Does Dietary Creatine Supplementation Have a Role to Play in Exercise Metabolism?

The Biosynthesis and Distribution of Creatine

Creatine (Cr), or methyl guanidine-acetic acid, is a naturally occurring compound.
In healthy individuals, the total body Cr pool is approximately 120 g, 95
percent of which is found in skeletal muscle. This pool is replenished at a
rate of approximately 2 g ^. d^-1 by endogenous Cr synthesis and/or dietary Cr
intake, e.g., meat and fish. Oral ingestion of Cr has been demonstrated to
suppress biosynthesis, but this response is abolished upon the cessation of
supplementation.¹

The precursors of Cr synthesis were first determined by labeling nitrogenous compounds with ¹⁵N and isolating subsequent creatine formed.²³ Synthesis proceeds via two successive reactions involving two enzymes. The first reaction is catalyzed by glycine transamidinase and results in an amidine group being reversibly transferred from arginine to glycine, forming guanidinoacetic acid. The second reaction involves the irreversible transfer of a methyl group from S-adenosylmethionine (SAM) catalyzed by guanidinoacetate methyltransferase, resulting in the methylation of guanidinoacetate and the formation of Cr. The distribution of the two enzymes across tissues varies between mammalian species. In man, the liver is the major site of de novo Cr synthesis; however, the pancreas is also known to have some capability. As little Cr is found in the major sites of synthesis, it is logical to assume that transport of Cr from its sites of synthesis to storage must occur, thus allowing a separation of biosynthesis from utilization.¹

Muscle Cr uptake occurs actively against a concentration gradient, possibly involving Cr interacting with a specific membrane site that recognizes the amidine group.⁴ Two mechanisms have been proposed to explain the very high Cr concentration within skeletal muscle. The first involves the transport of Cr into muscle by a specific saturable entry process. In this respect, a specific Cr transporter has recently been identified in skeletal muscle, heart, and brain.⁵ The second mechanism necessitates the intracellular trapping of Cr. Approximately 60 percent of muscle total Cr store exists in the form of phosphocreatine (PCr), which, due to its phosphorylated state, is unable to pass through membranes, thus trapping Cr. This entrapment results in the generation of a concentration gradient. However, phosphorylation is not the sole mechanism of cellular Cr retention. For example, the binding of Cr to intracellular components and the existence of restrictive cellular membranes have been proposed as mechanisms that facilitate muscle Cr retention.¹

Creatinine has been established as the sole end product of Cr degradation being formed nonenzymatically in an irreversible reaction.^3,6 As skeletal muscle is the major store of the body creatine pool, this is the major site of creatinine production. The daily renal creatinine excretion is relatively constant in an individual, but can vary between individuals, depending on the total muscle mass in healthy individuals.^3,6

The Role of Creatine in Muscle Energy Metabolism and Fatigue

In human skeletal muscle, Cr is present at a concentration of about 125 mmol ^. kg^-1 dry muscle (dm), of which approximately 60 percent is in the form of PCr at rest. A reversible equilibrium exists between Cr and PCr (PCr + ADP + H⁺ = ATP + Cr), and together they function to maintain intracellular adenosine triphosphate (ATP) availability, modulate metabolism, and buffer hydrogen ion accumulation during contraction. The availability of PCr has been proposed as one of the most likely limitations to muscle performance during intense, fatiguing, short-lasting exercise, i.e., where the anaerobic ATP demand is very high.⁷ This conclusion has been drawn from studies involving short bouts of maximal electrically evoked muscle contraction and/or voluntary dynamic exercise, and from animal studies in which the muscle Cr store has been depleted, prior to maximal electrical stimulation, using the Cr analog ß-guanidinopropionate (ß-GPA).
More recent studies have taken the complexity of investigation one step further
by implicating the availability of PCr specifically in type II muscle fibers as
being of critical importance to the maintenance of performance during maximal
short-lasting exercise.⁷ The availability of free Cr has also been
ascribed a central role in the control of phosphocreatine resynthesis, its
function in the regulation of mitochondrial ATP resynthesis, and thereby PCr
resynthesis, having been the subject of much debate.^8,9

The Effect of Creatine Ingestion on Muscle Creatine Concentration in Humans

Studies earlier this century demonstrated that Cr administration resulted in a small increase in urinary creatinine excretions.^2,3,6 However, there was no increase in excretion until a significant amount of the administered Cr had been retained. It was also noted that Cr retention was greatest during the initial stages of administration. In general, urinary creatinine excretion rose slowly during prolonged Cr administration and, upon cessation, approximately 5 weeks elapsed before a significant fall in creatinine excretion was observed.

These studies invariably involved periods of chronic Cr ingestion. However, with the application of the muscle biopsy technique, it has become apparent that the ingestion of 20 g of Cr each day in solution for 5 days (4 x 5 g doses) can lead to an average increase in muscle total Cr concentration of about 20 percent, of which approximately 30 percent is in the form of PCr.^10,11 In agreement with earlier urinary excretion studies, it appears that the majority of muscle Cr retention occurs during the initial days of supplementation; e.g., about 30 percent of the administered dose is retained during the initial 2 days of supplementation, compared with 15 percent from days 2-4.¹⁰ The natural time-course of muscle Cr decay following 5 days of 20 g ^. d^-1 ingestion occurs over the course of several weeks rather than days.¹²

As might be expected, lower dose Cr supplementation (e.g.,3 g ^. d^-1 for 2 weeks) is less effective at raising muscle Cr concentration than is a 5-day regimen of 20 g day-‘.’2 However, following 4 weeks of supplementation at this lower dose, muscle Cr accumulation is no different when regimens are compared.¹² It also appears that muscle Cr stores remain elevated for several weeks when the supplementation regimen of 20 g ^. d^-1 for 5 days is followed by lower dose supplementation (2 g ^. d^-1),¹² which seems to agree with the earlier suggestion that Cr is trapped within skeletal muscle once absorbed and that muscle Cr degradation to creatinine occurs at a rate of about 2 g ^. d^-1.

The Effect of Creatine Ingestion on Exercise Performance

As stated previously a reversible equilibrium exists between Cr and PCr, and the
development of fatigue during maximal short-duration exercise has been associated
with the depletion of muscle PCr stores.⁷ Creatine in its free and phosphorylated
forms therefore occupies a pivotal role in the regulation and homeostasis of
skeletal muscle energy metabolism and fatigue. However, despite these important
roles for Cr during muscle contraction, little has been published relating
to Cr ingestion and exercise Derformance. In 1981, Sipila et al.¹³ reported
that, in a group of patients receiving 1 g of creatine ^. day^-1 as a treatment
for gyrate atrophy, there was a comment from some of a sensation of strength
gain following a 1-year period of supplementation. Indeed, Cr ingestion was
shown to reverse the type II muscle fiber atrophy associated with this
disease, and one athlete in the group of patients improved his personal
best record for the 100 meter sprint by 2 seconds.

Based on more recently published work, it would appear that the ingestion of
4 x 5 g of Cr ^. day^-1 for 5 days will significantly
increase exercise performance in healthy male volunteers during repeated bouts
of fatiguing, short-lasting maximal exercise, e.g., maximal isokinetic knee
extensor exercise, maximal dynamic cycling exercise, maximal isokinetic
cycling exercise, and controlled “track” running experiments.^14-17
The consistent finding from these studies is that Cr ingestion significantly
increased exercise performance by 5-7 percent by sustaining force or work
output during exercise. Some studies have also reported an increase in
maximal strength or torque at the onset of contraction.¹⁵ More
recently, however, results from our laboratory indicate that approximately
30 percent of subjects who have ingested Cr in an attempt to increase
muscle Cr concentration have failed to retain substantial quantities
of Cr.^10,18 These results have also shown that any improvement
in exercise performance as a consequence of Cr supplementation is clearly
associated with the extent of muscle Cr retention during supplementation;
i.e., those individuals who experienced the highest muscle Cr
accumulation during supplementation were those who demonstrated the
better improvements in exercise performance.¹⁸ It would appear therefore that the extent of muscle Cr retention during feeding is critical to subsequent exercise performance.

In support of this conclusion, further results from our laboratory have shown that those individuals who experienced more than a 25-percent increase in total Cr muscle concentration following 5 days of Cr ingestion also showed an accelerated rate of PCr resynthesis during 2 minutes of recovery from intense, fatiguing muscular contraction. Conversely, those individuals who experienced no or little Cr accumulation during ingestion (on average an 8-percent increase) showed no measurable change in PCr resynthesis during recovery.¹⁰

Creatine’s Mechanism of Action

The exact mechanism by which Cr ingestion improves performance during maximal
exercise is not yet clear. The available data indicate that it may be related
to the stimulatory effect that Cr has upon pre-exercise PCr availability¹⁸
and on PCr resynthesis during exercise and recovery.¹⁰ Given that
PCr availability is generally thought to limit exercise performance during
maximal exercise, both of these effects would increase muscle contractile
capability by maintaining ATP turnover during exercise. This suggestion is
supported by reports showing that the accumulation of plasma ammonia and
hypoxanthine (accepted markers of skeletal muscle adenine nucleotide loss)
are reduced during maximal exercise following Cr ingestion, despite a higher
work output being achieved.^14,16 More coDvincing evidence comes
from a recent study showing that Cr supplementation can reduce the extent
of muscle ATP degradation by 25 percent during maximal isokinetic cycling
exercise, while, at the same time, increasing work output.¹⁸

Future Work

The precise biochemical route by which an increase in muscle Cr concentration
produces an improvement in muscle contractile function during maximal exercise
remains to be clearly demonstrated and is currently the subject of intense
debate. This area of research should be a focus of future in vivo and in vitro
work.

Research has shown that the extent of muscle Cr accumulation during supplementation varies greatly between individuals. Given that improvements in maximal exercise performance and PCr resynthesis during recovery from exercise appear to be critically dependent on the extent of muscle Cr accumulation during Cr supplementation, future work should focus on elucidating the principal factors regulating muscle Cr transport in man. It would also be of interest to determine the mechanism by which muscle Cr transport is upregulated in vegetarians and downregulated during daily Cr feeding. This may offer some insight into the mechanisms regulating the exaggerated loss of muscle Cr during fasting and disease. Finally, the consequences of long-term Cr supplementation on health and lifestyle in normal and diseased states requires investigation.

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18. Casey A, Constantin-Teodosiu D, Howell S. Hultman E, Greenhaff PL. Creatine supplementation favourably affects performance and muscle metabolism during maximal intensity exercise in humans. Am J Physiol, in press.