Carbohydrate Metabolism in Exercise
Chapter 6 Carbohydrate Metabolism in Exercise ERIC HULTMAN AND PAUL L. GREENHAFF Introduction During exercise, the energy demands of muscle contraction will ﬂuctuate enormously. For muscle contraction to occur, chemical energy stored in the form of adenosine triphosphate (ATP) must be converted into mechanical energy at rates appropriate to the needs of the muscle. However, the muscle store of ATP is relatively small and therefore for exercise to continue beyond a few seconds ATP must be resynthesized from phosphocreatine, carbohydrate and fat. It is generally accepted that carbohydrate is the major substrate for ATP resynthesis during intense exercise. The carbohydrate stores of the body are principally located in skeletal muscle and liver, with small amounts also being found in the form of circulating glucose. The amount of energy stored as glycogen amounts to approximately 6000 kJ (1430 kcal) and 1500 kJ (360 kcal) in muscle and liver, respectively, which is very small compared with the body store of triacylglycerol (340 MJ, 81 200 kcal), the alternative fuel for ATP resynthesis. Triacylglycerol is the preferred substrate for energy production in resting muscle and can cover the energy demands of exercise up to 50% of maximal oxygen consumption. At higher exercise intensities, the relative contribution of fat to total energy production falls and carbohydrate oxidation increases, such that carbohydrate is the sole fuel oxidized at the highest exercise intensities. This is due to an increasing recruitment of the glycolytic type II muscle ﬁbres and an activa- tion of glycolytic enzymes when ATP turnover rate is increased. The maximal rate of ATP production from lipid is lower than that of carbohydrate. In addition, the ATP yield per mole of oxygen utilized is lower for lipid compared with carbohydrate. In contrast with lipid, carbohydrate can be metabolized anaerobically via glycolysis. The lactate accumulation that occurs almost instantaneously at the onset of contraction demonstrates that the activation of this pathway is extremely rapid. It should be noted that the anaerobic utilization of carbohydrate will be indispensable during the transition from rest to steady-state exercise and during maximal exercise. Furthermore, the relatively small store of body carbohydrate will limit exercise performance during prolonged intense exercise due to the depletion of muscle and liver glycogen stores. The body store and maximal rates of ATP resynthesis from phosphocreatine, carbohydrate and lipid are shown in Table 6.1. Regulation of muscle carbohydrate utilization during exercise Glycogenolysis is the hydrolysis of muscle glycogen to glucose-1-phosphate, which is transformed to glucose-6-phosphate via a phosphoglucomutase reaction. The glucose-6phosphate formed, together with that derived from the phosphorylation of blood glucose by hexokinase at the muscle cell membrane, enters the glycolytic pathway which is a series of 85 86 nutrition and exercise Table 6.1 The amounts of substrate available and the maximal rates of energy production from phosphocreatine, carbohydrate and lipid in a 70-kg man (estimated muscle mass, 28 kg). ATP, PCr Æ ADP, Cr Muscle glycogen Æ Lactate Muscle glycogen Æ CO2 Liver glycogen Æ CO2 Fatty acids Æ CO2 Amount available (mol) Production rate (mol · min-1) 0.67 6.70* 84 19 4000* 4.40 2.35 0.85–1.14 0.37 0.40 * These pathways of substrate utilization will not be fully utilized during exercise. reactions involved in the degradation of glucose6-phosphate to pyruvate. Glycogenolysis The integrative nature of energy metabolism ensures that the activation of muscle contraction by Ca2+ and the accumulation of the products of ATP and phosphocreatinine (PCr) hydrolysis (ADP, AMP, IMP, NH3 and Pi) act as stimulators of glycogenolysis, and in this way attempt to match the ATP production to the demand. The control of glycogenolysis during muscle contraction is a highly complex mechanism which can no longer be considered to centre only around the degree of Ca2+ induced transformation of less active glycogen phosphorylase b to the more active a form, as is suggested in many textbooks. For some time it has been known that glycogenolysis can proceed at a negligible rate, despite almost total transformation of phosphorylase to the a form; for example, following adrenaline infusion (Chasiotis et al. 1983). Conversely, an increase in glycogenolytic rate has been observed during circulatory occlusion, despite a relatively low mole fraction of the phosphorylase a form (Chasiotis 1983). From this and other related work, it was concluded that inorganic phosphate (Pi) accumulation arising from ATP and PCr hydrolysis played a key role in the regulation of the glycogenolytic activity of phosphorylase a, and by doing so served as a link between the energy demand of the contraction and the rate of carbohydrate utilization (Chasiotis 1983). However, the ﬁndings that high rates of glycogenolysis can occur within 2 s of the onset of muscle contraction in conjunction with only a small increase in Pi and, more recently, that glycogenolysis can proceed at a low rate despite a high phosphorylase a form and Pi concentration, suggest that factors other than the degree of Ca2+ induced phosphorylase transformation and Pi availability are involved in the regulation of glycogenolysis (Ren & Hultman 1989, 1990). Classically, both inosine monophosphate (IMP) and adenosine monophosphate (AMP) have been associated with the regulation of glycogenolysis during exercise (Lowry et al. 1964; Aragon et al. 1980). IMP is thought to exert its effect by increasing the activity of phosphorylase b during contraction (the apparent Km (Michaeli’s constant) of phosphorylase b for IMP is about 1.2 mmol · l–1 intracellular water). AMP has also been shown to increase the activity of phosphorylase b, but it is thought to require an unphysiological accumulation of free AMP to do so (the apparent Km of phosphorylase b for AMP is about 1.0 mmol · l–1 intracellular water). In vitro experiments have demonstrated that AMP can bring about a more marked effect on glycogenolysis by increasing the glycogenolytic activity of phosphorylase a (Lowry et al. 1964). Because 90% or more of the total cell content of AMP may be bound to cell proteins in vivo, it has in the past been questioned whether the increase in free AMP during contraction is of a sufﬁcient magnitude to affect the kinetics of phosphorylase a. More recent work, however, demonstrates that a carbohydrate metabolism in exercise small increase in AMP concentration (10 mmol · l–1) can markedly increase the in vitro activity of phosphorylase a (Ren & Hultman 1990). Furthermore, in vivo evidence demonstrating a close relationship between muscle ATP turnover and glycogen utilization suggests that an exerciseinduced increase in free AMP and inorganic phosphate may be the key regulators of glycogen degradation during muscle contraction (Ren & Hultman 1990). Glycolysis From the preceding discussions it can be seen that the rate of glycogenolysis is determined by the activity of glycogen phosphorylase. However, it is the activity of phosphofructokinase (PFK) that dictates the overall rate of glycolytic ﬂux (Tornheim & Lowenstein 1976). PFK acts as a gate to the ﬂow of hexose units through glycolysis and there is no other enzyme subsequent to PFK that is capable of matching ﬂux rate with the physiological demand for ATP. Stimulation of glycogen phosphorylase by adrenaline and/or exercise results in the accumulation of glucose-6-phosphate demonstrating that PFK is the rate limiting step in the degradation of hexose units to pyruvate (Richter et al. 1986). ATP is known to be the most potent allosteric inhibitor of PFK. The most important activators or deinhibitors of PFK are adenosine diphosphate (ADP), AMP, Pi, fructose-6-phosphate, glucose 1–6 bisphosphate, fructose 1–6 and 2–6 bisphosphates and, under extreme conditions, ammonia. Removal of the ATP-mediated inhibition of PFK during contraction, together with the accumulation of the positive modulators of PFK, is responsible for the increase in ﬂux through the enzyme during exercise and thereby is responsible for matching glycolytic ﬂux with the energy demand of contraction. Hydrogen ion and citrate accumulation during contraction have been suggested to be capable of decreasing the activity of PFK and, thereby, the rate of glycolysis during intense exercise. However, it is now generally accepted that the extent of this inhibition of glycolysis during exer- 87 cise is overcome in the in vivo situation by the accumulation of PFK activators (Spriet et al. 1987). Pyruvate oxidation It has been accepted for some time that the rate limiting step in carbohydrate oxidation is the decarboxylation of pyruvate to acetyl-coenzyme A (CoA), which is controlled by the pyruvate dehydrogenase complex (PDC), and is essentially an irreversible reaction committing pyruvate to entry into the tricarboxylic acid (TCA) cycle and oxidation (Wieland 1983). The PDC is a conglomerate of three enzymes located within the inner mitochondrial membrane. Adding to its complexity, PDC also has two regulatory enzymes: a phosphatase and a kinase which regulate an activation–inactivation cycle. Increased ratios of ATP/ADP, acetyl-CoA/CoA and NADH/NAD+ activate the kinase, resulting in the inactivation of the enzyme. Conversely, decreases in the above ratios and the presence of pyruvate will inactivate the kinase, whilst increases in calcium will activate the phosphatase, together resulting in the activation of PDC. Thus, it can be seen that the increases in calcium and pyruvate availability at the onset of contraction will result in the rapid activation of PDC. These factors, together with the subsequent decrease in the ATP/ADP ratio as contraction continues, will result in continued ﬂux through the reaction (Constantin-Teodosiu et al. 1991). Following decarboxylation of pyruvate by the PDC reaction, acetyl-CoA enters the TCA cycle, resulting in the formation of citrate, in a reaction catalysed by citrate synthase. The rate of ﬂux through the TCA cycle is thought to be regulated by citrate synthase, isocitrate dehydrogenase, and a-ketoglutarate dehydrogenase. The activity of these enzymes is controlled by the mitochondrial ratios of ATP/ADP and NADH/NAD+. Good agreement has been found between the maximal activity of a-ketoglutarate dehydrogenase and ﬂux through PDC and the TCA cycle. The last stage in pyruvate oxidation involves NADH and FADH generated in the TCA cycle 88 nutrition and exercise entering the electron transport chain. In the electron transport chain, NADH and FADH are oxidized and the energy generated is used to rephosphorylate ADP to ATP. The rate of ﬂux through the electron transport chain will be regulated by the availability of NADH, oxygen and ADP (Chance & Williams 1955). Finally, the translocation of ATP and ADP across the mitochondrial membrane is thought to be effected by creatine by way of the mitochondrial creatine kinase reaction (Moreadith & Jacobus 1982), thereby linking mitochondrial ATP production to the ATPase activity in the contractile system. Lactate production Considerable controversy exists concerning the mechanism responsible for lactate accumulation during intense muscle contraction. The most widely accepted theory attributes this to a high rate of energy demand coupled with an inadequate oxygen supply. In short, when tissue oxygen supply begins to limit oxidative ATP production, resulting in the accumulation of mitochondrial and cytosolic NADH, ﬂux through glycolysis and a high cytosolic NAD+/NADH ratio are maintained by the reduction of pyruvate to lactate. However, it has been suggested that the reduction in mitochondrial redox state during contraction is insigniﬁcant, thereby indicating that reduced oxygen availability is not the only cause of lactate accumulation during contraction (Graham & Saltin 1989). In addition, there are data to indicate that it is the activation of the PDC and the rate of acetyl group production, and not oxygen availability, which primarily regulates lactate production during intense muscle contraction (Timmons et al. 1996). Furthermore, it has also been shown that for any given workload, lactate accumulation can be signiﬁcantly altered by pre-exercise dietary manipulation (Jansson 1980; Putman et al. 1993). Taken together, these ﬁndings suggest that an imbalance between pyruvate formation and decarboxylation to acetyl-CoA will dictate the extent of lactate formation during exercise as seen, for example, during the transition period from rest to steady-state exercise. Glycogen utilization with respect to exercise intensity Maximal exercise During submaximal (steady-state) exercise, ATP resynthesis can be adequately achieved by oxidative combustion of fat and carbohydrate stores. However, during high-intensity (non-steady state) exercise, the relatively slow activation and rate of energy delivery of oxidative phosphorylation cannot meet the energy requirements of contraction. In this situation, anaerobic energy delivery is essential for contraction to continue. Typically, oxidative energy delivery requires several minutes to reach a steady state, due principally to the number and complexity of the reactions involved. Once achieved, the maximal rate of ATP production is in the region of approximately 2.5 mmol · kg–1 dry matter (dm) · s–1. On the other hand, anaerobic energy delivery is restricted to the cytosol, its activation is almost instantaneous and it can deliver ATP at a rate in excess of 11 mmol · kg–1 dm · s–1. The downside, however, is that this can be maintained for only a few seconds before beginning to decline. Of course, oxidative and anaerobic ATP resynthesis should not be considered to function independently of one another. It has been demonstrated that as the duration of exercise increases, the contribution from anaerobic energy delivery decreases, whilst that from aerobic is seen to increase. Figure 6.1 shows that maximal rates of ATP resynthesis from PCr and glycogen degradation can only be maintained for short time periods during maximal contraction in man (Hultman et al. 1991). The rate of PCr degradation is at its maximum immediately after the initiation of contraction and begins to decline after only 1.3 s. Conversely, the corresponding rate of glycolysis does not peak until after approximately 5 s of contraction and does not begin to decline until carbohydrate metabolism in exercise 89 10 Fig. 6.1 Rates of anaerobic ATP formation from phosphocreatine and glycolysis during maximal intermittent electrically evoked isometric contraction in man (see Hultman et al. 1991). Note that the reference base for the muscle data in the ﬁgures and text is dry muscle. This is because the muscle samples were freeze-dried prior to biochemical analysis. To convert to wet weight, values should be divided by 4.3. This assumes 1 kg of wet muscle contains 70 ml of extracellular water and 700 ml of intracellular water. , phosphocreatine; , glycolysis. ATP production (mmol.kg–1 muscle.s–1) 8 6 4 2 0 0–1.3 after 20 s of contraction. This suggests that the rapid utilization of PCr may buffer the momentary lag in energy provision from glycolysis, and that the contribution of the latter to ATP resynthesis rises as exercise duration increases and PCr availability declines. This point exempliﬁes the critical importance of PCr at the onset of contraction. Without this large hydrolysis of PCr, it is likely that muscle force production would almost instantaneously be impaired, which is indeed the case in muscles in which the PCr store has been replaced with a Cr analogue (Meyer et al. 1986). It is also important to note that ultimately there is a progressive decline in the rate of ATP resynthesis from both substrates during this type of exercise. For example, during the last 10 s of exercise depicted in Fig. 6.1, the rate of ATP production from PCr hydrolysis had declined to approximately 2% of the peak rate. Similarly, the corresponding rate of ATP resynthesis from glycogen hydrolysis had fallen to approximately 40%. The above example concerns exercise of maximal intensity lasting about 30 s. However, 0–2.6 0–5 0–10 10–20 20–30 Exercise time (s) non-steady-state exercise, albeit less intense, can be sustained for durations approaching 5–15 min before fatigue is evident. Under these conditions, carbohydrate oxidation can make a major contribution to ATP production and therefore its importance should not be underestimated. It has been demonstrated that during 3.2 min of fatiguing exercise, oxidative phosphorylation can contribute as much as 55% of total energy production (Bangsbo et al. 1990). This indicates the importance of substrate oxidation during high-intensity exercise, a point which is often overlooked. Under these conditions, muscle glycogen is the principal fuel utilized as muscle glucose uptake is inhibited by glucose 6phosphate accumulation and adipose tissue lipolysis is inhibited by lactate accumulation. Submaximal exercise The term submaximal exercise is typically used to deﬁne exercise intensities which can be sustained for durations falling between 30 and 180 min. 90 nutrition and exercise In practice, this is usually exercise intensities between 60% and 85% of maximal oxygen consumption. Continuous exercise of any longer duration (i.e. an intensity of less than 60% of maximal oxygen consumption) is probably not limited by substrate availability and, providing adequate hydration is maintained, can probably be sustained for several hours or even days! Unlike maximal intensity exercise, the rate of muscle ATP production required during prolonged exercise is relatively low (< 2.5 mmol · kg–1 dm · s–1) and therefore PCr, carbohydrate and fat can all contribute to ATP resynthesis. However, carbohydrate is without question the most important fuel source. It can be calculated that the maximum rate of ATP production from carbohydrate oxidation will be approximately 2.0–2.8 mmol · kg–1 dm · s–1 (based upon a maximum oxygen consumption of 3–4 l · min–1), which corresponds to a glycogen utilization rate of approximately 4 mmol · kg–1 dm · min–1. Therefore, it can be seen that carbohydrate could meet the energy requirements of prolonged exercise. However, because the muscle store of glycogen is in the region of 350 mmol · kg–1 dm, under normal conditions, it can be calculated that it could only sustain in the region of 80 min of exercise. This was demonstrated in the 1960s by Bergström and Hultman (1967). The authors also demonstrated that if the glycogen store of muscle was increased by dietary means, exercise duration increased in parallel (Bergström et al. 1967). Of course, carbohydrate is also delivered to skeletal muscle from hepatic stores in the form of blood glucose and this can generate ATP at a maximum rate of approximately 1 mmol · kg–1 dm · s–1. The majority of hepatic glucose release during exercise (1.5–5.5 mmol · min–1) is utilized by skeletal muscle. Only 0.5 mmol · min–1 is utilized by extramuscular tissue during exercise. Muscle glucose utilization is dependent on glucose supply, transport and metabolism. If blood glucose is unchanged, as in the majority of exercise conditions, glucose supply to muscle is dictated by muscle blood ﬂow, which increases linearly with exercise intensity and can increase by 20-fold from rest to maximal exercise. The increase in muscle glucose delivery as a result of the exercise- mediated increase in blood ﬂow is probably more important for muscle glucose uptake during exercise than the insulin and contraction-induced increase in membrane glucose transport capacity (see Richter & Hespel 1996). As exercise continues, plasma insulin concentration declines, which facilitates hepatic glucose release and reduces glucose utilization by extramuscular tissue. However, insulin supply to muscle probably remains elevated above basal supply due to the contraction-induced elevation in muscle blood ﬂow. Hexokinase is responsible for the phosphorylation of glucose by ATP when it enters the muscle cell. The enzyme is allosterically inhibited by glucose-6-phosphate, the product of the hexokinase reaction and an intermediate of glycolysis. Thus, during short-term high-intensity exercise and at the onset of prolonged submaximal exercise, glucose phosphorylation by hexokinase will be inhibited by glucose-6phosphate accumulation. This will increase the concentration of glucose in the extra- and intracellular water and will contribute to the increase in blood glucose observed during high-intensity exercise. However, as submaximal exercise continues, the decline in muscle glucose-6phosphate results in an increase in glucose phosphorylation. In comparison with muscle glycogen metabolism, relatively little is known about the interaction between exercise and hepatic glycogen metabolism in man. This is not because of a lack of interest but because of the invasive nature of the liver biopsy technique. The few studies that have been performed in healthy volunteers using this technique have demonstrated that the rate of liver glucose release in the postabsorptive state is in the region of 0.8 mmol glucose · min–1, which is sufﬁcient to meet the carbohydrate demands of the brain and obligatory glucolytic tissues. Approximately 60% of this release (0.5 mmol · min–1) is derived from liver glycogen stores and the remainder is synthesized by gluconeogenesis in the liver using lactate, pyruvate, glycerol and carbohydrate metabolism in exercise 91 6 85% 4 75% 3 55% 2 30% 1 0 amino acids as substrates (Hultman & Nilson 1971; Nilsson & Hultman 1973). The rate of hepatic glucose release during exercise in the postabsorptive state has been shown to be mainly a function of exercise intensity (Fig. 6.2) (Hultman 1967; Wahren et al. 1971; Ahlborg et al. 1974; Ahlborg & Felig 1982). The uptake of gluconeogenic precursors by the liver is only marginally increased during the initial 40 min of submaximal exercise but increases further as exercise continues (Ahlborg et al. 1974). Most (more than 90%) of the glucose release is derived from liver glycogenolysis resulting in a decline and ultimately depletion of liver glycogen stores. Direct measurements of liver glycogen concentration in the postabsorptive state and following 60 min of exercise at 75% of maximal oxygen consumption showed a 50% decrease in the liver glycogen concentration with exercise (Fig. 6.3). This corresponded to a glycogen degradation rate of 4.2 mmol · min–1 (assuming 1.8 kg of liver) and suggested that the liver glycogen store would have been depleted within 120 min of exercise at this intensity. The exact mechanisms responsible for the regulation of liver glucose release at the onset and during exercise are still unresolved. However, it is known that the decline in blood 0 (rest) 10 20 30 40 120 180 240 Work time (min) 400 Liver glycogen (mmol.kg–1 wet wt) Fig. 6.2 Hepatic glucose release during exercise at 30%, 55%, 75% and 85% of maximal oxygen consumption in men. From Ahlborg et al. (1974), Ahlborg and Felig (1982), Wahren et al. (1971) and Hultman (1967). Hepatic glucose production (mmol.min–1) 5 300 200 100 0 Rest Exercise Fig. 6.3 Hepatic glycogen concentration in men at rest following an overnight fast (n = 33) and following an overnight fast and 60 min of exercise at approximately 75% of maximal oxygen consumption in a second group of subjects (n = 14). Biopsy samples were obtained at the same time of day in both groups of subjects. From Hultman and Nilsson (1971). insulin concentration and increases in adrenaline and glucagon with increasing exercise duration together with afferent nervous feedback from contracting muscle will stimulate liver glucose release (for more complete information, see Kjaer 1995). 92 nutrition and exercise Muscle ﬁbre type responses The conclusions presented so far have been based on metabolite changes measured in biopsy samples obtained from the quadriceps femoris muscle group. However, it is known that human skeletal muscle is composed of at least two functionally and metabolically different ﬁbre types. Type I ﬁbres are characterized as being slow contracting, fatigue resistant, having a low peak power output and favouring aerobic metabolism for ATP resynthesis during contraction. Conversely, in comparison, type II ﬁbres are fast contracting, fatigue rapidly, have a high peak power output and favour mainly anaerobic metabolism for ATP resynthesis (Burke & Edgerton 1975). Maximal exercise Evidence from animal studies performed on muscles composed of predominantly type I or type II ﬁbres and from one study performed using bundles of similar human muscle ﬁbre types, suggest that the rapid and marked rise and subsequent decline in maximal power output observed during intense muscle contraction in man may be closely related to activation and rapid fatigue of type II ﬁbres during contraction (Faulkner et al. 1986). Figure 6.4 demonstrates glycogen degradation in type I and type II muscle ﬁbres during maximal exercise under four different experimental conditions. Notice that during intense contraction the rates of glycogenolysis are higher in type II than in type I ﬁbres. This is true for both dynamic exercise (Greenhaff et al. 1994; treadmill sprinting) and electrically induced isometric contractions (Greenhaff et al. 1991, 1993). The rates of glycogenolysis observed in both ﬁbre types during treadmill sprinting and intermittent isometric contraction with circulation . occluded, are in good agreement with the Vmax. of phosphorylase measured in both ﬁbre types (Harris et al. 1976), suggesting that glycogenoly- Glycogenolytic rate (glucose units, mmol.s–1.kg–1) 5 4 3 2 1 0 Open circ. Open circ. and adrenaline Electrical stimulation Occluded circ. Sprinting Fig. 6.4 Glycogenolytic rates in type I ( ) and type II (䊐) human muscle ﬁbres during 30 s of intermittent electrically evoked maximal isometric contraction with intact circulation (circ.), intact circulation with adrenaline infusion, occluded circulation and during 30 s of maximal sprint running. Adapted from Greenhaff et al. (1991, 1993, 1994). carbohydrate metabolism in exercise sis is occurring at a near maximal rate during intense exercise. Surprisingly, during intermittent isometric contraction with circulation intact, when the rest interval between contractions is of the order of 1.6 s, the rate of glycogenolysis in type I ﬁbres is almost negligible. The corresponding rate in type II ﬁbres is almost maximal and similar to that seen during contraction with circulatory occlusion. This suggests that during maximal exercise glycogenolysis in type II ﬁbres is invariably occurring at a maximal rate, irrespective of the experimental conditions, while the rate in type I ﬁbres is probably very much related to cellular oxygen availability. Submaximal exercise In contrast to maximal exercise, the rate of glycogenolysis during submaximal exercise is greatest in type I ﬁbres, especially during the initial period of exercise (Ball-Burnett et al. 1990). This phenomenon is likely to be the result of differences in the recruitment pattern between muscle ﬁbre types. If exercise is continued, glycogen utilization occurs in both ﬁbre types but depletion is observed ﬁrst in the type I muscle ﬁbres. The consumption of carbohydrate during exhaustive submaximal exercise has been shown to offset the depletion of glycogen speciﬁcally in type I ﬁbres (Tsintzas et al. 1996). Fatigue mechanisms related to carbohydrate metabolism What is clear from the literature is that glycogen availability per se is not usually considered to be responsible for fatigue development during maximal exercise, providing the preexercise glycogen store is not depleted to below 100 mmol · kg -1 dm. It is even unlikely that glycogen availability will limit performance during repeated bouts of exercise, due to the decline in glycogenolysis and lactate production that occurs under these conditions. It is more probable that fatigue development during maximal exercise will be caused by a gradual decline in anaerobic ATP production caused by the 93 depletion of PCr and a fall in the rate of glycogenolysis. Lactic acid accumulation during highintensity exercise is considered to produce muscle fatigue as a result of H+ and Pi accumulation. An increase in hydrogen ion concentration will negatively affect phosphorylase activity, thereby delaying the rate of glycogenolysis, by delaying transformation of the b form to the a form (Danforth 1965; Chasiotis 1983) and by decreasing the HPO42+, the dibasic form of Pi, which is the substrate for phosphorylase. The inhibition of PFK discussed previously seems to be at least partly offset by an increase in the activators of PFK, especially ADP, AMP and Pi, when the rate of ATP utilization is higher than the rate of oxidative ATP resynthesis. The increase in ADP and Pi, especially the H2PO4– form, in acidotic muscle is known to have inhibitory effects on contractile function (Cook & Pate 1985; Nosek et al. 1987). However, there is no evidence of a direct relationship between the decline in muscle force during contraction and H+ accumulation. For example, studies involving human volunteers have demonstrated that muscle-force generation following fatiguing exercise can recover rapidly, despite having a very low muscle pH value (Sahlin & Ren 1989). The general consensus at the moment appears to be that the initial generation of muscle force production is dependent on the capacity to generate ATP but the maintenance of force generation is also pH dependent. Despite the wealth of information showing that carbohydrate availability is essential to performance during submaximal exercise, the biochemical mechanism(s) by which fatigue is brought about in the carbohydrate depleted state are still unclear. Recent evidence suggests that carbohydrate depletion will result in an inability to rephosphorylate ADP to ATP at the required rate, possibly because of a decrease in the rate of ﬂux through the TCA cycle as a result of a decline in muscle TCA cycle intermediates (Sahlin et al. 1990). The consequent rise in ADP concentration will bring about fatigue, perhaps as a direct inhibitory effect of ADP and/or Pi on contraction coupling. 94 nutrition and exercise Conclusion The carbohydrate stores of the body, liver and muscle glycogen, are utilized immediately at start of exercise. Glucose output from the liver closely matches the increased glucose requirement of the contracting muscles, keeping the blood glucose concentration unchanged during submaximal exercise. Blood glucose levels are normally seen only to increase in the initial period of intense exercise and to fall when the hepatic glycogen store is depleted near to exhaustion. The regulation of the hepatic glucose release is a complex process dependent on both hormonal control and feedback signals from contracting muscles. Glucose uptake by exercising muscle is directly related to exercise intensity and regulated by muscle blood ﬂow and facilitated by increased glucose transport capacity of the plasma membrane of the contracting muscle. The maximal rate of glucose uptake at a normal blood glucose concentration is about 0.4 mmol · min–1 · kg–1 exercising muscle. Glucose utilization is also dependent on the glucose phosphorylation capacity mediated by the activity of hexokinase. The major carbohydrate store of the body is muscle glycogen, which is used in concert with the hepatic glycogen store to provide the exercising muscle with energy. The rate of utilization is low at rest and during low-intensity exercise, when blood-borne glucose and free fatty acids are the major sources of fuel for ATP resynthesis. With increasing exercise intensity, the use of carbohydrate as an energy substrate increases gradually to cover almost all the energy demand of contraction at exercise intensities near the subject’s maximal oxygen uptake. The maximal rate of oxidative energy production from muscle glycogen is of the order of 35 mmol ATP · min–1 · kg–1 exercising muscle, corresponding to a glycogen degradation rate of 1 mmol · min–1 · kg–1 wet muscle. The mechanism(s) controlling the integration of fat and carbohydrate utilization during exercise are poorly understood and, as yet, unresolved. The muscle glycogen store can also produce ATP anaerobically and at a rate that is twice that of oxidative ATP regeneration. Anaerobic energy delivery can be activated within milliseconds, while the aerobic energy production needs several minutes to reach a steady state. Thus, anaerobic carbohydrate utilization will be important as an energy provider during the transition period between rest and exercise and during periods of intense exercise when the energy demand of contraction exceeds the capacity of oxidative ATP regeneration. It can be concluded that carbohydrate is used as fuel at onset of exercise at all intensities and is an obligatory fuel for the continuation of exercise at intensities above 50–60% of the subject’s maximal oxygen uptake. Depletion of the muscle carbohydrate stores will impair exercise performance at this range of exercise intensities. Exhaustion of the liver glycogen store during prolonged exercise results in hypoglycaemia which also impairs continued exercise performance. Carbohydrate metabolism in exercising muscle is initiated by Ca2+ release from the sarcoplasmic reticulum and thereafter is regulated by the rate of ATP degradation via the phosphorylation state of the high-energy phosphate pool (ATP, ADP, AMP, PCr) and Pi. AMP and Pi concentrations regulate the ﬂux through the glycolytic pathway while Ca2+ and pyruvate concentrations are the main regulators of PDH activity which, together with the intramitochondrial concentration of ADP, determines the rate of carbohydrate oxidation. The result is a tight matching of ATP generation from carbohydrate sources with the ATP demand of contracting muscle. Other inﬂuences on carbohydrate metabolism during exercise include diet, training status and hormonal balance. References Ahlborg, G. & Felig, P. (1982) Lactate and glucose exchange across the forearm, legs and splanchnic bed during and after prolonged leg exercise. Journal of Clinical Investigation 69, 45–54. 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