Carbohydrate and fat are the main substrates for the muscle used during aerobic exercise. Carbohydrates are stored in the body as muscle glycogen, liver glycogen (1) and circulating as plasma glucose (2). Fat is stored as adipose tissue and as intramuscular triglyceride (IMTG) (1). In addition, some fat is present in the circulation as plasma free fatty acids (FFA) (3) and as triglycerides (TG) incorporated in lipoproteins (2). An overview of the energy stores is provided in table 1.
Table 1 Fuel stores in an average man
|In weight (g)||
In energy (kJ)
Based on estimates for a normal, non-obese male with a body mass of ±70 kg (4). Fat provides 39 kJ • g-1 and carbohydrate 17 kJ • g-1.
Liverglycogen is converted to glucose-6-P and dephosphorylated by glucose-6-phosphatase to form glucose (4). Glut-4 is only present in skeletal muscle and can translocate from an intracellular microsomal Glut-4 pool to the cellsurface following insulin release and/or contraction, thereby enabling a rapid increase in plasma glucose uptake (12). Glucose is converted to glucose-6-P by hexokinase (4). During muscle glycogenolysis, muscle glycogen is reduced to glucose-1-P by glycogenphosphorylase. Glucose-1-P is also converted to glucose-6-P by hexokinase (5).
Intracellular glucose is metabolized in the glycolytic pathway to form pyruvate. Pyruvate is converted into lactate when the glycolytic rate exceeds the entry of pyruvate into the tricarboxylic (TCA)-cycle (5). When oxygen is present, pyruvate is converted into acetyl-CoA and oxidized within the mitochondria to form CO2 and H2O. Whereas the anaerobic metabolism of glucose yields only 2 ATP per glucose molecule the complete oxidative metabolism of glucose yields 38 ATP per glucose molecule (5). Thus the oxidative metabolism of glucose is a much more energy efficient mechanism.
Also fats are available to meet cellular needs. Triglyceride in fat tissue, IMTG and plasma TG are hydrolyzed into fatty acids. Fatty acids are activated by acyl-CoA synthetase to form acyl-CoA. A carnitine-dependent transport system transports the activated fatty acids over the mitochondrial membrane (6). In a four step process known as the ß-oxidation, fatty acids are broken down into acetyl-CoA (7). Acetyl-CoA which comes from the metabolism of fats is also oxidized within the mitochondria to form CO2 and H2O.
During aerobic exercise there is a mixture of fat and carbohydrate utilisation. However, the relative contribution of fat and carbohydrate utilisation to total substrate metabolism is dependent on exercise intensity, exercise duration, dietary and training status (1)
During exercise there is a dramatic increase in energy requirements because of the metabolic need of working muscles. The rate of fat and carbohydrate oxidation increases 5-10 fold during prolonged low to moderate-intensity exercise (25-65%VO2max) (1). During low intensity exercise (25%VO2max) plasma FFA oxidation provides most of the energy needed (8). As exercise intensity is increased up to 65%VO2max the absolute contribution of fat oxidation to total energy expenditure reaches maximum rates (9). About 40-60% of total fat oxidation is provided by the oxidation of plasma FFA, as such the oxidation of other fat sources (IMTG and plasma TG) provides the remaining 40-60% of total fat oxidation (1).
The relative and absolute contribution of fat oxidation during high intensity aerobic exercise (>75%VO2max) is substantially declined compared to low to moderate intensity exercise (25-65% VO2max) (1). The reduction in fat oxidation can not entirely be explained by a reduced plasma FFA availability. The latter has become evident due to the fact that increasing plasma FFA concentration with an intravenous infusion with lipid and heparin does not fully restore fat oxidation during high intensity exercise (10). This implies that an intracellular mechanism must contribute to the inhibition of fat oxidation during high intensity exercise (>75%VO2max). This is in accordance with Sidossis et al. (11) who found that the oxidation of long-chain acids (LCFA) but not medium chain fatty acids (MCFA) is inhibited at high-intensity exercise (80%VO2max) compared to exercise at 40%VO2max (11). LCFA are transported over the mitochondrial membrane by carnitine palmitoyl transferase 1 (CPT-1) (6). It is suggested by Sidossis et al. (11) that the decline in fat oxidation during high intensity exercise is limited by the decreased transport of LCFA over the mitochondrial membrane (11).
For the observed reduction in fat oxidative capacity during high intensity exercise (>75%VO2max) are different explanations. The decrease in fat oxidation is related to an increase in glycolytic rate. The increased glycolytic rate leads to an increase in acetyl-CoA, which subsequently increases the amount of malonyl-CoA. The latter inhibits CPT-1. However, muscle malonyl-CoA levels do not seem to increase in rat nor in human skeletal muscle during high intensity exercise, which makes the regulatory role of malonyl-CoA during high intensity exercise unlikely (4). An alternative explanation for the reduced fat oxidation during high intensity exercise is proposed by Van Loon et al. (12) and involves the availability of muscle carnitine. Carnitine is a cofactor that is required for the transport of LCFA across the inner mitochondrial membrane. During high intensity exercise (>75%VO2max) carnitine acts as a sink for acetyl group storage. During high intensity exercise the acetyl group production exceeds its flux through the tricarboxylic (TCA) cycle thereby markedly increasing muscle acetylcarnitine concentrations and reducing free carnitine availability (12). Another mechanism that could explain the decrease in fat oxidation during high intensity exercise (>75%VO2max) involves muscle pH (4). High intensity exercise (>75%VO2max) effectively lowers muscle pH. The decrease in muscle pH could increase the sensitivity of CPT-1 for malonyl-CoA (12). An interesting alternative mechanism is that pyruvate-derived acetyl-CoA effectively competes with fatty acid-derived acetyl-CoA for entry into the TCA-cycle (1).
In either case there seems to be a cross-over-point where carbohydrate utilisation predominates over the energy derived from fat, with further increments in power output increasing carbohydrate utilisation and reducing the relative contribution of fat oxidation (13). This shift is due partly to the relatively greater abundance of a glycolytic to lipolytic enzyme system in skeletal muscle (14). The increased CHO-utilisation with higher power outputs is also a consequence of a change in fibre type recruitment shifting from a predominant use of type 1 muscle fibres towards type 2 muscle fibres (14).
During low to moderate intensity exercise, the contribution of fat oxidation to total energy expenditure increases with the duration of exercise (15). With the duration of exercise plasma FFA availability increases (16). The increased plasma FFA availability results in a shift towards enhanced oxidation of plasma FFA (16). The increased plasma FFA oxidation leads to a reduced oxidation in both muscle glycogen and IMTG (16).
During the early part of moderate intensity exercise (40-65%VO2max), plasma glucose provides approximately one-third and muscle glycogen approximately two-thirds of the carbohydrate oxidized. However, as exercise continues the relative contribution to total energy expenditure from plasma glucose utilisation increases and that from muscle glycogen utilisation decreases (16).
Consumption of a high carbohydrate diet is related to an increased carbohydrate oxidation during exercise (6). This stimulatory effect on carbohydrate oxidation does not appear to have negative effects on endurance performance, which is enhanced by increased muscle glycogen availability (6).
In contrast, carbohydrate oxidation decreases and fat oxidation increases during exercise following a low-carbohydrate (high-fat) diet (17). Increasing fat availability immediate before exercise (by acute fat feeding) can increase endurance performance (18).
This shift towards fat oxidation spares muscle glycogen. However, low-carbohydrate diet leads to a decreased muscle glycogen content. Thus controversy exists whether the use of a high fat diet can increase endurance performance
Endurance trained humans rely less on muscle glycogen and plasma glucose and more on fats as an energy source during exercise at any given absolute or even relative intensity as compared to untrained humans (13,14). These differences are due to physiologic, biochemical and hormonal adaptations to endurance training.
It is evident that the metabolic adaptations to training are largely mediated by an increase in mitochondrial density and muscle capillary density (13). The increase in mitochondrial density is explained by an increase in both number and size of the mitochondria. This increase following endurance training leads to an increase of the mitochondrial enzymes responsible for the activation, mitochondrial transport, β-oxidation of fatty acids, and enzymes of the tricarboxylic (TCA)-cycle. Also increases of the enzymes of the electronic transport chain (ETC) are observed after endurance training (4). These enzymes include NADH dehydrogenase, cytochrome c reductase and oxidase (6).
The alterations in substrate utilisation with endurance training are likely explained by a lesser disturbance of energetic homeostasis. With a greater mitochondrial volume after training, smaller decreases in ATP, phosphocreatine (PCr) and smaller increases in ADP and inorganic phosphate (Pi) are required during exercise to balance the rate of ATP synthesis with the rate of ATP hydrolysis. The smaller increase in ADP, results in less of an increase in AMP formation by AMPK (=adenylate kinase) and, therefore, also less of an increase in IMP and NH4+ formation by AMP deaminase. These metabolic alterations, especially the smaller increases in Pi and AMP, play a major role in reducing glycogenolytic rate in muscle that has been adapted to endurance training (6).
An important metabolic regulator is AMPK. AMPK causes two ADP molecules to interact and make one AMP and one ATP molecule. This reaction keeps the ADP concentration from building up to the extent it would without this reaction. High ADP concentrations would inhibit the activation of the CPT-1 system and would augment fat oxidation. However, AMP also inhibits the CPT-1 system (6). In either case endurance training leads to a reduced rise in ADP and AMP concentration and diminished activation of glycolysis (6). However, activation of AMPK can inactivate acetyl-CoA carboxylase (ACC). The latter catalyzes the formation of malonyl-CoA. However, the precise biochemical mechanism by which malonyl-CoA affects fat oxidation have only been partly elucidated.
Although important, the augmention of muscle respiratory capacity is not the only mechanism by which training affects substrate metabolism during exercise. Neuro-endocrine responses, which are modified by endurance training, play a major role in regulating substrate mobilisation and utilisation. For example plasma norepinephrine, epinephrine, growth hormone, cortisol, and adrenocorticotropic hormone increase less during exercise at the same absolute exercise intensity in the trained state, compared to the untrained state. Reduced catecholamine levels after training may contribute to many of the previously described alterations in substrate metabolism during exercise. For example, the slower rate of muscle glycogenolysis during exercise after training may be due, in part, to lower catecholamine levels (6). Similarly, lower catecholamine levels, in conjunction with less of a decline in insulin secretion, seem likely to contribute to the reduced rates of hepatic glucose production during exercise after training (6).
It is now quite clear that endurance training leads to decreased reliance on carbohydrate oxidation and an increased reliance on fat oxidation at any given exercise intensity. However, the relative contribution of the different fat sources used during exercise remains debated (6).
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