Creatine (Cr)—methylguanidino acetic acid is a naturally
occurring compound that was first described by
Chevreul in 1832. Its name is derived from the Greek
word kreas (flesh).
Creatine is found in abundance in skeletal muscle (red meat) and fish.
It is essential in energy transmission and storage via creatine kinase (CK). The
daily Cr dosage is obtained by both endogenous synthesis
and via nutritional intake, followed by absorption in
the intestine (1). Creatine supplementation is widespread
among sportspersons because of its documented and/or
presumed ergogenic effects (2–4). In addition, supplementation
with Cr has proven to be instrumental for the treatment
of rare inborn errors of metabolism due to defects in
Cr biosynthesis enzymes (5–8).
Creatine is stored in high concentrations in skeletal
and heart muscles and to a lesser extent in the
brain. It exists in both free and phosphorylated form
[phosphocreatine (PCr)] and is important for maintaining
high ratios between adenosine triphosphate (ATP) and
adenosine diphosphate (ADP). Upon increases in workload,
ATP hydrolysis is initially buffered by PCr via the
CKreaction. During high-intensity exercise, PCr in muscle
is depleted within several seconds. Whether de novo Cr
biosynthesis occurs in the brain or whether Cr is taken up
into the brain through the blood–brain barrier, is currently
a matter of debate.
DEFICIENCY AND SUPPLEMENTATION
Patients with Cr deficiency syndromes (CDS), that is, patients
with a Cr biosynthesis defect or a Cr transporter
defect, have developmental delay and mental retardation
(MR), indicating that Cr is crucial for proper brain function.
Surprisingly, however, CDS patients do not suffer
from muscular or heart problems. Those with a Cr biosynthesis
defect, in contrast to Cr transporter-deficient subjects,
can partly restore their Cr pool in brain upon Cr
Creatine supplementation, due to its ergogenic effects,
has become a multimillion dollar business (3). In the
Western world, Cr has received wide public interest. A
simple search on “creatine” in the World Wide Web using
common database search engines results in more than
500,000 entries. Besides the use by sportspersons, Cr supplementation
is explored in several animal models of neuromuscular
disease (i.e., Huntington and Parkinson disease,
amyotrophic lateral sclerosis) and in human disease
(3,6,11,12). A recent study suggests that Cr supplementation
increases intelligence and memory performance tasks
The goal of this entry is to provide an overview on Cr
and its metabolism in health and disease. The functions of
Cr and PCr, Cr biosynthesis, its degradation, tissue distribution,
transport and molecular aspects, as well as the benefits
and risks of Cr supplementation are discussed. (For
in-depth reviews, see Refs. 2, 3, 6 and references therein.)
BIOCHEMISTRY AND FUNCTION
Creatine is a naturally occurring guanidino compound.
Its chemical structure is depicted in Figure 1. Creatine is a
hydrophilic, polar molecule. Phosphocreatine is zwitterionic,
with negatively charged phosphate and carboxylate
groups and a positively charged guanidino group.
Creatine Synthesis Biosynthesis
The transfer of the amidino group of arginine to glycine
yielding L-ornithine and guanidinoacetic acid (GAA) represents
the first step in the biosynthesis of Cr and is performed
by L-arginine:glycine amidinotransferase (AGAT;
EC 18.104.22.168). This reaction is reversible and occurs in mitochondria,
into which arginine has to be taken up for guanidinoacetate
biosynthesis. The human AGAT mRNA encodes
a 423-amino acid polypeptide including a 37-amino
acid mitochondrial targeting sequence. The AGAT gene is
located on chromosome 15q15.3, is approximately 17 kb
long, and consists of 9 exons.
The second step involves the methylation of GAA
at the amidino group by (S)-adenosyl-L-methionine:Nguanidinoacetate
methyltransferase (GAMT; EC 22.214.171.124),
whereby Cr is formed. The methyl group is provided
by (S)-adenosylmethionine. The human GAMT mRNA
encodes a 236-amino acid polypeptide. The gene is located
on chromosome 19p13.3, is approximately 12 kb long, and
consists of 6 exons.
Creatine is produced by chemical synthesis, mostly from
sarcosine and cyanamide. This reaction is prone to generation
of contaminants such as dicyandiamide, dihydrotriazines,
or Crn (14). Some manufacturers may fail to separate
these contaminants from Cr. The toxicological profiles
of these contaminants are often not known. Dicyandiamide
liberates hydrocyanic acid (HCN) when exposed
to strongly acidic conditions (such as in the stomach). For
human consumption, only pure preparations of Cr should
thus be allowed. Unfortunately, no generally accepted and
Schematic representation of the creatine kinase (CK) reaction,
and chemical structures of creatine (Cr) and phosphocreatine (PCr).
meaningful quality labels are yet in place that would allow
a consumer to judge the origin and quality of Cr in
a given commercial product. Moreover, for most studies
published so far, it is not possible to correlate the presence
or lack of ergogenic, preventive, or adverse side effects
with the quality of the many Cr preparations used.
Creatine Function (CK Reaction)
Creatine is involved in ATP regeneration via the CK reaction.
The phosphate group of PCr is transferred to ADP
to yield Cr and ATP, the “universal energy currency” in
all living cells. The CK reaction serves as an energy and
pH buffer and has a transport/shuttle function for high energy
Several CK subunits exist that are expressed in a
tissue- and/or spatial-specific manner. In mammals, four
CK isoforms exist: the cytosolic M-CK (M for muscle) and
B-CK (B for brain) subunits form dimeric molecules, that
is, the MM-, MB-, and BB-CK isoenzymes. The two mitochondrial
CK isoforms, ubiquitous Mi-CK and sarcomeric
Mi-CK, are located in the mitochondrial intermembrane
space and form both homodimeric and homo-octameric
In fast-twitch skeletal muscles, a sizeable pool of PCr
is available for immediate regeneration of ATP, which is
hydrolyzed during short periods of intense work. In these
muscles, the cytosolic CK activity is high and “buffers”
the cytosolic phosphorylation potential that seems to be
crucial for the proper functioning of a variety of reactions
driven by ATP. Slow-twitch skeletal muscles, the heart,
and spermatozoa depend on a more continuous delivery
of high-energy phosphates to the sites of ATP utilization.
In these tissues, distinct CK isoenzymes are associated
with sites of ATP production (e.g., Mi-CK in the mitochondrial
intermembrane space) and ATP consumption
[e.g., cytosolic CK bound to the myofibrillar M line, the
sarcoplasmic reticulum , or the plasma membrane] and
fulfill the function of a “transport device” for high-energy
phosphates. The -phosphate group of ATP, synthesized
within the mitochondrial matrix, is transferred by Mi-CK
in the mitochondrial intermembrane space to Cr to yield
ADP and PCr. ADP may directly be transported back to
the matrix where it is phosphorylated to ATP. Phosphocreatine
leaves the mitochondria and diffuses through the
cytosol to the sites of ATP consumption. There, cytosolic
CK isoenzymes locally regenerate ATP and thus warrant
a high phosphorylation potential in the vicinity of the respective
ATPases. Subsequently, Cr diffuses back to the
mitochondria, thereby closing the cycle. According to this
hypothesis, transport of high-energy phosphates between
sites ofATP production andATP consumption is achieved
mainly by PCr and Cr. The CK system is required to allow
most efficient high-energy phosphate transport, especially
if diffusion of adenine nucleotides across the outer mitochondrial
membrane is limited.
Tissue Distribution of Creatine and of Its Biosynthesis Enzymes
In a 70-kg man, the total body creatine pool amounts to
approximately 120 g (1). Creatine and PCr are found in
tissues with high and fluctuating energy demands such
as skeletal muscle, heart, brain, spermatozoa, and retina.
In skeletal and cardiac muscle, approximately 95% of the
total bodily Cr is stored, and the concentration of total
creatine may reach up to 35 mM. Intermediate levels are
present in brain, brown adipose tissue, intestine, seminal
vesicles and fluid, endothelial cells, and macrophages.
Low levels are found in lung, spleen, kidney, liver, white
adipose tissue, blood cells, and serum (25–100 M) (2).
Until recently, GAA biosynthesis was presumed to
occur mainly in the kidney (and pancreas), where AGAT
is highly expressed, followed by its transport via the blood
and uptake of GAA into the liver, the presumed major site
of the second reaction, the methylation of GAA by GAMT.
Current knowledge suggests that AGAT and GAMT expression
is not limited to these organs. Synthesis outside
of these organs may allow local supply of Cr (e.g., in brain;
see creatine biosynthesis in mammalian brain) and may,
to a minor extent, contribute to the total Cr content in the
Creatine Accumulation: Transporter-Mediated
Cellular transport is of fundamental importance for creatine
homeostasis in tissues devoid of Cr biosynthesis. Creatine
needs to be taken up against a steep concentration
gradient [muscle (mM), serum (M)]. The Cr transporter
gene (SLC6A8) (MIM300036) has been mapped to chromosome
Xq28. Northern blots indicated that this gene is expressed
in most tissues, with the highest levels in skeletal
muscle and kidney, and somewhat lower levels in colon,
brain, heart, testis, and prostate. The SLC6A8 gene product
is a member of a superfamily of proteins, which includes
the Na+-dependent and Cl−-dependent transporters responsible
for uptake of certain neurotransmitters. The Cr
transporter gene spans approximately 8.4 kb, consists of
13 exons, and encodes a protein of 635-amino acids.
Creatine can be cleared from the blood via either uptake
into different organs by the Cr transporter or by excretion
via the kidney. There is evidence that tissue uptake
204 Salomons et al.
of Cr may be influenced by carbohydrates, insulin, caffeine,
and exercise and that transporter molecules located
in kidney are able to reabsorb Cr. Nevertheless, Cr is found
under normal conditions in urine in various amounts. The
main route for clearance of Cr is via creatinine excretion.
Creatine and PCr are nonenzymatically converted to creatinine.
The rate of creatinine formation, which mainly
occurs intracellularly, is almost constant (∼1.7% per day
of the Cr pool). Because muscle is the major site of creatinine
production, the rate of creatinine formation is mostly
a reflection of the total muscle mass. Creatinine enters the
circulation most likely by passive transport or diffusion
through the plasma membrane, followed by filtration in
kidney glomeruli and excretion in urine.
Creatine Deficiency Syndromes
Both AGAT and GAMT deficiencies are autosomal recessive
inborn errors of metabolism. This is in contrast to
the third disorder of Cr metabolism, which is an X-linked
inborn error due to a defect in the Cr transporter (Table 1).
The first inborn error of Cr biosynthesis,GAMTdeficiency
(MIM601240), was identified in 1994. The absence of a
Cr signal in the proton magnetic resonance spectroscopy
(1H-MR) spectrum of brain, the low amounts of urinary
creatinine, and the increased levels of GAA in plasma and
urine led to the diagnosis of this disease. In addition to creatinine,
Cr is also reduced in body fluids. Clinical symptoms
are usually noted within the first eights months of
life. Possibly Cr is provided in high amounts in utero via
the umbilical cord and in newborns via the mother’s milk,
thereby delaying the clinical signs. All patients identified
so far have developmental delay, MR to various degrees,
expressive speech and language delay, epilepsy, autistiform
behavior, and very mild-to-severe involuntary extrapyramidal
movements. The disorder has a highly heterogeneous
presentation, varying from very mild signs to
severe MR, accompanied by self-injurious behavior.
In 2001, the first family with AGAT deficiency
(MIM602360) was identified. The two sisters, four and six
years old presented with MR, developmental delay from
the age of eight months, and speech delay. GAMT deficiency
was ruled out because GAA was not increased in
urine and plasma. Creatine supplementation (400 mg/kg
body weight per day) increased the Cr content in the
brain to 40% and 80% of controls within three and nine
months, respectively. A homozygous nonsense mutation
in the AGAT gene, predicting a truncated dysfunctional
enzyme, was finally identified. Lymphoblasts and fibroblasts
of the patients indicated impaired AGAT activity. A
third related patient was identified with similar clinical
presentation. The biochemical hints to detect this disorder
are reduced levels ofGAA(and creatinine) in plasma, cerebrospinal
fluid (CSF) and possibly urine, together with
reduced undetectable levels of Cr in the brain.
SLC6A8 Deficiency (Creatine Transporter Deficiency)
Like AGAT deficiency, the X-linked Cr transporter defect
was unraveled in 2001. An X-linked Cr transporter
(MIM300352) defect was presumed because of: (i) the
absence of Cr in the brain as indicated by proton magnetic
resonance spectroscopy (MRS); (ii) elevated Cr levels in
urine and normal GAA levels in plasma, ruling out a Cr
biosynthesis defect; (iii) the absence of an improvement
on Cr supplementation; and (iv) the fact that the pedigree
suggested an X-linked disease. The hypothesis was
proven by the presence of a hemizygous nonsense mutation
in the male index patient and by impaired Cr uptake
by cultured fibroblasts. The hallmarks of this disorder are
MR, expressive speech and language delay, epilepsy, developmental
delay, and autistiform behavior.
Unfavorable skewed X-inactivation is likely the cause of the difference
in severity of the clinical signs in females.
Intriguing Questions Linked to CDS
Does a Muscle-Specific Creatine Transporter Exist?
It is noteworthy that the SLC6A8-deficient patients do not
seem to suffer from muscle and/or cardiac failure. This
could indicate sufficient endogenous Cr biosynthesis in
muscle. Alternatively, Cr uptake is taken over by other
transporters, or a yet unknown Cr transporter exists that
is specifically expressed in skeletal and cardiac muscle.
Creatine Biosynthesis in Mammalian Brain
It is a matter of debate whether Cr biosynthesis occurs in
mammalian brain. The following findings suggest that it
actually does: (i) In rat brain, AGAT and GAMT mRNA
and protein were detected (16), (ii) The Cr content in brain
of mice treated with guanidinopropionic acid, an inhibitor
of the Cr transporter, was—in contrast to muscle tissues—
hardly decreased. (iii) In contrast to skeletal muscle, Cr
supplementation in AGAT- and GAMT-deficient patients
requires months to result in an increment in Cr concentration
in the brain. These findings make it unlikely that the
brain is entirely dependent on Cr biosynthesis in the liver
or on its nutritional intake, followed by transport through
the blood–brain barrier into the brain.
However, why do Cr transporter deficient patients
also reveal Cr deficiency in the brain? One explanation
could be that Cr synthesis in the brain, although present,
is too low to be relevant physiologically. Alternatively, the
expression of AGAT and GAMT may be separated spatially
(i.e., AGAT and GAMT molecules may be found
in the same or different cell types, but may not be expressed
in one and the same cell). This is in line with
data of Braissant et al. (17) showing such spatial separation
in rat brain at both the mRNA and protein level.
These findings suggest thatGAAneeds to be taken up into
the appropriate cells prior to GAA methylation, which in
case of the transporter defect is not feasible. This would
explain the incapability to synthesize Cr in the brain of
SLC6A8-deficient patients. Clearly, more thorough investigations
are needed to study these discrepancies toward
a better understanding of Cr metabolism in the human
Significance of CDS/relevance for Health Care
Mental retardation occurs at a frequency of 2% to 3% in
the Western population. In 25% of MR cases, a genetic
cause is suspected, of which Down syndrome and fragile
X syndrome are the most common. Mutations in the
SLC6A8 gene may be, together with other X-linked MR
genes, partly responsible for the skewed ratio in sex distribution
in MR, autism, and individuals with learning
disabilities. SLC6A8 deficiency appears to be a relatively
common cause of X-linked MR, though not as common as
fragile X. Creatine biosynthesis defects may be less common.
Because the damage incurred in these three diseases
is irreversible to a large part and an effective treatment
is available at least for the Cr biosynthesis defects, early
diagnosis of these patients is highly important.
To date, the clinical phenotype appears to be nonspecific
and suggests that allMRpatients should be tested
in diagnostic centers by 1H-MRS, metabolite screening,
and/or sequence analysis of the SLC6A8 gene. In the case
of X-linked MR or X-linked autism due to a genetic, but
unknown, cause, the parents are confronted with a risk of
recurrence (50% chance that the mother passes the mutant
allele on to her child). The diagnosis of SLC6A8 deficiency
or a Cr biosynthesis defect allows prenatal diagnosis for
Creatine Supplementation/Therapeutic Use
Creatine is present in high amounts in meat
(4.5 g/kg in beef, 5 g/kg in pork) and fish (10 g/kg in herring, 4.5 g/kg
in salmon), which are the main exogenous Cr sources in
the human diet. Low amounts of Cr can be found in milk
(0.1 g/kg) and cranberries (0.02 g/kg) (17). As discussed
earlier, Cr is also synthesized endogenously, which supplies
around 50% of the daily requirement of approximately
2 g. This suggests that in vegetarians, who have a
low intake of Cr, the bodily Cr content is reduced, unless
its endogenous biosynthesis is largely increased. Indeed,
in vegetarians, the Cr concentration in muscle biopsies
was reported to be reduced (18).
Dosing as an Ergogenic Aid
Creatine can be obtained as nutritional supplement in the
form of various over-the-counter creatine monohydrate
products, which are supplied by many manufacturers.
Commercial Cr is chemically produced. The majority of
consumers are sportspersons, due to Cr’s documented
and/or presumed ergogenic and muscle mass increasing
effects. Usually, a loading phase of five to seven days of
20 g/day (in four portions of 5 g) is recommended, followed
by a maintenance phase with 3–5 g Cr per day.
Benefits in Sportspersons
Creatine supplementation is common among cyclists,
mountain bikers, rowers, ski jumpers and tennis, handball,
football, rugby, and ice hockey players.
While there is a large body of evidence supporting the ergogenic effects
of Cr in high-intensity, intermittent exercise, the situation
is more controversial in sports involving single bouts of
high-intensity exercise, such as sprint running or swimming
(2,19). In endurance exercise, there is currently no
reason to believe that Cr supplementation has any benefit.
There is a widespread contention that Cr supplementation,
by accelerating recovery between exercise bouts, may
allow more intensive training sessions. Similarly, supplementation
seems to enhance recovery after injury.
In most studies, a significant weight gain has been
noted upon Cr supplementation. The underlying basis for
this weight gain is still not entirely clear, and may be due
to stimulation of muscle protein synthesis or increased
water retention. The proportion of fat tends to decrease.
Most likely, the increase in body weight reflects a corresponding
increase in actual muscle mass and/or volume.
Therefore, it is not surprising that Cr use is popular among
206 Salomons et al.
bodybuilders and wrestlers. On the other hand, in masssensitive
sports like swimming and running, weight gain
due to Cr supplementation may impede the performance,
or may at least counteract the ergogenic effects of Cr.
Creatine supplementation may improve muscle performance,
especially during high-intensity, intermittent
exercise, in four different ways by: (i) increasing PCr
stores, which is the most important energy source for immediate
regeneration of ATP in the first few seconds of
intense exercise; (ii) accelerating PCr resynthesis during
recovery periods; (iii) depressing the degradation of adenine
nucleotides and possibly also the accumulation of
lactate; and (iv) enhancing glycogen storage in skeletal
Benefits in Neuromuscular Disease
Besides its ergogenic effects, supplementary Cr has a neuroprotective
function in several animal models of neurological
disease, such as Huntington disease, Parkinson
disease, and amyotrophic lateral sclerosis (ALS) (2,3,6,11).
The rationale could be that these disorders, due to different
causes, hamper cellular energy metabolism in the
brain. In animal studies, Cr also protected against hypoxic
and hypoxic-ischemic events. Therefore, Cr may be
useful in the treatment of a number of diseases, for example,
mitochondrial disorders, neuromuscular diseases,
myopathies, and cardiopathies. Currently, the first clinical
studies with Cr supplementation in neuromuscular
disease are emerging. In two studies on patients with mitochondrial
myopathies or other neuromuscular diseases,
Tarnopolsky’s group showed increased muscle strength
upon Cr supplementation (11). A randomized, doubleblind,
placebo-controlled trial to determine the efficacy
of creatine supplementation did not show a significant
beneficial effect on survival and disease progression in a
group of 175 ALS patients. These data are in contrast to
what was suggested from animal models of ALS and tissue
specimens of ALS patients (12). Studies on single subjects
and small groups of neuromuscular disease patients
have been reported to show both the presence and absence
of beneficial effects of Cr supplementation. Recent publications
on Cr supplementation in Huntington disease
showed difficulty in proving the effect of Cr on the deterioration
of cognitive function (20,21). In Duchenne muscular
dystrophy, enhanced muscle strength upon treatment
was shown; whereas, for example, in myotonic dystrophy
type 2/proximal myotonic myopathy, no significant
results were seen (22,23). Future studies with enough statistical
power are warranted to unravel the relevance of
Cr supplementation in these disorders. Clinical trials of
patients with ALS, Parkinson, and other neurological diseases
are currently ongoing (http://clinicaltrials.gov/).
Benefits in Creatine Biosynthesis Disorders
Oral supplementation with 350 mg to 2 g/kg body weight
per day has been used in patients with GAMT and AGAT
deficiencies. In these patients, the Cr concentration in
their brains increased over a period of several months (5).
In GAMT deficiency, the GAA concentration in plasma,
urine, and CSF decreased with Cr supplementation, but
still remained highly elevated. Guanidinoacetic acid was
found to be toxic in animals and may be partly responsible
for some of the clinical signs (i.e., involuntary extrapyramidal
movements). Combination therapy of Cr plus
ornithine supplementation with protein (arginine) restriction
reduced GAA in CSF, plasma, and urine, and almost
completely suppressed epileptic seizures (7). In general,
all patients with a Cr biosynthesis defect who were treated
with Cr alone or in combination therapy showed improvements.
Clearly, younger patients will experience the
largest benefits, because less irreversible damage is to be
expected. However, even older patients showed remarkable
Weight gain is the only consistent side effect reported.
Gastrointestinal distress, muscle cramps, dehydration,
and heat intolerance have been reported repeatedly.
Most of these complaints may be due to water retention
in muscle during the loading phase of Cr supplementation.
Although a causal relationship with fluid
intake has not been proven yet, subjects should take
care to hydrate properly to prevent these side effects.
The French Agency of Medical Security of Food
(www.afssa.fr/ftp/basedoc/2000sa0086.pdf) released a
statement in January 2001 that the health risk associated
with oral Cr supplementation is not sufficiently evaluated,
and that Cr may be a potential carcinogen. Because
at present there is no scientific basis for the assertion (both
Cr and Cr analogs were actually reported to display anticancer
activity), this in turn has resulted in a wave of
protest from suppliers and defenders of oral Cr supplementation.
In fact, based on the current scientific knowledge
in healthy individuals, Cr supplementation at the
recommended dosages (see dosing as an ergogenic aid)
should be considered safe. Unfortunately, almost nothing
is known about the use of Cr in pregnancy, nor are
appropriate studies in children available. Furthermore, a
potential health hazard is the possible presence of contaminants
in some commercial Cr preparations (see chemical
Oral Cr supplementation is known or presumed to have
a number of favorable effects. For example, it prevents or
ameliorates clinical symptoms associated with inherited
Cr biosynthesis defects, it may protect against neurological
and atherosclerotic disease, (2,6) and it increases sports
performance, particularly in high-intensity, intermittent
exercise. Despite widespread use of Cr as an ergogenic aid
and the significant public interest, the majority of studies
on the properties, metabolism, and function of Cr have
focused on physiological questions rather than on pharmacokinetics.
As yet, the pharmacokinetics is difficult to
interpret due to different (and incomplete) study designs.
Currently, therefore, it is not adequately known whether
Cr supplementation causes any long-term harmful effects.
Some precaution is warranted based on the fact that the
daily recommended dosage for ergogenic effects (i.e., 20 g
during the loading phase, 3–5 g during the maintenance
phase) cannot be met by normal food intake.