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Creatine Supplementation: Abstract The use of creatine (Cr) in its supplemental form, Cr monohydrate, has become rather widespread. The discovery that the Cr and phosphocreatine (PCr) content in human muscle can be increased by oral ingestion of supplemental Cr has led to numerous studies examining its benefits on exercise performance. Cr monohydrate supplementation appears to result in an increased ability to maintain power output during high-intensity exercise and increase the rate of PCr resynthesis during the recovery phase of intermittent high-intensity exercise. Subjects supplemented with Cr monohydrate demonstrate a reduction in the accumulation of plasma lactate, ammonia, and hypoxanthine, indicating an alteration in energy metabolism and an attenuation of ATP degradation. Thus, higher concentrations of Cr seem to enhance the muscle's ability to sustain the high ATP turnover rates encountered during strenuous exercise. Another potential benefit is an increase in body mass which results from the ingestion of Cr monohydrate; however, the composition of the weight gain remains undetermined. This article discusses the theoretical basis for Cr supplementation and reviews what is known about its effects on performance. I. Introduction Creatine has an integral role in energy metabolism as a substrate (in the form of PCr) for the formation of ATP through the Cr kinase reaction. Therefore, an increase in this energy source may be advantageous for athletes who rely on this energy system. Evidence that the intramuscular stores of Cr can be increased by ingesting Cr in greater than normal amounts dates back to 1926. Recently
it was established that the total Cr content of the quadriceps femoris is significantly increased after supplementation of 20-25 g Cr monohydrate per day for >2 days. Based on this finding, further research has explored the potential of this nutritional ergogenic aid in enhancing performance, with promising results. The supplemental form of Cr used in these studies is a synthetic, odorless, and tasteless white powder, Cr monohydrate. To our knowledge, the only side effect cited in the literature associated with ingestion of large amounts of Cr monohydrate is an increase in body mass (BM). II. Creatine: Sources, Transport, Metabolism, and Excretion Creatine is a nitrogenous organic compound obtained predominantly from the ingestion of meat or fish, which contains approximately 5 g Cr
kg-l. Creatine is also synthesized primarily in the liver, pancreas, and kidney from the precursor amino acids arginine, glycine, and methionine. However, approximately 98% of the total Cr is contained in skeletal muscle, of which about 40% is free Cr and 60% is in the phosphorylated form, phosphocreatine
(PCr). Creatine is then transported from its site of synthesis to its primary site of storage via the circulation where active uptake by skeletal muscle against a concentration gradient occurs. Normal concentration of Cr in plasma is 50 to 100 mmol
L-1. The concentration of Cr in skeletal muscle is approximately 124 mmol per
kg-1 dry muscle.
III. Functions of Creatine and Phosphocreatine A. Temporal Energy Buffer Phosphocreatine serves a major role in energy metabolism, specifically as a donor of phosphate for energy production. The maintenance of ATP during rapid in- creases in energy demands is met by breakdown of PCr. The only known enzymatic reaction involving Cr and PCr is the reversible Cr kinase reaction, which is controlled by creatine kinase (CPK). PCr + ADP + H' <- (CPK) -> Cr + ATP The reaction is driven to the right by removal of ATP at sites where energy utilization is taking place, and driven to the left by removal of ADP at sites of energy generation at the mitochondria. B. Spatial Energy Buffer Over the years, evidence has accumulated supporting the role of diffusion of Cr and PCr between mitochondrial sites of production and sites of utilization (e.g., myosin head, cation pumps). This is achieved by a process termed the PCr energy shuttle. The PCr energy shuttle includes 3 areas:
At the peripheral terminus lie specific isoenzymes of CPK which act to rephosphorylate the ADP produced from muscular activity from breakdown of PCr by the previously mentioned Cr kinase reaction. The liberated free Cr then diffuses into the intervening space, traveling in the opposite direction as PCr, where it finally arrives at the energy-generating terminus in the mitochondria. Here the free Cr interacts with a specific isoenzyme of CPK, and PCr is formed from mitochondrial ATP. The PCr is then shuttled back to the sites of utilization and the process continues. Thus the PCr energy shuttle connects sites of energy production with sites of utilization by carrying energy via PCr. C. Proton Buffer Furthermore PCr acts to buffer protons (H+) which are products of ATP hydrolysis. When the Cr kinase reaction is in favor of ATP regeneration, H+ are utilized. This buffering capacity helps prevent acidification of cells and maintains a normal pH. D. Modulator of Glycolysis Another possible function of Cr is to modulate glycolysis. During sudden intense muscular activity, the glycolytic flux may increase several hundredfold. Also, during this time the concentration of PCr is decreased due to the rapid need for replenishing ATP in the active muscle via the Cr kinase reaction. In vitro evidence suggests that glycolysis may be stimulated by the decline in PCr levels. It is postulated that the key glycolytic enzyme, phosphofructokinase (PFK), is partially inhibited by physiological concentrations of PCr. During strenuous activity, when the PCr concentration is decreased, PFK is disinhibited and glycolysis is increased. The elevated glycolytic rate produces more available ATP for the increased energy demands required by the active muscle. IV. Anaerobic Glycolysis and Phosphocreatine Degradation During High-Intensity Exercise During high-intensity exercise, energy for the resynthesis of ATP is met primarily by the simultaneous break-down of PCr (Reaction III.A) and anaerobic glycolysis. Phosphocreatine depletion occurs faster than glycogen depletion, and as exercise continues, the high ATP demands are met primarily by anaerobic glycolysis. In fact, Boobis has estimated that the ATP from PCr declines more than 3 times as fast as the rate of glycolysis. PCr depletion occurs rapidly at the onset of all-out exercise. During 10 sec of all-out cycle ergometry, the stored phosphagens in the muscle have been estimated to contribute about 50% of the total supply of ATP, and peak power is reached during the first 5 sec. Table 1 shows the magnitude of PCr depletion during various forms of high-intensity exercise.
V. Fatigue During High-Intensity Exercise A general definition of fatigue proposed by Gibson and Edwards is a failure to maintain the required or expected force or power output. The onset of fatigue during high-intensity exercise may be linked to PCr depletion, pH alterations, or lactate accumulation. A. Phosphocreatine and Fatigue The concentration of PCr appears to correlate with the development of force and may contribute to fatigue. Studies using animal models support this notion. Infante demonstrated a direct relationship between external work performed and PCr breakdown in frog rectus abdominus muscles. Spande and Schottelius tetanically stimulated isolated mouse soleus muscles to determine the association between PCr degradation and fatigue and the relationship between force development and the PCr level in the muscle. A direct proportional relationship was observed between PCr concentration in the muscle and the force it can develop in an isometric tetanus. The reported relationship was postulated to be mediated through the reduction or insufficiency in the excitation-contraction coupling process caused by the reduced PCr levels. In humans, the importance of PCr in maintaining power output during sprinting has been demonstrated by Hirvonen et al., who observed that PCr stores were depleted in subjects after 5 to 7 sec following sprints of 40, 60, 80, and 100 m. Hirvonen et al. concluded that the decrease in running speed that occurred after 5 sec of exercise may be related to the decline in energy production via the lack of energy supply from the high-energy phosphate stores. This inference was based on correlations between sprinting performance and depletion of PCr stores in muscle. It is known that Type II muscle fibers possess higher initial levels and greater rates of utilization of PCr and glycogen than Type I muscle fibers. Also, PCr recovery seems to be slower in Type II fibers. The greater loss and slower recovery of PCr and glycogen following high-intensity exercise in Type II muscle fibers very likely contribute to the reduction in force during high-intensity exercise, as these muscle fibers are predominantly recruited in this type of activity. B. Importance of Phosphocreatine Resynthesis During Recovery The resynthesis of PCr during recovery has been shown to be an oxygen-dependent process exhibiting a biphasic pattern consisting of a fast and slow component. The half-time for the fast component is approximately 21-22 sec; it is greater than 170 sec for the slow component. C. pH Alterations and Lactate Accumulation The decline in power output observed during high-intensity exercise may be a result of lactic acid accumulation in the muscle which dissociates to the lactate anion and hydrogen cation [H+], thereby decreasing the pH of the muscle. A decreased pH has numerous effects on the intracellular and extracellular environments, and has been associated with contributing to the onset of fatigue by various mechanisms. Increased [H+] may have a direct action on the contractile apparatus by affecting either calcium or myosin ATPase. Acidosis may also affect equilibrium reactions such as the Cr kinase reaction, resulting in a more rapid depletion of PCr. Elevated [H+] and a reduced pH inhibit the two key enzymes involved in glycolysis, phosphorylase and phosphofructokinase. Finally, tension is reduced when arterial blood and muscle lactate concentration is in- creased independent of a change in pH. All these effects caused by an accumulation of [H+] and lactate result in a decline in the muscle's force capabilities. VI. Change in Total Muscle Creatine Concentration Following Cr Supplementation Harris et al. investigated whether supplemental Cr could increase plasma levels and ultimately lead to an increase in the total Cr pool. They first determined a dosage of Cr that would result in a significant increase in plasma Cr concentration. This was established at 5 g, which after 1 hour produced a mean peak of 795 μmol*L-l. The plasma Cr concentration then declined toward preingestion levels over the subsequent 6 to 7 hrs. Normal concentration in plasma is 50-100 μmol per L. To determine whether the total Cr content of muscle could be increased, supplementation with 5 g Cr monohydrate 4 to 6 times*day-1 for ≥ 2 days was attempted in 17 subjects. Before and after supplementation, a single muscle biopsy of the vastus lateralis was taken along with venous blood samples to analyze ATP, PCr, and Cr content along with routine hematology and biochemistry. Mean total Cr content in all subjects was 126.8 mmol*kg-l dry muscle prior to supplementation and148.6 mmol*kg-1 dry muscle afterward. Increase in total Cr was a result of both an increase in Cr and PCr, the latter representing anywhere from 20 to 40% of the increase. There was no increase in the ATP content in muscle associated with the increase in PCr content. VII. Phosphocreatine Resynthesis Following Cr Supplementation To investigate the effect of oral Cr ingestion on muscle PCr resynthesis after intense isometric muscle actions, Greenhaff et al. supplemented subjects with Cr monohydrate (20 g*day-1 for 5 days). Subjects then underwent 20 intense, percutaneous, electrically evoked (50 Hz) isometric muscle actions of the quadriceps lasting 1.6 sec interspersed with 1.6 sec rest periods. After 2 min recovery, the Cr supplemented group had PCr values 20% higher than controls, indicating an accelerated rate of PCr resynthesis following Cr ingestion. These findings were later supported by Greenhaff et al.. Eight subjects first underwent a protocol similar to that in the previous study (electrically evoked isometric muscle actions) followed by muscle biopsy samples taken after 0, 20, 60, and 120 sec recovery. Ten days later the same subjects repeated this protocol using the other leg, after ingesting 20 g Cr monohydrate*day-1 for the preceding 5 days. Results revealed that the 5 subjects with the lowest initial total Cr content had the greatest increase in total Cr concentration after Cr ingestion, 15 to 33% of initial total Cr concentration. The 3 subjects with initial total Cr concentrations ≥125 mmol*kg-1 dry matter experienced a relatively small increase in total Cr concentration after Cr supplementation (≈5%). The 5 subjects who had significant increases in total Cr concentration also had an increased rate of PCr resynthesis during recovery after Cr ingestion. Interestingly, during the first 40 sec of recovery, PCr resynthesis was almost identical in both treatments. However, the rate of PCr resynthesis was greater after Cr ingestion during the remainder of recovery, resulting in the mean muscle concentration being 30% higher at the end of recovery. Greenhaff et al. attributed the increased PCr resynthesis rate observed after 40 sec to an increased ability to maintain the free Cr concentration higher than the Km of creatine kinase for Cr (19 mmol*L-1) , pushing the creatine kinase reaction toward PCr resynthesis and ADP formation. VIII. Performance During High-Intensity Exercise Following Cr Supplementation The idea of delaying fatigue and achieving a high force production for a longer period of time has wide application. Many athletes rely heavily on the energy generation and resynthesis rate of PCr, for example, body-builders, power lifters, weight lifters, sprinters, rowers, swimmers, long jumpers, high jumpers, and football, basketball, hockey, volleyball, and soccer players. All these athletes may benefit from an increased store of PCr. There is evidence that supplementation with Cr monohydrate, 20-25 g*day-1 for 5-6 days, may be of benefit to athletes as indicated by improvements in performance compared to controls who received a placebo.
IX. Side Effects Associated With Cr Supplementation The only known side effect of Cr ingestion (20-25 g) above levels normally consumed in the diet (≈1 g), as noted in the literature, is an increase in body mass (BM). Most of the published studies consisted of a relatively short period of Cr supplementation, <1 week. There is little documented data on the effects of long-term Cr administration on BM. As early as 1926, Chanutin and Guy examined the fate of Cr when administered to humans. Serving as their own subjects, Chanutin consumed 250 g of Cr for 29 days while Guy consumed 340 g Cr for 44 days. Chanutin gained 3.2 kg and Guy gained 3.8 kg. Their diet consisted of ≈3,400 kcal per day and was Cr-free. In further support of the association between Cr ingestion and increases in BM, Earnest et al. supplemented 4 weight-trained men with 20 g Cr monohydrate per day for 28 days. Mean BM increased 1.7 kg. Increased BM associated with a shorter Cr ingestion period has also been demonstrated. All the studies used 20-30 g Cr monohydrate per day and the supplementation period was 5-7 days. Mean BM increases ranged from 0.9 to 1.8 kg in these investigations, which is in agreement with results obtained in our laboratory after supplementing subjects with Cr for 1 week. Table 3 summarizes the studies demonstrating an increased BM associated with Cr monohydrate ingestion.
The composition of weight gain in these studies was not determined, except for the study by Earnest et al. involving weight trained subjects. The authors reported no significant change in percent body fat or fat-free mass as assessed by hydrostatic weighing. Therefore, questions arise as to the body components contributing to the observed increase in BM recorded in the other studies, and also the mechanisms behind these increases. Balsom et al. proposed that the increase in BM may be due in part to an increase in total body water content. However, they postulated that part of the weight gain may be due to an increased diameter of the fast-twitch glycolytic muscle fibers, based on the fact that rats depleted of Cr exhibit a decreased diameter, and gyrate atrophy patients supplemented with low dosages Cr for 1 year exhibit an increased diameter of Type II muscle fibers. Balsom et al.
postulated that increased synthesis of contractile protein may be responsible for this increased "diameter. Several theories support the role of Cr as a positive effector in regulating muscle protein synthesis; however, the mechanism by which Cr acts is unknown.
lngwall suggests that Cr may act as a transcriptional or translational factor, or it may alter levels of chargerl tRNA's or amino acid pools specific for muscle protein synthesis. X. Practical Application The ergogenic effect of Cr ingestion is most likely a result of increased preexercise availability of Cr and PCr and enhanced PCr resynthesis during recovery from exercise. Therefore, athletes involved in activities that are predominantly anaerobic in nature (high-intensity, short duration) are most likely to benefit from Cr supplementation. In fact, subjects demonstrated a decrease in performance following a 6 km run after supplementation with Cr monohydrate. Individuals with lower basal levels of muscle Cr tend to gain the largest increases in muscle Cr, and hence are more likely to increase performance versus someone with already high concentrations of muscle Cr. Bodybuilders and weightlifters especially may benefit from the increased BM associated with Cr ingestion. If increased PCr levels are maintained for an extended time, individuals in training may benefit from being able to train at higher intensities for longer periods of time before fatigue sets in. More intense training should promote a better training response, assuming recovery is adequate to meet the increased demands placed on the individual. Muscle Cr appears to be "trapped" in the muscle and may stay elevated for an extended time (> 1 month) once the tissue levels have been elevated by prior Cr monohydrate supplementation. Recent evidence suggests that once the muscle Cr stores have been elevated by Cr supplementation (i.e., 20 g per day for 5 days), the dosage may be reduced to ≈3 g per day to maintain elevated Cr stores in the muscle. XI. Summary of Cr Supplementation Judging from the current research, Cr monohydrate supplementation in healthy individuals has an impact on performance during highintensity, intermittent exercise. Improvements in performance are particularly evident in individuals with already low levels of muscle Cr and PCr (<120 mmol per kg). In these subjects, Cr monohydrate supplementation has been proven to increase both Cr and PCr muscle concentration by as much as 30%. Increased Cr and PCr levels attenuate ATP degradation during high-intensity muscular activity. This is likely a result of an increased rate of ATP resynthesis from ADP. The increased availability of PCr as an energy source may decrease the usual dependence on anaerobic glycolysis for resynthesis of ATP. Thus the accumulation of lactate and [H+] associated with maximal rates of glycolysis are delayed, allowing the muscle to generate a high force for an extended time. Increases in performance during intermittent exercise are most likely explained by a greater availability of PCr in the activated muscles, particularly the Type II muscle fibers, as a result of a higher rate of resynthesis during recovery periods. A higher preexercise Cr and PCr concentration, as well as a smaller decrease in muscle pH, may also increase the amount of PCr available at the next exercise bout following a rest period. Figure 2 demonstrates possible mechanisms by which elevated levels of Cr and PCr in muscle may enhance muscular performance.
Although the length of the Cr ingestion period in most Cr supplementation studies was relatively short (5-6 days), the weight gain in subjects after supplementation was significant compared to subjects who received a placebo. The most logical explanation is an in- crease in total body water; however, an increase in lean tissue cannot be ruled out. There is evidence that Cr is a positive effector in regulating the biosynthesis of actin, myosin, and creatine kinase by developing muscle cells. In general, Cr supplementation may be advantageous to individuals in a broad spectrum of physical activities. Any athlete involved in a sport or activity that relies heavily on PCr as an energy source, especially those intermittent in nature, may benefit from elevated Cr stores in the muscle. Cr supplementation may also enhance the quality of training sessions by allowing athletes to complete their workouts at a higher intensity. Increased training intensity will help promote performance improvements in the shortest time possible within the individual's genetic limitations. Finally, the increased body mass associated with Cr supplementation could be especially beneficial to athletes involved in sports that require strength and power, or simply in- creased muscle mass (i.e., bodybuilding). Research should focus on the physiological consequences of long-term Cr supplementation and the optimal timing and administration of dosages required to maximize and maintain elevated muscle concentrations of Cr. The magnitude of Cr uptake into muscle is related to the observed increase in body mass and exercise performance, as well as to the resynthesis rate of PCr. Therefore it would be beneficial to find a way to enhance Cr uptake into muscle above that currently attainable (≈160 mmol per kg). Since the functional and structural characteristics of the Cr transporter in muscle has been described, it may be possible to achieve higher levels of Cr in the muscle by combining other substances with Cr to enhance its uptake into muscle. In devising the optimal schedule of dosages and timing of Cr administration to maximize 'storage in the muscle, it is important to re- member that the level of expression of the Cr transporter is regulated by the extracellular Cr concentration (i.e., increased extracellular Cr leads to down-regulation of the Cr transporter). Combining other nutritional ergogenic aids that may promote a synergistic effect with Cr should therefore be explored. Examples might include ingesting Cr in combination with substances that (a) enhance insulin release, (b) up-regulate the Na+ / K+-ATPase pumps involved in Cr transport, (c) increase the percentage of PCr in muscle (i.e., phosphate loading), (d) enhance the buffering capacity of the muscle, or (e) promote cellular swelling (i.e., taurine, glutamine, and glycine). There should also be focus on gaining a better understanding of the mechanisms of action that elevated Cr stores have on energetics and cell metabolism, specifically, the mechanisms by which Cr may (a) delay fatigue, (b) reduce lactate, ammonia, and hypoxanthine accumulation, (c) increase the rate of PCr resynthesis, (d) enhance protein synthesis, (e) increase the hydration status of the cell, and (f) act as a repartitioning agent in altering body composition. By Jeff S. Volek and William J. Kraemer Center for Sports Medicine, The Penn State University, University Park, PA As published in the Journal of Strength and Conditioning Research
Copyright 2001 - http://thefastlane.borghoms.com Last updated on 02 May 2002 |
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