Fat Metabolism during Exercise
Chapter 13 Fat Metabolism during Exercise JOHN A. HAWLEY, ASKER E. JEUKENDRUP AND FRED BROUNS Introduction In the search for strategies to improve athletic performance, recent interest has focused on several nutritional procedures which may, theoretically, promote fatty acid (FA) oxidation, attenuate the rate of muscle glycogen utilization and improve exercise capacity (for reviews, see Chapter 14 and Lambert et al. 1997; Hawley et al. 1998). The aim of this chapter is to provide the reader with a general overview of the role of endogenous fat as an energy source for muscular contraction, to discuss the effects of exercise intensity and duration on the regulation of fat metabolism, and to give a synopsis of some of the factors which may potentially limit FA mobilization, uptake and oxidation by human skeletal muscle during exercise. Fat as an energy source for physical activity The human body utilizes carbohydrate (CHO), fat and, to a lesser, extent protein as fuel for muscular work. Fat as an energy source has several advantages over CHO: the energy density of fat is higher (37.5 kJ · g–1 (9 kcal · g–1) for stearic acid vs. 16.9 kJ · g–1 (4 kcal · g–1) for glucose), therefore the relative weight as stored energy is lower. FAs provide more adenosine triphosphate (ATP) per molecule than glucose (147 vs. 38 ATP). However, in order to produce the equivalent amount of ATP, the complete oxidation of FA requires more oxygen than the oxidation of CHO 184 (6 vs. 26 molecules of oxygen per molecule of substrate for the complete oxidation of glucose and stearic acid, respectively). Using CHO as a fuel, 21 kJ (5 kcal) of energy are available for each litre of oxygen used, whereas only 19.7 kJ (4.7 kcal) per litre of oxgyen are available when fat is the sole fuel oxidized: this may be important when the oxygen supply is limited. On the other hand, for every gram of CHO stored as glycogen, approximately 2 g of water are stored (Holloszy 1990). Consequently, the amount of glycogen stored in muscle and liver is limited to about 450 g in an average-sized adult. Of interest is that although skeletal muscle comprises up to 40% of body mass in well-trained individuals, CHO utilization by muscle in the resting or postabsorptive state is minimal, accounting for less than 10% of total glucose turnover (Felig & Wahren 1975). Fat can be stored in much greater amounts. In a healthy, untrained male, up to 20 kg of fat can be stored, mainly in adipose tissue: in the obese individual, the fat store may exceed 100 kg. Even in highly trained athletes with much lower levels of adipose tissue, endogenous fat stores still far exceed the requirements of all athletic pursuits. Both FA stored in adipose tissue and fat entering the circulation after a meal can serve as potential energy sources for the muscle cell (Fig. 13.1). For humans ingesting a typical Western diet (approximately 35% of energy from fat), FAs are comprised of approximately 40% oleate, 25% palmitate, 15% stearate and 10% linoleate. The remainder is thought to be a mixture of both fat metabolism during exercise Adipose tissue Fig. 13.1 The storage and mobilization of peripheral adipose and intramuscular triacylglycerol (TG). TG from peripheral adipose tissue can be broken down to glycerol and free fatty acids (FFAs). FFAs can be mobilized by binding to plasma albumin for transport into the systemic circulation to skeletal muscle. Intramuscular TG can also be broken down to glycerol plus fatty acids, which can enter the mitochondria for oxidation during exercise. TCA, tricarboxylic acid. From Coyle (1997), with permission. Blood plasma Triacylglycerol (210 000 kJ) 185 Muscle Intramuscular TG (12 600 kJ) Glycogen (c. 8400 kJ) Glycerol FFA saturated and unsaturated FAs with chain lengths of 12–20 carbon atoms (Havel et al. 1964). Small but physiologically important amounts of FA are also stored as triacylglycerols (TG) inside the muscle cells: the total muscle mass may contain up to 300 g of fat of which the major part is stored within the myocyte as small lipid droplets (Björkman 1986). FAs liberated from TG stored in adipocytes are released to blood, where they are bound to albumin. The albumin concentration of blood is about 6 mm, while the concentration of FA is about 0.2–1.0 mm. As albumin can bind up to eight FAs, the albumin transport capacity is far in excess of the amount of FAs bound under physiological circumstances and therefore cannot be the limiting factor for FA oxidation by muscle. FA can also be derived from the triacylglycerol core of circulating chylomicrons and very low density lipoproteins (VLDL), which are both formed from dietary fat in the postabsorptive state. Chylomicrons are formed in the epithelial wall of the intestine and reach the blood stream after passage through the lymphatic system. VLDLs are synthesized in the liver after which they are released directly into the blood stream. FFA Albumin FFA Fatty acids FFA Mitochondria Acetyl-CoA TCA cycle and electron transport Glucose (420 kJ) ATP Energy O2 Effects of exercise intensity and duration of fat metabolism More than 50 years ago, Christensen and Hansen (1939) provided evidence from respiratory gas exchange measurements that fat was a major fuel for exercise metabolism. Since that time, a number of investigations have provided evidence that plasma FAs contribute a signiﬁcant portion to the energy demands of mild-tomoderate exercise. However, until recently the rates of whole-body lipolysis had only been measured during very low-intensity exercise, and in untrained or moderately active individuals. Our understanding of the regulation of endogenous fat and CHO metabolism in relation to exercise intensity and duration has been advanced considerably by modern-day studies which have used a combination of stable isotope techniques in association with conventional indirect calorimetry (Romijn et al. 1993, 1995; Sidossis & Wolfe 1996; Siddossis et al. 1996, 1997). As the three most abundant FAs are oxidized in proportion to their relative presence in the total plasma FA pool (Havel et al. 1964), total plasma FA kinetics can be reliably estimated from stable isotope studies using infusions of either palmitate or oleate (when the concentrations of total FAs and 186 nutrition and exercise palmitate and oleate are known). Palmitate is a saturated 16-C FA (CH3(CH2)14COOH) whose kinetics closely resemble those of most other long-chain FAs (Havel et al. 1964). As such, the rate of appearance (Ra) of palmitate gives an index of the release of FAs into the plasma. The Ra of glycerol, on the other hand, gives an index of whole-body lipolysis. Rates of total fat and CHO oxidation are determined by indirect calorimetry. The use of these methods has allowed estimates to be made of the rates of lipid kinetics, including the contribution to energy expenditure from peripheral lipolysis occurring in the adipocytes and from intramuscular lipolysis. During low-intensity exercise (25% of maxi. mum oxygen uptake (Vo2max.)), peripheral lipolysis is strongly stimulated, with little lipolysis of intramuscular TG (Fig. 13.2). Similarly, CHO oxidation appears to be met exclusively by blood glucose with little or no muscle glycogen utilization. Ra of FA into the plasma and their oxidation . are highest during exercise at 25% of Vo2max., and decline progressively as the exercise intensity Energy expenditure (J.kg–1.min–1) 1400 Muscle glycogen 1200 Muscle triglyceride 1000 Plasma FFA 800 Plasma glucose 600 400 200 0 25 65 . Intensity (% VO2max) 85 Fig. 13.2 The maximal contribution to energy expenditure from endogenous fat and carbohydrate, expressed as a function . of increasing exercise intensity. FFA, free fatty acids; Vo2max., maximal oxygen uptake. From Romijn et al. (1993), with permission from the American Physiological Society. increases. Conversely, although intramuscular TG (and glycogen) do not contribute signiﬁcantly to energy production during low-intensity work, fat oxidation is highest during exercise at . about 65% of Vo2max. (Fig. 13.2). At this intensity, lipolysis in both peripheral adipocytes and intramuscular TG stores attains its highest rates, and these two sources contribute about equally to the rate of total fat oxidation. With an increase . in exercise intensity to 85% of Vo2max., total fat oxidation falls. This is mainly due to a suppression in the Ra of FA into the plasma, presumably caused by the increases in circulating plasma catecholamines, which stimulate muscle glycogenolysis and glucose uptake. Lipolysis of intramuscular TG does not increase substantially with an increase in exercise intensity from 65% . to 85% of Vo2max., indicating that lipolysis of peripheral adipose tissue and lipolysis of intramuscular TG are regulated differently. Further evidence for this hypothesis comes from studies which have increased FA delivery (by intravenous infusion of lipid and heparin) during . intense (85% of Vo2max.) exercise in well-trained subjects (Romijn et al. 1995). These data reveal that even when plasma FA concentration is artiﬁcially maintained above 1 mm, this only partly restores fat oxidation to those (higher) levels seen . at more moderate intensity (65% of Vo2max.) exercise. Taken collectively, these observations indicate that factors other than FA availability play an important role in the regulation of FA oxidation during high-intensity exercise (see following sections). With regard to the effects of exercise duration on fat metabolism, there is little change in either the rates of total fat or total CHO oxidation after 2 h compared with the ﬁrst 30 min of exercise at . 25% of Vo2max.. However, at an intensity of 65% of . Vo2max., there is a progressive increase in the Ra of FA into the plasma (and presumably their oxidation) and glucose availability over time. After 2 h of cycling at this intensity, there is no change in either the rates of total fat and total CHO oxidation compared with the situation after 30 min of exercise. Thus, it is likely that the contribution of intramuscular substrates (TG and glycogen) fat metabolism during exercise to total energy expenditure decreases with increasing exercise duration during prolonged . (> 90 min) moderate-intensity (65% of Vo2max.) exercise. Factors limiting fat oxidation by muscle Factors limiting fatty acid uptake by muscle cells As previously discussed, the metabolism of FA derived from adipose tissue lipolysis constitutes a major substrate for oxidative metabolism, especially during prolonged, low-intensity exercise. The metabolism of long-chain FA is a complex and integrated process that involves a number of events: FA mobilization from peripheral adipose tissue, transport in the plasma, transport and permeation across muscle cell membranes and interstitium, cytoplasmic transport, and intracellular metabolism. The ﬁrst stage in this process, the mobilization of lipids, plays a key role in the subsequent regulation of FA utilization during both the resting state and exercise. During perfusion of the muscle capillaries, FA bound to albumin or stored in the core of chylomicrons and VLDL have to be released prior to transport across the vascular membrane. In the case of VLDL and chylomicrons, this is achieved by the action of the enzyme lipoprotein lipase (LPL). LPL is synthesized within the muscle cell and, after an activation process, is translocated to the vascular endothelial cell membrane where it exerts its enzymatic action on TG. LPL also expresses phospholipase A2 activity which is necessary for the breakdown of the phospholipid surface layer of the chylomicrons and lipoproteins. LPL activity is upregulated by caffeine, catecholamines and adrenocorticotrophic hormone (ACTH), and downregulated by insulin (for review, see Jeukendrup 1997). After TG hydrolysis, most of the FA will be taken up by muscle, whereas glycerol will be taken away via the bloodstream to the liver, where it may serve as a gluconeogenic precursor. During the postab- 187 sorptive state, the concentration of circulating TG in plasma is usually higher than that of FA, in contrast to the fasting state when chylomicrons are practically absent from the circulation. Nevertheless, the quantitative contribution of circulating TG to FA oxidation by the exercising muscle cells in humans is somewhat uncertain. Due to technical limitations, no reliable data are available to determine whether FA derived from the TG core of VLDL or chylomicrons substantially contribute to overall FA utilization. However, it is interesting to note that even a small extraction ratio of the order of 2–3% of FA/TG could potentially cover over up to 50% of total exogenous FA uptake and subsequent oxidation (Havel et al. 1967). The arterial concentration of FA strongly affects FA uptake into muscle both at rest and during low-intensity exercise (for review, see Bulow 1988). This implies an FA gradient from blood to muscle in these conditions, which is achieved by a relatively rapid conversion of FA, taken up by the muscle cell, to fatty acyl-CoA. The rate of the latter reaction step is controlled by fatty acyl-CoA synthetase. During transport of FA from blood to muscle, several barriers may limit FA uptake, including the membranes of the vascular endothelial cell, the interstitial space between endothelium and muscle cell, and ﬁnally the muscle cell membrane (for review, see van der Vusse & Reneman 1996). Uptake by endothelial cells is most likely protein mediated. Both albumin-binding protein and membrane-associated FA-binding proteins (FABP) may play a role. After uptake, most FAs will diffuse from the luminal to the abluminal membrane of the endothelial cells as free molecules. Although small amounts of FABP are present at this site, their role in transmembrane FA transport is assumed to be unimportant. Once in the interstitial space, albumin will bind the FAs for transport to the muscle cell membrane. Here the FAs are taken over by a fatty-acidtransporting protein, or will cross the membrane directly because of their lipophilic nature. In the sarcoplasm, FABP, which is present in relatively high concentrations, is crucial for FA transport to 188 nutrition and exercise the mitochondria. This transport is assumed not to ultimately limit FA oxidation. As indicated earlier, an alternative source of FA are TGs present inside the skeletal muscle cells. For the storage of FA, glycerol is obtained from glycolysis (as glycerol-3-phosphate) which reacts with fatty acyl-CoA, after which further condensation to and storage as TG take place in small fat droplets, mainly located in the proximity of the mitochondrial system. It has been suggested that adipocytes, positioned between muscle cells may also supply FA for oxidation, although the physiological signiﬁcance of this has never been accurately quantiﬁed. During periods of increased muscle contractile activity, muscle lipase is activated by hormonal actions which leads to the release of FA from the intramuscular TG. Noradrenaline infusion has been observed to cause a signiﬁcant reduction in muscle TG, and insulin counteracts this effect. Apart from hormonal stimuli, there is also local muscular control of lipase activity, shown by the observation that electrical stimulation of muscle enhances TG breakdown. Compared to fast twitch (type II) muscle ﬁbres, slow twitch (type I) ﬁbres have a high lipase activity (Gorski 1992) as well as TG content (Essen 1977). Interestingly, TG storage within the muscle cell can be increased by regular endurance training (Morgan et al. 1969; Howald et al. 1985; Martin 1996). However, whereas some studies report an increased utilization of intramuscular triacylglycerol after endurance training (Hurley et al. 1986; Martin et al. 1993), others (Kiens 1993) ﬁnd no change. These conﬂicting results may simply be a reﬂection of the different type of exercise modes employed (cycling vs. dynamic knee-extension exercise), which result in marked differences in circulating catecholamine levels. On the other hand, an inability to detect exercise-induced perturbations in intramuscular TG content does not exclude the possibility that while FAs are being hydrolysed from the intramuscular TG pool, TG is also being synthesized, with the net result that there is no change in concentration (Turcotte et al. 1995). If indeed the intramuscular TG pool is in a state of constant turnover, a net decline in stores would only be observed when the rate of utilization of intramuscular TG is greater than the rate of TG synthesis. Factors limiting fatty acid oxidation by muscle cells As previously discussed, a relatively high percentage of the total energy production is derived from FA oxidation at rest and during lowintensity exercise. However, with increasing exercise intensities, particularly above 70–80% of . Vo2max., there is a progressive shift from fat to CHO (Gollnick 1985), indicating a limitation to the rate of FA oxidation. Several explanations for this shift from fat to CHO have been proposed, including an increase in circulating catecholamines, which stimulates glycogen breakdown in both the muscle and liver. However, the increased lactate formation (and accompanying hydrogen ion accumulation) which occurs when glycogen breakdown and glycolytic ﬂux are increased also suppresses lipolysis. The net result will be a decrease in plasma FA concentration and hence in the supply of FA to muscle cells. As a consequence, enhanced CHO oxidation will most likely compensate for the reduced FA oxidation. Another reason for this substrate shift is the lower ATP production rate per unit of time from fat compared with that from CHO, combined with the fact that more oxygen is needed for the production of any given amount of ATP from fat than from CHO, as previously noted. Finally, limitations in the FA ﬂux from blood to mitochondria might explain the shift from fat to CHO at higher exercise intensities. This ﬂux is dependent on the concentration of FA in the blood, capillary density, transport capacity across vascular and muscle cell membranes, mitochondrial density and mitochondrial capacity to take up and oxidize FA. The latter depends on the action of the carnitine transport system across the mitochondrial membrane which is fat metabolism during exercise regulated by malonyl-CoA (Winder et al. 1989). During exercise, malonyl-CoA formation is reduced and therefore the capacity to transport FAs across the mitochondrial inner membrane is enhanced. The rate of oxidation of FA is the result of three processes: 1 Lipolysis of TG in adipose tissue and circulating TG and transport of FA from blood plasma to the sarcoplasm. 2 Availability and rate of hydrolysis of intramuscular TG. 3 Activation of the FA and transport across the mitochondrial membrane. It is likely that the ﬁrst two processes pose the ultimate limitations to fat oxidation observed during conditions of maximal FA ﬂux. This is most evident during both short-term intense exercise or during the initial phase of a long-term exercise. In this condition, lipolysis in adipose tissue and in muscle TG is insufﬁciently upregulated to result in enhanced FA supply. The result will be that the rate of FA oxidation exceeds the rate at which FAs are mobilized, leading to a fall in plasma FAs and intracellular FAs in muscle. As a consequence, the use of CHO from glycogen must be increased to cover the increased energy demand. Direct evidence that the rate of FA oxidation can be limited by a suppression of lipolysis, at least during low-to-moderate intensity (44% of . Vo2max.) exercise, comes from a recent investigation by Horowitz et al. (1997). They showed that CHO ingestion (0.8 g · kg–1 body mass) before exercise, which resulted in a 10–30 mU · ml–1 elevation in plasma insulin concentration, was enough to reduce fat oxidation during exercise, primarily by a suppression of lipolysis. They also showed that fat oxidation could be elevated (by about 30%) when plasma FA concentration was increased via Intralipid and heparin infusion, even when CHO was ingested. However, the increase in lipolysis was not sufﬁcient to restore fat oxidation to those levels observed after fasting. Taken collectively, these results suggest that CHO ingestion (and the concomitant eleva- 189 tion in plasma insulin concentration) has another (additional) effect on reducing the rates of FA oxidation by exercising skeletal muscle. Conclusion In contrast to body CHO reserves, fat stores are abundant in humans and represent a vast source of fuel for exercising muscle. FAs stored both in peripheral adipose tissue and inside the muscle cells serve as quantitatively important energy sources for exercise metabolism. During low. intensity work (25% of Vo2max.), plasma FA liberated from adipose tissue represents the main source of fuel for contracting muscle, with little or no contribution from intramuscular lipolysis to total energy metabolism. On the other hand, during moderate-intensity exercise (65% of . Vo2max.), fat metabolism is highest, with the contribution of lipolysis from peripheral adipocytes and of intramuscular TG stores contributing about equally to total fat oxidation. During high. intensity exercise (85% of Vo2max.), there is a marked reduction in the rate of entry of FA into the plasma, but no further increase in intramuscular TG utilization. At such workrates, muscle glycogenolysis and the accompanying increased lactate concentration suppress the rates of whole-body lipolysis. The major hormonal changes which promote lipolysis during exercise are an increase in catecholamine concentration and a decline in insulin levels, both of which facilitate activation of LPL. The rate of FA oxidation is also regulated indirectly by the oxidative capacity of the working muscles and the intramuscular concentration of malonyl-CoA. The muscle tissue level of malonyl-CoA is dependent on the prevailing concentrations of plasma glucose and insulin: elevated circulating levels of these two compounds is associated with elevated concentrations of malonyl-CoA. Any increase in glycolytic ﬂux therefore may directly inhibit long-chain FA oxidation, possibly by inhibiting its transport into the mitochondria (Sidossis & Wolfe 1996; Sidossis et al. 1996; Coyle et al. 1997). 190 nutrition and exercise References Björkman, O. 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