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Glossary, sports nutritionSuccess Chemistry Staff

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.


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

treatment (5–10).

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.)


Creatine Structure

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 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,

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.

Chemical synthesis

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

interconvertible molecules.

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

Creatine Uptake

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/Creatinine Clearance

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).

GAMT Deficiency

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.

AGAT Deficiency

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

subsequent pregnancies.

Creatine Supplementation/Therapeutic Use

Creatine Sources

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 (

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

improvements (7).


Adverse Effects

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

( 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.