Creatine Supplementation
Beth Lulinski, M.S., R.D.
Creatine is marketed as "nature's muscle builder" and "the most legitimate sports supplement around." Professional and amateur athletes alike are gobbling up this alleged ergogenic aid, hoping to increase their strength and performance. Creatine supplementation is claimed to increase muscle power by playing a role in the transfer of energy to help the muscle contract. Supplement labels state that "creatine is converted to phosphocreatine, which is important for short energy bursts such as sprinting and weight lifting" and that "depletion of phosphocreatine can result in muscle fatigue and fading muscle power." Claims are also made that supplementation increases muscle body mass.
Health-food stores sell creatine supplements in capsule, chewable, and powdered form, the most popular being the powder. One teaspoon of powder contains 5 grams (g) of creatine monohydrate. The recommended daily dose is 1-2 teaspoons dissolved in 8 ounces of water or sweetened beverage. Manufacturers and distributors suggest a five- to seven-day loading phase with intake of 10-20 g (2-4 scoops) daily to fill up the muscle. The maintenance phase of 5-10 g/day is recommended before and/or immediately following a workout. This protocol is claimed to increase creatine muscle stores by 20-50%.
Role In Muscle Contraction
To meet the demands of a high-intensity exercise, such as a sprint, muscles derive their energy from a series of reactions involving adenosine triphosphate (ATP), phosphocreatine (PCr), adenosine diphosphate (ADP), and creatine. ATP, the amount of which is relatively constant, provides energy when it releases a phosphate molecule and becomes ADP. ATP is regenerated when PCr donates a phosphate molecule that combines with ADP. Stored PCr can fuel the first 4-5 seconds of a sprint, but another fuel source must provide the energy to sustain the activity. Creatine supplements increase the storage of PCr, thus making more ATP available to fuel the working muscles and enable them to work harder before becoming fatigued [1].
Sources
The body's pool of creatine can be replenished either from food (or supplements) or through synthesis from precursor amino acids. Dietary sources include beef, tuna, cod, salmon, herring, and pork [2]. The normal dietary intake of creatine is 1-2 g/day, although vegetarians may consume less [3,4]. Dietary creatine is absorbed from the intestines into the bloodstream. If the dietary supply is limited, creatine can be synthesized from the body stores of the amino acids glycine, arginine, and methionine. The kidneys use glycine and arginine to make guanidinoacetate, which the liver methylates to form creatine [1], which is transported to the muscle cells for storage. It is also stored in the kidneys, sperm cells, and brain tissue [5].
The maximum amount of creatine the body can store is about 0.3 gram per kilogram of body weight [6]. The creatine content of skeletal (voluntary) muscles averages 125 millimoles per kilogram of dry matter (mmol/kg/dm) and ranges from about 60 to 160 mmol/kg/dm. Approximately 60% of muscle creatine is in the form of PCr. Human muscle seems to have an upper limit of creatine storage of 150 to 160 mmol/kg/dm. Athletes with high creatine stores don't appear to benefit from supplementation, whereas individuals with the lowest levels, such as vegetarians, have the most pronounced increases following supplementation. Without supplementation, the body can
replenish muscle creatine at the rate of about 2 g/day [7].
Although creatine is a natural component of food, the amount of food required to supersaturate the muscle with PCr may not be feasible. For example, it could require 22 pounds of meat daily [8]. If creatine monohydrate is proven to be a safe and effective ergogenic aid, creatine supplementation may be the simplest way to increasing muscle stores. It may be beneficial to avoid caffeine if taking creatine supplements. One study showed that caffeine diminished strength gains seen with creatine use [9].
Evidence Supporting Use
Several studies support the use of creatine supplementation for enhancing activities that require short periods of high-intensity power and strength. These include weightlifting, sprinting, and rowing.
One study demonstrated that daily supplementation with 5 g of creatine monohydrate increased the intracellular creatine and PCr content of quadriceps muscle in 17 human subjects. Those with the lowest initial total creatine content had the greatest increase. In addition, exercise enhanced creatine uptake in muscle. No adverse effects were reported [5].
Another study found that one week of creatine supplementation at 25 g/day enhanced muscular performance during repeated sets of bench press and jump squat exercise. Creatine supplementation appeared to allow the subjects to complete their workouts at a higher intensity. The researchers concluded that, over time, working at higher intensities may provide a more intense training stimulus and improved muscular adaptations [10].
Another study demonstrated that females receiving 4 days of high-dose creatine intake (20 g/day) followed by low-dose creatine intake (5 g/day) during 10 weeks of resistance training (3hours/week) increased muscle PCr concentrations by 6%. Also, maximal strength of the muscle groups trained increased by 20-25%, maximal intermittent exercise capacity of the arm flexors increased by 10-25% and fat-free mass increased by 60% [11].
A double-blind study provided 20 g/day of creatine monohydrate for 5 days to qualified sprinters and jumpers who performed 45 seconds of continuous jumping and 60 seconds of continuous treadmill running. Supplementation enhanced performance in the jumping test by 7% for the first 15 seconds and 12% for the next 15 seconds, but there was no difference for the final 15 seconds. There was a 13% improvement in the time of intensive running to exhaustion [12].
Another double-blind study supplemented with 18.75 g/day of creatine monohydrate for 5 days prior to high-intensity intermittent work to exhaustion, and then 2.25 g/day during testing. The workouts consisted of cycling to exhaustion using several protocols: (a) nonstop, (b) 60 seconds work/120 seconds rest, (c) 20 seconds work/40 seconds rest, and (d) 10 seconds work/20 seconds rest. Creatine supplementation significantly increased the total work time for all four protocols [13].
Another study tested male subjects performing two bouts of 30 second isokinetic cycling before and after ingesting 20 g creatine monohydrate daily for 5 days. Work production improved about 4%. Cumulative increases in both peak and total work production over the two exercise bouts were positively correlated with the increase in muscle creatine [14].
A 12-week placebo-controlled study of 19 weightlifters in their mid-twenties found that the creatine group could lift more weight and had greater increases in fat-free mass and muscle-fiber size than did the placebo group. The researchers thought that the creatine let the athletes who used it train harder [15].
Three additional studies suggest that creatine supplementation may not be beneficial for running velocity, sprint swimming performance, or a maximal cycling effort [7]. Short bouts of repeated anaerobic activity have shown some potential benefits with creatine supplementation use in a laboratory setting. However, creatine supplementation has not been shown to enhance single-event performance such as stationary cycling [16-19]. Taken together, these studies do not support creatine supplementation to enhance aerobic activities such as distance running.
Other areas of research include therapeutic uses of creatine to help patients with muscle wasting caused by disease states such as muscular dystrophy and amyotrophic lateral sclerosis (ALS). Small-scale preliminary studies show some gains in strength may be possible for these patients, which could improve their quality of life. One study of 81 patients with various neurologic diseases found that giving 10 g/day of creatine for five days, followed by 5 grams for another week, increases their muscle strength by about 10% [20]. Large-scale studies should be done before recommendations are made to such patients.
Adverse Effects
Creatine supplementation often causes weight gain that can be mistaken for increase in muscle mass. Increasing intracellular creatine may cause an osmotic influx of water into the cell because creatine is an osmotically active
substance [10]. It is possible that the weight gained is water retention and not increased muscle. The retention of water may be connected to reports of muscle cramps, dehydration, and heat intolerance when taking creatine supplements. It would be prudent to encourage proper hydration for creatine users. Further research is needed to investigate these and other possible side effects.
Creatine is classified as a "dietary supplement" under the 1994 Dietary Supplement Health and Education Act and is available without a prescription. Creatine is not subjected to FDA testing, and the purity and hygienic condition of commercial creatine products may be questionable [21]. A 1998 FDA report lists 32 adverse creatine-associated events that had been reported to FDA. These include seizure, vomiting, diarrhea, anxiety, myopathy, cardiac arrhythmia, deep vein thromboses and death. However, there is no certainty that a reported adverse event can be attributed to a particular product [22]. A recent survey of 28 male baseball players and 24 male football players, ages 18 to 23, found that 16 (31%) experienced diarrhea, 13 (25%) experienced muscle cramps, 7 (13%) reported unwanted weight gain, 7 (13%) reported dehydration, and 12 reported various other adverse effects [23].
Caution Urged
There appears to be some potential for creatine supplementation. However, many questions remain. Are there any long-term harmful effects from supplementation? Is there a point where enhanced performance levels off from long-term supplement usage? What effect does "stacking" or taking two ergogenic aids simultaneously have on the body? What happens if you immediately stop taking the creatine supplement? Is the enhanced performance great enough to warrant the expense of the supplement? Until further research answers these questions, creatine is not recommended for the average athlete.
References
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Juhn MS and others. Oral creatine supplementation in male collegiate athletes: A survey of dosing habits and side effects. Journal of the American Dietetic Association 99:593-594, 1999.
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Ms. Lulinski is a medical nutritionist. Susan Moore, MS, RD, Susan Parry Mandell, MS, RD, Thomas J. Wheeler, PhD, and Manfred Kroger, PhD, also helped produce this article.
