normal glycogen levels on a day-to-day basis have been deﬁned, these procedures do not address the problem of athletic competitions that require the rapid resynthesis of muscle glycogen within hours. Although it is unlikely that muscle glycogen stores could be completely resynthesized within a few hours by nutritional supplementation alone, it would be of beneﬁt to the athlete if supplementation procedures which maximized the rate of muscle glycogen storage after exercise were deﬁned. Factors that inﬂuence the rate of muscle glycogen storage immediately following exercise are the timing, amount and type of carbohydrate supplement consumed, the frequency of feeding, and the type of exercise performed. timing of carbohydrate consumption after exercise The time elapsed between competition or a prolonged exercise bout and the consumption of a carbohydrate supplement will critically inﬂuence the rate of muscle glycogen resynthesis (Ivy et al. 1988a). When carbohydrate supplements are provided immediately after exercise, they generally result in a rate of glycogen resynthesis of between 5 and 6 mmol · g–1 wet weight · h–1 (Maehlum et al. 1977; Blom et al. 1987; Ivy et al. 1988a, 1988b). This rate is maintained for approximately 2 h and then declines by approximately 50% over the next 2 h as the blood glucose and insulin levels decline to after-exercise levels (Ivy et al. 1988a). If the supplement is delayed for 2 h, the rate of glycogen resynthesis during the 2 h immediately after consumption ranges between 3 and 4 mmol · g–1 wet weight · h–1, or about 50% as fast as when the supplement is provided immediately after exercise (Fig. 7.4). This lower rate of glycogen resynthesis occurs despite normal increases in blood glucose and insulin levels. It appears that when the carbohydrate supplement is delayed for several hours after exercise, the muscle becomes insulin resistant, reducing the rate of muscle glucose uptake and glycogen 20 Glycogen synthesis (µmol.g–1 wet wt) exercise was conducted by Burke et al. (1993). Subjects were exercised to deplete the muscle glycogen stores on two separate occasions and provided a diet composed of carbohydrate with a high glycaemic index on one occasion, and a diet composed of carbohydrate with a low glycaemic index on the other. Total carbohydrate intake over the 24-h recovery period was 10 g · kg–1 body weight, evenly distributed between meals eaten at 0, 4, 8 and 21 h after exercise. The increase in muscle glycogen averaged 106 mmol · g–1 wet weight for the high glycaemic index carbohydrate and 71.5 mmol · g–1 wet weight for the low glycaemic index. The difference in glycogen storage was signiﬁcant. This ﬁnding suggests that the increase in muscle glycogen content during long-term recovery is affected by the amount and glycaemic index of the carbohydrate consumed. 103 15 10 5 0 0–120 120–240 Time after exercise (min) Fig. 7.4 Muscle glycogen storage during the ﬁrst 2 h and second 2 h of recovery from exercise. The open bar represents the glycogen storage when the carbohydrate supplement was provided immediately after exercise, and the black bar represents the glycogen storage when the supplement was delayed until 2 h into recovery. The carbohydrate supplement consisted of a 23% solution of glucose polymers (2 g · kg–1 body weight). From Ivy et al. (1988a), with permission. 104 nutrition and exercise resynthesis. Once developed, this state of insulin resistance persists for several hours. Providing a carbohydrate supplement soon after exercise therefore appears to beneﬁt the muscle glycogen recovery process by preventing the development of muscle insulin resistance. Furthermore, during the time between the end of exercise and the consumption of a carbohydrate supplement, there is very little muscle glycogen resynthesis (approximately 1–2 mmol · g–1 wet weight · h–1) (Ivy et al. 1988a). Therefore, providing a carbohydrate supplement soon after exercise has the added beneﬁt of starting the muscle glycogen recovery process immediately. frequency and amount of carbohydrate consumption after exercise The frequency of carbohydrate supplementation as well as the amount of carbohydrate in each supplement is also of importance in the regulation of muscle glycogen resynthesis. When an adequate carbohydrate supplement is provided immediately after exercise, its effect on muscle glycogen recovery eventually declines as blood glucose and insulin levels decline. However, Blom et al. (1987) reported that providing a carbohydrate supplement immediately after exercise and at 2-h intervals for the next 4 h maintained an elevated blood glucose level and a rapid rate of muscle glycogen resynthesis during a 6-h recovery period. Blom et al. (1987) also found that a critical amount of carbohydrate must be consumed if the rate of muscle glycogen resynthesis was to be maximized. When carbohydrate supplements of 0.7 or 1.4 g glucose · kg–1 body weight were provided at 2-h intervals, the rate of glycogen storage did not differ between treatments and averaged 5.7 mmol · g–1 wet weight · h–1. However, when Blom et al. (1987) provided 0.35 g glucose · kg–1 body weight at 2-h intervals, the rate of muscle glycogen resynthesis was reduced by 50%. To better evaluate the critical level of carbohydrate supplementation required for maximal glycogen resynthesis, we tested the effects of supplements with different concentrations of carbohydrate during 4 h of recovery from exercise (Fig. 7.5). Very little muscle glycogen resynthesis was found when carbohydrate was withheld from the subjects (approximately 0.6 mmol · g–1 wet weight · h–1). With increasing concentration of carbohydrate supplementation, however, the rate of muscle glycogen resynthesis increased in a curvilinear pattern and then plateaued at a rate of 5.5 mmol · g–1 wet weight · h–1 as the carbohydrate concentration Protein + CHO 7 Increase in muscle glycogen concentration (µmol. g–1. h–1) 6 5 4 3 2 1 0 0 0.5 1.0 1.5 2.0 Carbohydrate supplement (g.kg–1 body wt) 2.5 3.0 Fig. 7.5 The average rate of muscle glycogen resynthesis during a 4-h exercise recovery period after oral consumption of different concentrations of carbohydrate (CHO) from a liquid supplement (ª 21% wt/vol). Supplements were provided immediately after exercise and 2 h after exercise. Protein + CHO represents the average muscle glycogen resynthesis rate when 1.5 g CHO · kg–1 body weight plus 0.53 g protein · kg–1 body weight (milk and whey protein isolate mixture, 7.6% wt/vol) was provided. optimization of glycogen stores approached 1–1.5 g · kg–1 body weight. These results imply that when carbohydrate supplements are provided at 2-h intervals in amounts below 1 g · kg–1 body weight, the rate of muscle glycogen resynthesis will be submaximal. The reduced rate of resynthesis is probably due to the inability of a small carbohydrate supplement to adequately increase and maintain blood glucose and insulin levels for a 2-h interval, as smaller supplements taken more frequently have been found to be adequate (Doyle et al. 1993). The reason for similar glycogen resynthesis rates when carbohydrate supplements exceed 1 g · kg–1 body weight was not immediately clear. Estimates of gastric emptying rates, based on the research of Hunt et al. (1985), suggest that carbohydrate available to the muscle was far in excess of the amount actually converted to glycogen. This would indicate that under conditions of high carbohydrate supplementation, the ratelimiting step in glycogen resynthesis is either glucose transport or the processing of glucose through the glycogen synthetic pathway. To test this hypothesis, Reed et al. (1989) continuously infused glycogen-depleted subjects with 3 g glucose · kg–1 body weight during the ﬁrst 3.75 h of a 4-h exercise recovery period. The rate of muscle glycogen resynthesis during infusion was then compared with that which occurred when a liquid supplement containing 1.5 g glucose · kg–1 body weight was consumed immediately after and 2 h after exercise. During infusion, blood glucose increased to 10 mm, whereas the blood glucose level only reached 6 mm when the liquid glucose supplement was consumed orally. Despite this large difference in blood glucose, the rates of muscle glycogen resynthesis were virtually identical at the end of the recovery periods. The results of Reed et al. (1989) therefore support the hypothesis that glycogen resynthesis is not limited by glucose availability when adequate carbohydrate is consumed. Prior research studies employing glucose infusion (Ahlborg et al. 1967b; Bergström & Hultman 1967c; Roch-Norlund et al. 1972), however, have generally demonstrated greater rates of glycogen synthesis than those reported by Reed et al. 105 (1989). Possibly accounting for the difference in synthesis rates are the different rates of glucose infusion. The rates of glucose infusion in the earlier studies were much faster and plasma glucose concentrations two to three times higher than those reported by Reed et al. (1989). It is likely that plasma insulin concentrations in the earlier studies were greater as well, although these results were not reported. It was of interest to note that in the study by Reed et al. (1989) the plasma insulin response during the infusion treatment was similar to that produced by the liquid supplement, and therefore could account for the similar rates of glycogen storage for these two treatments. The blood insulin concentration plays a major role in determining the rate of muscle glycogen storage. Insulin stimulates both muscle glucose transport and activation of glycogen synthase. The results raised the possibility that increasing the insulin response to a carbohydrate supplement could increase the rate of muscle glucose uptake and glycogen storage. protein plus carbohydrate Certain amino acids are effective secretagogues of insulin and have been found to synergistically increase the blood insulin response to a carbohydrate load when administered in combination (Floyd et al. 1966; Fajans et al. 1967). Of the 20 amino acids normally found in protein, the most effective insulin secretagogue is arginine (Fajans et al. 1967). When infused with carbohydrate, arginine has been found to increase the insulin response ﬁvefold above that produced by the carbohydrate or arginine alone. However, we have found the use of amino acids to be impractical when added to a carbohydrate supplement because they produce many unwanted sideeffects such as mild borborygmus and diarrhoea. Protein meals and supplements also have been found to enhance the insulin response to a carbohydrate load and do not produce the unwanted side-effects of the amino acids (Rabinowitz et al. 1966; Pallota & Kennedy 1968; Spiller et al. 1987). For example, Spiller et al. (1987) demonstrated an 106 nutrition and exercise Increase in muscle glycogen concentration (µmol.kg–1 wet wt) increased blood insulin response and decreased blood glucose response with the addition of protein to a 58 g carbohydrate supplement. The insulin response was found to be directly proportional and the glucose response inversely proportional to the protein content of the carbohydrate–protein supplement. No adverse side-effects were reported. We therefore investigated the effects of a carbohydrate–protein supplement on muscle glycogen resynthesis after exercise (Zawadzki et al. 1992). The supplements tested consisted of 112 g carbohydrate or 112 g carbohydrate plus 40.7 g protein (21% wt/vol mixture). The supplements were administered immediately after exercise and 2 h after exercise. It was found that the combination of carbohydrate plus protein resulted in a synergistic insulin response. In conjunction with the greater insulin response was a signiﬁcantly lower blood glucose response and a 38% faster rate of muscle glycogen storage compared with carbohydrate supplementation alone. Rates of muscle glycogen resynthesis averaged 7.1 mmol · g–1 wet weight · h–1 for the carbohydrate–protein treatment and 5.0 mmol · g–1 wet weight · h–1 for the carbohydrate treatment during the 4-h recovery period (see Fig. 7.5). It was also found that carbohydrate oxidation rates and blood lactate concentrations for the carbohy- drate–protein and carbohydrate treatments were similar. These results suggested that the increased rate of muscle glycogen resynthesis during the carbohydrate–protein treatment was the result of an increased clearance of glucose by the muscle due to the increased blood insulin response. Since the carbohydrate–protein supplement was palatable and there were no unwanted side-effects, it would appear to be a viable supplement for postexercise glycogen recovery. differences in simple carbohydrates The effect of supplements composed of predominately glucose, fructose and sucrose have also been investigated (Blom et al. 1987). Glucose and fructose are metabolized differently. They have different gastric emptying rates and are absorbed into the blood at different rates. Furthermore, the insulin response to a glucose supplement is generally much greater than that of a fructose supplement. Blom et al. (1987) found that ingestion of glucose or sucrose was twice as effective as fructose for restoration of muscle glycogen (Fig. 7.6). They suggested that the differences between the glucose and fructose supplementations were the result of the way the body metabolized these 40 Glucose Sucrose 20 Fructose 0 0 1 2 3 Time (h) 4 5 6 Fig. 7.6 Increases in muscle glycogen concentration when glucose, fructose and sucrose are provided in amounts of 0.7 g · kg–1 body weight immediately after exercise and at 2-h intervals. From Blom et al. (1987), with permission. optimization of glycogen stores sugars. Fructose metabolism takes place predominantly in the liver (Zakin et al. 1969), whereas the majority of glucose appears to bypass the liver and be stored or oxidized by the muscle (Maehlum et al. 1978). When infused, fructose has been found to result in a four times greater liver glycogen storage than glucose (Nilsson & Hultman 1974). On the other hand, a considerably higher glycogen storage rate has been demonstrated in skeletal muscle after glucose than after fructose infusion (Bergström & Hultman 1967c). The similar rates of glycogen storage for the sucrose and glucose supplements could not be accounted for by Blom et al. (1987). Sucrose contains equimolar amounts of glucose and fructose. If muscle glycogen storage was chieﬂy dependent on the glucose moiety of the disaccharide, one should expect a lower rate of glycogen storage from sucrose than from a similar amount of glucose. One possible explanation provided by Blom et al. (1987) was that fructose, by virtue of its rapid metabolism in the liver, compared with that of glucose, inhibits the postexercise hepatic glucose uptake, thereby rendering a large proportion of absorbed glucose available for muscle glycogen resynthesis. solid vs. liquid supplements The form in which the carbohydrate is provided has also been investigated. Keizer et al. (1986) found that providing approximately 300 g of carbohydrate in either liquid or solid form after exercise resulted in a glycogen storage rate of approximately 5 mmol · g–1 wet weight · h–1 over the ﬁrst 5 h of recovery. However, these solid feedings contained a substantial amount of fat and protein that is typically not found in liquid supplements. Therefore, Reed et al. (1989) compared the postexercise glycogen storage rates following liquid and solid carbohydrate supplements of similar compositions. Again there were no differences noted between the two treatments. The average glycogen storage rates for the liquid and solid supplements were 5.1 and 5.5 mmol · g–1 wet weight · h–1, respectively. 107 influence of type of exercise As previously indicated, during prolonged bouts of exercise in which muscle and liver glycogen concentrations are reduced and hypoglycaemia results, muscle glycogen synthesis is typically 5–6 mmol · g–1 wet weight · h–1, provided an adequate carbohydrate supplement is consumed. However, if the exercise rapidly reduces the muscle glycogen concentration, resulting in elevated blood and muscle lactate, synthesis of glycogen can be very rapid even in the absence of a carbohydrate supplement. Hermansen and Vaage (1977) depleted the muscle glycogen levels of their subjects by multiple 1-min maximal exercise bouts on a cycle ergometer. During the ﬁrst 30 min of recovery, the rate of muscle glycogen synthesis averaged 33.6 mmol · g–1 wet weight · h–1. The increase in muscle glycogen was found to parallel the decline in muscle lactate, which had increased to 26.4 mmol · g–1 wet weight after the last exercise bout. MacDougall et al. (1977) also found a relatively rapid rate of storage after muscle glycogen depletion when subjects per. formed 1-min cycling sprints at 150% of Vo2max. to exhaustion. The difference in storage rates following prolonged exercise, as opposed to highintensity exercise, can probably be explained by the availability of substrate for muscle glycogen synthesis. With multiple high-intensity sprints, glycogen depletion is accompanied by hyperglycaemia and elevated blood and muscle lactate concentrations, which can be used immediately as substrate for glycogen synthesis. By contrast, prolonged sustained exercise severely reduces the endogenous precursors of muscle glycogen, thereby requiring an exogenous carbohydrate source for rapid muscle glycogen synthesis. Exercise that results in muscle damage also affects muscle glycogen synthesis. Sherman et al. (1983) found that after a marathon, restoration of muscle glycogen was delayed and that this delay was related to muscle damage caused by the run (Hikida et al. 1983; Sherman et al. 1983). Eccentric exercise which involves the forced lengthening of active muscles and the transfer of external power from the environment to the muscle 108 nutrition and exercise causes severe muscle damage. O’Reilly et al. (1987) reported that muscle glycogen stores reduced by eccentric exercise were still signiﬁcantly below normal levels after 10 days of recovery. Costill et al. (1990) also found that the rate of muscle glycogen resynthesis was signiﬁcantly reduced following glycogen depletion by exercise that incorporated a substantial eccentric component, and that the reduced rate of resynthesis was associated with muscle damage. More recently, Asp et al. (1995) demonstrated that muscle damage induced by eccentric exercise resulted in a down regulation of GLUT-4 protein that lasted for several days. In addition, it was not until the GLUT-4 protein returned to the preexercise concentration that normal muscle glycogen levels were restored. These results suggest that muscle damage following exercise can limit glucose uptake due to a reduced GLUT-4 protein concentration, and that this limits the restoration of muscle glycogen. Recommendations From a dietary position, the ﬁrst concern of the endurance athlete is that energy consumption and energy expenditure be in balance (Sherman 1995). The endurance athlete may expend 15– 30 MJ · day–1 when training. If consumption is inadequate and not balanced with expenditure, the athlete’s training and competitive abilities will eventually be adversely affected. It is also important that a substantial percentage of the diet consist of carbohydrate. It was suggested by Sherman and Lamb (1988) that the endurance athelete’s diet consists of approximately 65% carbohydrate during strenuous training. However, this percentage can be modiﬁed according to actual energy consumption. What is important is the amount of carbohydrate consumed. Costill et al. (1981) recommended that endurance trained athletes consume approximately 8 g carbohydrate · kg–1 body weight · day–1 to maintain a normal muscle glycogen concentration during training. Similarly, a recommendation of 7 g carbohydrate · kg–1 body weight · day–1 was provided by Sherman (1995). Prior to competition, the muscle and liver glycogen stores should be maximized. For the best results with the least amount of stress, it is recommended that a hard training bout be performed 7 days prior to competition to reduce the muscle glycogen stores. During the next 3 days, training should be of moderate intensity and duration and a well-balanced mixed diet composed of about 45–50% carbohydrate consumed. During the next 3 days, training should be gradually tapered and the carbohydrate content of the diet should be increased to 70%. This should result in muscle glycogen stores similar to that normally produced by the classic glycogen supercompensation regimen, but with much less stress and fatigue. For the rapid replenishment of muscle glycogen stores, one should consume a carbohydrate supplement in excess of 1 g · kg–1 body weight immediately after competition or after a training bout. Continuation of supplementation every 2 h will maintain a maximal rate of storage up to 6 h after exercise; smaller supplements taken more frequently are also effective. Increasing the amount of carbohydrate consumption above 1.0–1.5 g · kg–1 body weight · supplement–1 appears to provide no additional beneﬁt, and may have the adverse effects of causing nausea and diarrhoea. Supplements composed of glucose or glucose polymers are more effective for the replenishment of muscle glycogen stores after exercise than supplements composed of predominantly fructose. However, some fructose is recommended because it is more effective than glucose in the replenishment of liver glycogen. It might also be of beneﬁt to include some protein with the carbohydrate supplement as this will enhance the rate of glycogen resynthesis. Finally, carbohydrates in solid or liquid form can be consumed immediately after exercise with similar results. However, a liquid supplement immediately after exercise is recommended because it is easier to digest and less ﬁlling, and therefore will not tend to adversely affect one’s normal appetite. 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