During the past decade, the nutritional supplement creatine has been gaining popularity exponentially. First introduced to the general public in the 1990s, shortly after the Atlanta Olympic Games, creatine has become one of the most widely used nutritional supplements in the world. Recently, many athletes and teams have implemented oral creatine (Cr) supplementation in an effort to enhance sports performance, as creatine is not presently on the banned substance list by the International Olympic Committee. Thus, using this supplement would not constitute anything illegal or unethical on behalf of the athletes or coaches.
As the pressures to succeed in today’s society increase, athletes are aware that they not only need specifically designed training programs to achieve peak physical and mental condition, but they also need appropriately prescribed methods of restoration and regeneration. Consequently creatine has risen to the top of the modern athletes shopping list. Therefore it is important to address the following questions:
- Are there ergogenic effects associated with oral creatine supplementation?
- Does creatine supplementation benefit specific sports?
Creatine supplementation has been suggested (Dawson et al. 1995; Finn et al. 2001; Snow et al. 1998) as a means to "load" the muscle with creatine (Cr) and phosphocreatine (PCr). Loading the muscle with Cr and PCr would theoretically serve to improve the ability to produce energy during high intensity exercise as well as improve the speed of recovery.
Since creatine supplementation (CrS) has been shown to increase intramuscular PCr concentrations (Stout et al. 2000; McKenna et al. 1999; Vandenberghe et al. 1997), CrS would theoretically enhance the availability of energy during explosive, high-intensity exercise bouts and/or enhance the ability to recover from intense exercise.
It has also been argued (Abt & Reaburn, 2000; Stout et al. 2000; Vandenberghe et al. 1997) that CrS can increase the PCr concentration in skeletal muscle, thus allowing it to act by ‘mopping up’ the acid producing hydrogen ions produced during the breakdown of ATP and other anaerobic processes. Thus PCr helps to maintain optimal acid levels within the muscle and allows continued performance with minimal fatigue.
The purpose of this paper is to examine the scientific evidence presented and report on CrS usefulness as a performance-enhancing aid by identifying any potential ergogenic effects that this supplement may have on performance. To achieve this it is important to have an understanding of exercise physiology regarding energy metabolism and the role of creatine throughout this process.
CREATINE’S ROLE IN ENERGY METABOLISM
A French scientist named Chevereual is credited with first discovering creatine in 1832, but it was not until 1926 that scientists quantified creatine storage and retention in the body. Creatine is a compound that is both made within the body from amino acids and obtained through diet. Most of the body’s creatine is stored within skeletal muscle where it plays a role in metabolism, with the daily turnover of creatine for the average sized person of about 2g (Abt & Reaburn, 2000).
Broad (1998) explains that the energy for all out maximal effort exercise (lasting to 6 to 8 seconds) is primarily derived from limited stores of adenosine triphosphate (ATP) in the muscle. In this regard, the phosphate from ATP is cleaved off liberating energy for muscle contraction. During explosive exercise, the phosphate from phosphocreatine (PCr) stored in the muscle is also cleaved off to provide energy for resynthesis of ATP. This allows the ATP pool to be turned over several dozen times during an all out maximal effort exercise bout lasting 6 to 8 seconds. Additionally, the energy derived from the breakdown of PCr during recovery helps restore the ATP depleted during maximal effort explosive exercise.
The degree to which skeletal muscle will use PCr depends primarily on how intense the activity is. During most exercise or sport situations, aerobic energy provides most of the ATP needed by the working muscles. However, when the intensity exceeds the power of the aerobic system the muscle begins to rely on the anaerobic system, which includes the use of PCr and muscle glycogen as fuels. Consequently, during the most intense periods of exercise or sport, the muscle will tax the PCr store most highly. Thus, we can see why some have argued (Schneider et al. 1997; Dawson et al. 1995; Meir, 1995) that CrS may benefit certain athletes in particular sports.
WHAT DOES THE RESEARCH INDICATE?
In recent years, a number of studies have examined the effects of creatine supplementation (CrS) on muscle metabolism and/or high-intensity exercise performance. All studies that have measured muscle total creatine (TCr) (phospho creatine + creatine ) have reported an elevation in TCr after CrS involving a dose of 20-30 g·d-1Cr/day for 3-6 days. Some studies (Snow et al. 1998; McKenna et al. 1999) found that both resting muscle Cr and PCr content increased, whereas others reported significant increases in only PCr (Stout et al. 2000) or Cr (Becque, Lochmann, & Melrose, 2000).
Theoretically, an increase in TCr stores may provide an ergogenic effect during high intensity exercise by enhancing the rate of ATP synthesis during contraction and by improving the rate of PCr resynthesis during recovery (which may be beneficial for sport training such as repeated sprint activity). A recent investigation by Mujika et al. (2000) supports this theory, they concluded that acute Cr supplementation favourably affected repeated sprint performance and limited the decay in jumping ability in highly trained soccer players.
On a whole, the experimental evidence supporting an ergogenic effect for CrS is somewhat mixed. Several studies have demonstrated an improved high-intensity exercise performance after CrS (Mujika et al., 2000; Vandenberghe et al., 1997; Van Leemputte, Vandenberghe, & Hespel, 1999 ), whereas others have reported no beneficial effects (Gilliam et al. 2000; Snow et al. 1998; Barnett, Hinds & Jenkins, 1996).
STUDIES REPORTING ERGOGENIC BENEFIT
Most studies that have investigated the ergogenic value of CrS have reported significant increases in strength/power, sprint performance, and/or work performed during multiple sets of maximal effort muscle contractions. The improvement in exercise capacity has been attributed to increased TCr and PCr content thus resulting in greater resynthesis of PCr, improved metabolic efficiency and/or an enhanced quality of training promoting greater training adaptations. The following overviews some of the literature reporting ergogenic benefits of creatine supplementation.
For a weight lifter/body builder, gains in strength/power are often accompanied by muscle hypertrophy. Consequently, ingesting a nutritional supplement that can promote strength gains during training may be particularly beneficial. Vandenburghe et al. (1997) reported that CrS 20 g·d-1Cr/day for 4 day followed by 5 g·d-1Cr/day for 66-day promoted a 20 to 25% greater gain in 1RM strength in untrained women than subjects receiving a placebo, all of whom participated in a 70 day resistance-training program.
Furthermore, the gains in strength observed were maintained in subjects ingesting creatine during a 70-day detraining period. These findings indicate that CrS during resistance-training promotes significantly greater gains in strength.
While it is understandable that if creatine allows an athlete to train harder, athletes may get stronger over time, studies also indicate that short-term CrS may enhance peak power. For example, Dawson et al. (1995) reported that CrS (20 g·d-1Cr/day for 5 days) significantly increased peak power during the first set of 6 x 6-set sprints performed on a cycle ergometer.
An investigation by Becque, Lochmann, & Melrose (2000) involving twenty-three male volunteers with at least 1 yr of weight training experience tested arm flexor 1RM, upper arm muscle area, and body composition. Subjects ingested 5 g·d-1Cr/day four times per day for 5 days. After 5 days, supplementation was reduced to 2 g·d-1. Results indicated that CrS during arm flexor strength training lead to greater increases in arm flexor muscular strength, upper arm muscle area, and fat-free mass than strength training alone.
Multiple Sets of Maximal Effort Muscle Contractions
One of the most potentially beneficial effects of CrS for the weight lifter/body builder is that CrS supplementation has been reported to increase the amount of work performed during a series of maximal effort muscle contractions. For example, Volek et al. (1997) reported that during their double-blind investigation, CrS (25 g·d-1Cr/day for 7 days) resulted in a significant increase in the amount of work performed during five sets of bench press and jump squats in comparison to a placebo group. Bench press and squat increases were greater in creatine (24% and 32%, respectively) than placebo groups (16% and 24%, respectively).
The results of a study performed by Vandenberghe et al. (1997) indicated that CrS (20 g·d-1Cr/day for 4 days) increased muscle PCr concentration by 6%. Thereafter, this increase was maintained during 10 weeks of training associated with low-dose creatine intake (5 g·d-1Cr/day). Compared with placebo, maximal strength of the muscle groups trained, maximal intermittent exercise capacity of the arm flexors, and fat-free mass were increased 20-25%, 10-25%, and 60% more, respectively, during CrS.
It has also been reported that CrS may improve single effort and/or repetitive sprint performance particularly in sprints lasting 6 to 30 seconds with 30 seconds to 5 minutes of rest recovery between sprints. For example, Dawson et al. (1995) found that creatine (5 g·d-1Cr/day four times per day for 5 days) significantly increased work performed during the first of 6 x 6-set cycle ergometer sprints with 30-second recovery between sprints. These results are supported by Schneider et al. (1997) who reported that CrS (5 g·d-1Cr/day five times per day for 7 days) significantly improved 5 x 15 set cycle ergometer sprints with 60-s recovery between sprints.
Finally, Meir (1995) investigated creatine supplementation (20 g·d-1Cr/day for 4 days) on seventeen graded professional rugby league players competing in the Winfield Cup competition. The work/rest ratio for athletes participating in this sport has been estimated at 1:6-8. This would suggest that professional rugby league could be considered an interval activity. On average for every 5 seconds of intense activity, 30 seconds are spent in less intense activities. The results concluded that CrS may be useful in sports such as rugby league that require repeated sprints and that CrS may be advantageous as an aid improving both training and performance.
STUDIES REPORTING NO ERGOGENIC BENEFIT
A number studies have reported no ergogenic benefit from CrS, although the reason for the lack of ergogenic effect observed in these studies is not clear. It is possible that individual variability in response to CrS may account for the lack of ergogenic benefit reported in these studies as well as differences in experimental design.
Snow et al. (1998) utilised a double-blind crossover design on untrained men performing a 20 second maximal sprint on an cycle ergometer after CrS (30 g·d-1Cr/day for 5 days). The data demonstrated that CrS increased muscle total Cr content, but the increase did not induce an improved sprint exercise performance or alterations in anaerobic muscle metabolism. In conclusion, Snow et al. (1998) reported that a small, yet significant, increase in muscle TCr content occurred but this increase, however, did not result in an improved sprint-exercise performance or any alterations in markers of muscle anaerobic energy metabolism during, and in recovery from, sprint exercise.
Similar results have been found (McKenna et al., 1999; Odland et al., 1997) that support these findings. In their investigation Odland et al. (1997) had subjects perform of a 30 second maximal cycling (Wingate) task after CrS (20 g·d-1Cr/day for 3 days). It was found that three days of CrS did not increase resting muscle PCr, nor did it affect the single short-term maximal cycling performance. The most likely explanation for this is that the increase in muscle TCr content after CrS was insufficient to induce an enhanced sprint performance and to allow an improved rate of PCr resynthesis after exercise. Alternatively, it is also possible that CrS does not enhance sprint performance during brief maximal exercise.
A more recent study by Gilliam et al. (2000) examined the effect of CrS (5 g·d-1Cr/day four times per day for 5 days) on the decline in peak isokinetic torque of the quadriceps muscle group during an endurance test. Subjects performed isokinetic strength tests that consisted of five sets of 30 maximum volitional contractions with a 1-minutes rest period between sets. Based on group comparisons they were unable to detect an ergogenic effect of oral CrS on the decline in peak torque during isokinetic exercise.
This review has discussed some of the actions of creatine supplementation on muscle metabolism and performance. The available research indicates that CrS can increase muscle PCr content (but not in all individuals), which may improve performance involving short periods of extremely powerful activity, especially during repeated bouts of activity. It does not appear that CrS increases maximal isometric strength, the rate of maximal force production, or aerobic exercise performance.
However, not all studies have reported ergogenic benefit - due possibly to differences in subject response to CrS, length of supplementation, evaluated exercise criteria, and/or the amount of recovery observed during repeated bouts of exercise. A possible explanation for conflicting findings may also relate to inconsistencies and variances of experimental design used to examine the effects of CrS on exercise performance.
There is no definitive evidence that CrS causes gastrointestinal, renal, and/or muscle-cramping complications. The only significant side effect reported was that of weight gain within the first few days, which is likely due to water retention related to creatine uptake in the muscle.
At the completion of this review the author is of the opinion that another possible contributing factor in the apparent improvements in performance resulting from CrS, is enhanced psychological state. It is – in the opinion of the author - plausible that the apparent high expectations for performance enhancement through use of creatine, may have an influence on the actual result achieved. If an athlete believes that CrS will enhance their performance, this expected ergogenic benefit may in fact result in improved performance. Regardless, at this point in time CrS appears to be a safe and effective nutritional strategy to enhance exercise performance in specific sports.
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