Creatine phosphate (phosphocreatine) functions as a phosphate reservoir. It is found in high levels in the skeletal muscles and the heart, but also to some degree in almost every cell of all vertebrates and various invertebrates.
Some (uncooked) meats have high levels of creatine:
Beef, with minimal visible connective tissue: 5 g per 1.1 kg, or 2.15-2.5 g/lb (4.74-5.51 g/kg)
Chicken: 3.4 g/kg
Rabbit: 3.4 g/kg
Cardiac tissue (ox): 2.5 g/kg
Cardiac tissue (pig): 1.5 g/kg
Some (uncooked) meats have low levels of creatine:
Liver: 0.2 g/kg
Kidney: 0.23 g/kg
Lung: 0.19 g/kg
Creatine accumulates in the same organs in meat products as in humans. Tissues with a high creatine content include the heart and the skeletal muscles.
Other compounds containing creatine include:
Dairy products have minimal creatine content, but beyond meat products they are the only significant source of dietary creatine.
According to the NHANES III survey, the average daily consumption of creatine from food sources among Americans (19-39 years old) is about 7.9 mmol (1.08 g) for men and 5 mmol (0.64 g) for women. This is below the “2 g/day consumed via the diet” estimate that many studies reference.
Creatine from food is digested more slowly than creatine taken as a supplement, but total bioavailability is identical.
1.2. Properties and Structure
Creatine is a small peptide — a structure composed of amino acids. Specifically, creatine is composed of L-arginine, Glycine, and methionine. Its molecular structure is depicted below.
1.3. Food Processing
Depending on the cooking temperature and the presence of a reducing sugar, such as glycogen, carnosine and aspartic acid will degrade into acrylic acid and acrylamides. At the same time, creatine will degrade into methylamine, which will then bind to acrylic acid and acrylamides to incorporate into the toxic substance N-methylacrylamide (C4H7NO).
Creatine can also be converted to the biologically inactive creatinine through the removal of a water molecule. Approximately 30% of meat-bound creatine can be lost in exudate or degraded into creatinine when cooking to medium-well.
Finally, creatine can also participate in the formation of heterocyclic amines, a process that can be partially inhibited by marination.
1.4. Biological Significance
Carbohydrates provide quick energy in an anaerobic environment (high-intensity exercise), while fats provide sustained energy during periods of high oxygen availability (low-intensity exercise or rest). The breakdown of carbohydrates, fats, and ketones produces ATP (adenosine triphosphate). When the cells use ATP for energy, this molecule is converted into adenosine diphosphate (ADP) and adenosine monophosphate (AMP). Creatine exists in cells to donate a phosphate group (energy) to ADP, turning this molecule back into ATP.
By increasing the overall pool of cellular phosphocreatine, creatine supplementation can accelerate the reycling of ADP into ATP. Since ATP stores are rapidly depleted during intense muscular effort, one of the major benefits of creatine supplementation is its ability to regenerate ATP stores faster, which can promote increased strength and power output. Over 95% of creatine is stored in muscle at a maximum cellular concentration of 30uM. Creatine storage capacity is limited, though it increases as muscle mass increases. If we were are to assume a 70 kg male with an average physique, his total creatine stores would be about 120 g. The body can store a lot more energy as glycogen (in the liver, brain, and muscles), and even more as fat.
Creatine is an energy substrate: a small peptide serving as a reservoir for high-energy phosphate groups that can regenerate ATP, the main currency of cellular energy. An increase in creatine intake (through food or supplementation) increases cellular energy stores, promoting the regeneration of ATP on the short term. Stores are limited, however, and glucose or fatty acids are responsible for ATP replenishment over the longer term.
Without supplementation, creatine is formed primarily in the liver, with minor contribution from the pancreas and kidneys. The two amino acids, glycine and Arginine, combine via the enzyme Arginine:Glycine amidinotransferase (AGAT) to form Ornithine and guanidoacetate. This is the first of two steps in creatine synthesis, and although rare, any deficiency of this enzyme can result in mild mental retardation and muscular weakness. AGAT is also the primary regulatory step, and an excess of dietary creatine can suppress activity of AGAT to reduce creatine synthesis by reducing AGAT mRNA levels, rather than competitive inhibition.
Guanidoacetate (made by AGAT) then receives a methyl donation from S-Adenosyl Methionine via the enzyme guanidinoacetate methyltransferase (GAMT), which produces S-adenosylhomocysteine (as a byproduct) and creatine. Deficiencies in GAMT are a bit more severe (although equally rare) relative to AGAT, resulting in severe mental retardation and autism-like symptoms.
For the most part, the above reactions occur in the liver, where most systemic creatine is synthesized, but the AGAT and GAMT enzymes have been located in lesser amounts in kidney and pancreatic tissue (the extra-hepatic synthesis locales). Neurons also possess the capability to synthesize their own creatine.
To form creatine, the amino acids glycine and arginine are enzymatically combined to form guanidoacetate, which is then methylated to form creatine. Diseases associated with errors in creatine synthesis can result in muscle disorders and mental retardation.
As mentioned above, S-adenylmethionine must be converted to S-adenylhomocysteine in order for guanidoacetate to convert into creatine, during a process known as methylation. It has been suggested that the production of creatine accounts for up to 40% of the S-adenylmethionine used in the body for methylation processes.
Creatine supplementation alleviates the intrinsic burden of producing creatine. Supplementation reduces the expected increase in homocysteine after intense exercise and may be a reason why creatine is seen as cardioprotective around the time of exercise.
After supplementation of creatine monohydrate (loading phase, followed by 19 weeks maintenance), creatine precursors are decreased by up to 50% (loading) or 30% (maintenance), which suggests a decrease in endogenous creatine synthesis during supplementation. This appears to occur through creatine’s own positive feedback and suppression of the l-arginine:glycine amidinotransferase enzyme, the rate-limiting step in creatine synthesis, as levels of intermediates before this stage are typically elevated by up to 75%.
A suppression of creatine synthesis is seen when enough creatine is supplemented to cover the vital needs (approximately 4g daily, 2g of which would have been synthesized). This suppression may be beneficial to health, due to the inherent costs associated with creatine synthesis.
Creatine is stored in the body in the form of creatine and as creatine phosphate, otherwise known as phosphocreatine, which is the creatine molecule bound to a phosphate group. Creatine phosphate is thought to maintain the ATP/ADP ratio by acting as a high-energy phosphate reservoir. The more ATP a muscle has relative to ADP, the higher its contractility is, and thus its potential strength output in vivo. This pro-energetic mechanism also affects nearly all body systems, not just skeletal muscle.  During periods of rest and anabolism, creatine can gain a phosphate group through the creatine-kinase enzyme pathway, up to a cellular concentration of 30uM to be later used for quick ATP resupply when needed.
Creatine kinase enzymes (of which there are numerous isozymes) exist in both the mitochondria and the cytosol of the cell. The four isozymes of creatine kinase include the Muscle Creatine Kinase (MCK), present in contractile muscle and cardiac muscle, and the Brain Creatine Kinase (BCK), expressed in neuron and glial cells and some other non-muscle cells. These two Creatine Kinases are met with Sarcolemmic Mitochondrial Creatine Kinase (sMitCK), expressed alongside MCK, and the ubiquitous Mitochondrial Creatine Kinase (uMitCK), which is expressed alongside BCK everywhere else.
Supplementation of creatine monohydrate increases stores of both of these compounds in myocytes, neurons, eyes, kidneys and testes; of which muscle comprises >95% of bodily creatine stores.
Creatine and creatine phosphate form a couplet in cells, which sequesters phosphate groups. These phosphate groups are then donated to ADP to regenerate ATP. This donation is faster than any other process in a cell for replenishing energy, and higher cellular creatine levels result in more phosphate donation and subsequent energy replenishment.
Increasing cellular survival (by preventing ATP depletion, cells survive longer) against hypoxia, oxidative damage, and some toxins against neurons and skeletal muscle cells is a mechanism of creatine supplementation mediated via creatine-kinase. This has also been shown to have efficacy against toxin-induced seizures.
Expressing the creatine-kinase enzyme in cells that do not normally express it (and thus enabling these cells to use creatine) exerts protective effects, while inhibiting this enzyme reduces survival rates.
Creatine and phosphocreatine surplus within a cell serves as an energy reservoir that can protect cells under periods of acute stress, and may enhance cell survival secondary to its bioenergetic effects.
Creatine kinase appears to be subject to sexual dimorphism, where differences exist in males and females, with males having increased enzyme activity.
When looking at race interactions, black people appear to have higher activity of the creatine kinase system when compared to both white and hispanic people, with hispanic people having greater levels than whites. The differences between races are more pronounced in men.
When splitting a sample into exercisers and non-exercisers, it appears that exercise as a pre-requisite precedes a higher range of activity. Those who are inactive tend to be on the lower end of creatine kinase activity and relatively clustered in magnitude, while exercises generally increases activity, but introduces a larger range of possible activity.
Men appear to have higher active Creatine-Kinase systems, and racial differences favor Blacks over Hispanics over Whites for the activity of the Creatine-Kinase system. This system is more variable in men (independent of supplementation). Exercise may increase the activity of the Creatine-Kinase system independent of supplementation
1.5. Deficiency States
Creatine is also a neurological nutrient. Individuals who cannot produce endogenous creatine suffer from a form of mental retardation with autistic-like symptoms due to deficiencies in the enzymes of creatine synthesis (AGAT or GAMT).
The main storage area of creatine in the human body is the skeletal (contractile) muscle, which holds true for other animals. Therefore, consumption of skeletal muscle (meat products) is the main human dietary source of creatine. Since vegetarians and vegans lack the main source of dietary creatine intake, which has been estimated to supply half of the daily requirements of creatine in normal persons, both vegetarians and vegans have been reported to have lower levels of creatine. This also applies to other meat-exclusive nutrients, such as L-Carnitine.
Due to this relative deficiency-state in vegetarians and vegans, some aspects of creatine supplementation are seen as more akin to normalizing a deficiency, rather than providing the benefits of supplementation. In young vegetarians, but not omnivores, creatine supplementation can enhance cognition. The increased gain in lean mass may be more significant in vegetarians, relative to omnivores. Supplementation of creatine in vegetarians appears to normalize the gap in storage between vegetarians and omnivores. This is possibly related to a correlation seen in survey research where vegetarianism and veganism appear to be more commonly affected by some mental disorders like anxiety and depression.
The importance of supplemental creatine is elevated in vegetarian and vegan diets due to the elimination of creatine’s main dietary sources.
1.6. Formulations and Variants
Creatine monohydrate is the most common form of creatine, and if not otherwise mentioned is the default form of creatine used in most studies on creatine. It has fairly decent intestinal absorption (covered more in depth in the pharmacology section) and is the standard form or “reference” form of creatine, which all other variants are pitted against.
This basic form of creatine comes in two forms, one of which involves the removal of the monohydrate (which results in creatine anhydrous) that converts to creatine monohydrate in an aqueous environment, but due to the exclusion of the monohydrate it is 100% creatine by weight despite creatine monohydrate being 88% creatine by weight (as the monohydrate is 12%). This allows more creatine to be present in a concentrated formula, like capsules.
Creatine monohydrate can also be micronized (commonly sold as “Micronized Creatine”) which is a mechanical process to reduce particulate size and increase the water solubility of creatine. In regards to supplementation, it is equivalent to creatine monohydrate.
Creatine is most commonly found in the basic form of creatine monohydrate, which is the standard form and usually recommended due to the low price. It can also be micronized to improve water solubility or the monohydrate can be temporarily removed to concentrate creatine in a small volume supplement. Neither alteration changes the properties of creatine
Creatine hydrochloride (Creatine HCl) is a form of creatine where the molecule is bound to a hydrochloric acid moiety. It is claimed to require a lower dosage than creatine monohydrate, although this claim has not been tested.
Creatine hydrochloride likely forms into free creatine and free hydrochloric acid in the aqueous environment of the stomach, which would mean it is approximately bioequivalent to creatine monohydrate.
Creatine HCl is touted to require a lower dosage, but this has not been proven through studies and seems unlikely, since the stomach has an abundance of HCl anyway and creatine will freely dissociate with HCl in the stomach. Thus, both creatine HCl and creatine monohydrate form free creatine in the stomach.
Liquid creatine has been shown to be less effective than creatine monohydrate. This reduced effect is due to the passive breakdown of creatine over a period of days into creatinine, which occurs when it is suspended in solution. This breakdown is not an issue for at-home use when creatine is added to shakes, but it is a concern from a manufacturing perspective in regards to shelf-life before use.
Liquid creatine is ineffective as a creatine supplement due to its limited stability in solution. This shouldn’t be an issue if you’re preparing a creatine solution at home, since it takes a few days to for creatine to degrade. This is a problem from the manufacturing side, where creatine in solution has a limited shelf-life.
Buffered Creatine (Kre-Alkylyn as brand name) is touted to enhance the effects of creatine monohydrate due to a higher pH level, which enables better translocation across the cytoplasmic membrane and more accumulation in muscle tissues.
This claim has not been demonstrated at this time, and a recent comparative study of buffered creatine against basic creatine monohydrate found no significant differences between the two in 36 resistance trained individuals, in regards to the effects or the accumulation of creatine in muscle tissue. There also were no significant differences in the amount of adverse side-effects reported.
“Buffered” creatine (Kre-Alkylyn) is suggested to be a better absorbed form of creatine supplementation, but it can be rapidly neutralized in the stomach if not in an enteric coating. Even if it is enteric coated, there is no evidence to support its efficacy above creatine monohydrate.
Creatine ethyl ester increases muscle levels of creatine to a lesser degree than creatine monohydrate. It may also result in higher serum creatinine levels due to creatine ethyl ester being converted into creatinine via non-enzymatic means in an environment similar to the digestive tract. At equal doses to creatine monohydrate, ethyl ester has failed to increase water weight after 28 days of administration (indicative of muscle deposition rates of creatine, which are seemingly absent with ethyl ester).
Creatine ethyl ester is more a pronutrient for creatinine rather than creatine, and was originally created in an attempt to bypass the creatine transporter. It is currently being studied for its potential as treatment for situations in which there is a lack of creatine transporters (alongside cyclocreatine as another possible example). Its efficacy may rely on intravenous administration, however.
Direct studies on creatine ethyl ester show it to be less effective than creatine monohydrate, on par with a placebo.
Creatine ethyl ester is 82.4% creatine by weight, and thus would provide 8.24g of active creatine for a dosage of 10 grams.
Creatine ethyl ether is likely ineffective as a creatine supplement for general use. Despite being able to passively diffuse through cell membranes in vitro, it degrades into creatinine rapidly in the intestines.
Magnesium-chelated creatine typically exerts the same ergogenic effects as creatine monohydrate at low doses. It was made because carbohydrates tend to beneficially influence creatine metabolism and magnesium is also implicated in carbohydrate metabolism and creatine metabolism.referenc|url=http://www.ncbi.nlm.nih.gov/pubmed/14506619|title=Magnesium-creatine supplementation effects on body water|published=2003 Sep|authors=Brilla LR, Giroux MS, Taylor A, Knutzen KM|journal=Metabolism Magnesium chelated creatine may be useful for increasing muscle strength output with a similar potency to creatine monohydrate, but without the water weight gain (noted differences, but statistically insignificant).
Creatine magnesium chelate has some limited evidence for it being better than creatine monohydrate, but this has not been investigated further.
Creatine nitrate is a form of creatine where a Nitrate (NO3) moiety is bound to the creatine molecule, which has been demonstrated to enhance solubility in water by approximately 10-fold with the pH of 2.5 or 7.5 not significantly affecting the solubility. Beyond increased solubility, no other studies have been conducted using creatine nitrate.
Creatine nitrate is a highly water soluble form of creatine, and while it is theoretically possible that this possesses benefits of both creatine and nitrate, this has not been investigated.
Creatine citrate is creatine bound to citric acid, or citrate. Creatine Citrate does not differ greatly from monohydrate in regards to absorption or kinetics. Note that creatine citrate is more water-soluble than monohydrate, but creatine absorption is generally not limited by solubility. The increased water solubility may play a factor in palatability.
It can be found in varying ratios of creatine:citrate, including 1:1 (creatine citrate), 2:1 (dicreatine citrate), and 3:1 (tricreatine citrate).
Creatine malate is the creatine molecule bound to malic acid. There might be some ergogenic benefits of malic acid on its own, but this has not been investigated in conjunction with creatine. Malic acid/Malate also confers a sour taste and may negate the sensation of bitterness, common among some supplements.
Creatine citrate and creatine malate are variants of creatine with increased water solubility.
Creatine pyruvate (also known as Creatine 2-oxopropanoate) in an isomolar dose relative to creatine monohydrate has been shown to produce higher plasma levels of creatine (peak and AUC) with no discernible differences in absorption or excretion values. The same study noted increased performance from creatine pyruvate at low (4.4g creatine equivalence) doses relative to citrate and monohydrate, possibly due to the pyruvate group.
Creatine pyruvate is 60% creatine by weight.
Creatine pyruvate has once been noted to reach higher levels of plasma creatine relative to an isomolar dose of creatine monohydrate. The lone study failed to note differences in absorption, however, which conflicts with the observation of increased serum levels. This result has not been replicated.
Creatine α-ketoglutarate is the creatine molecule bound to an alpha-ketoglutaric acid moiety. Little research has been done with creatine α-ketoglutarate.
Creatine α-ketoglutarate is 53.8% creatine by weight.
Creatine α-ketoglutarate is thought to be an enhanced form of creatine supplementation (similar to Arginine α-ketoglutarate, which has an increased rate of absorption) but this has not been investigated.
Sodium creatine phosphate is 51.4% creatine by weight.
Sodium creatine phosphate appears to be about half creatine by weight, and it is not certain if this variant offers any advantages over conventional forms.
Polyethylene glycosylated creatine seems to be as effective as creatine monohydrate at a lower dose (1.25-2.5g relative to 5g monohydrate), but does not seem to be comparable in all aspects.
Polyethylene glycosylated creatine (PEG creatine) appears to be somewhat comparable to creatine monohydrate
Creatine gluconate is a form of creatine supplementation where the creatine molecule is bound to a glucose molecule. It currently does not have any studies conducted on it.
Creatine gluconate is sort of a glycoside of creatine, and it is thought to be better absorbed when taken alongside food (since many other gluconate molecules, particular on minerals like Magnesium, are absorbed better with food). There are currently no studies on this particular variant, however.
Cyclocreatine (1-carboxymethyl-2-iminoimidazolidine) is an analogue of creatine in a cyclic form, and synthetically made. It serves as a substrate for the creatine kinase enzyme system, acting as a creatine mimetic. Cyclocreatine may compete with creatine in the CK enzyme system to transfer phosphate groups to ADP, as coincubation of both can reduce cyclocreatine’s anti-motility effects on some cancer cells.
The structure of Cyclocreatine is fairly flat (planar), which aids in passive diffusion across membranes. It has been used with success in an animal study, where mice suffered from a SLC6A8 (creatine transporter at the blood brain barrier) deficiency, which is not responsive to standard creatine supplementation. This study failed to report increases in creatine stores in the brain, but noted a reduction of mental retardation associated with increased cyclocreatine and phosphorylated cyclocreatine storages. As demonstrated by this animal study and previous ones, cyclocreatine is bioactive after oral ingestion and may merely be a creatine mimetic, able to phosphorylate ADP via the creatine kinase system.
This increased permeability is noted in glioma cells, where it exerts anti-cancer effects related to cell swelling and in other membranes, such as breast cancer cells and skeletal (contractile) muscle cells. The kinetics of cyclocreatine appear to be first-order, with a relative Vmax of 90, Km of 25mM and a KD of 1.2mM.
In regards to bioenergetics, phosphorylated cyclocreatine appears to have less affinity for the creatine kinase enzyme than phosphorylated creatine in donating the high energy phosphate group (about 160-fold less affinity) despite the process of receiving phosphorylation being similar. When fed to chickens, phosphorylated cyclocreatine can accumulate up to 60mM in skeletal muscle which suggests a sequestering of phosphate groups before reaching equilibrium. Cyclocreatine still has the capacity to donate phosphate, however, as beta-adrenergic stimulated skeletal muscle (which depletes ATP and glycogen) had an attenuation of glycogen depletion (indicative of preservation of ATP) with phosphocreatine.
When looking at kinetics, cyclocreatine appears to be passively diffused through membranes and not subject to the creatine transporter, which can be beneficial for cases where creatine transporter function is compromised (creatine non-response and SLG6A8 deficiency). Similar to other forms of creatine, it buffers ATP concentrations although its efficacy as a supplement in otherwise healthy persons is currently unknown.
2.1. Cellular Hydration
When creatine is absorbed it pulls water in with it, causing cells to swell. This “cell volumization” is known to promote a cellular anabolic state associated with less protein breakdown and increased DNA synthesis. An increase in cellular viability assessed via phase angle (measuring body cell mass) has been noted in humans during supplementation of creatine.
Glycogen synthesis is known to respond directly and positively to cellular swelling. This was demonstrated in an earlier study where rat muscle cells were exposed to a hypotonic solution in vitro to induce cell swelling, which increased glycogen synthesis by 75%. In contrast, exposing these same cells to a hypertonic solution hindered glycogen synthesis by 31%. These changes were not due to alterations in glucose uptake, but are blocked by hindering the PI3K/mTOR signalling pathway. It was later noted that stress proteins of the MAPK class (p38 and JNK) as well as heat shock protein 27 (Hsp27) are activated in response to increasing osmolarity. Furthermore, activation of MAPK signaling in skeletal muscle cells is known to induce myocyte differentiation via GSK3β and MEF2 signaling, which can induce muscle cell growth.
The increase in cellular swelling (water retention within the cell) per se appears to have a positive influence on muscle cell growth, since the increase in swelling is met with the activation of stress response proteins of the MAPK class, which then influence muscle protein synthesis. These mechanisms do not involve the creatine kinase cycle.
Inducing hypertonicity (a reduction in cellular swelling) is known to actually increase the mRNA of the creatine transporter, thought to be due to increasing cellular creatine uptake to normalize creatine levels. This has been noted in both muscle cells and endothelial cells, but is thought to apply to all cells.
This regulation of creatine uptake is similar to other osmolytic agents such as myo-Inositol or Taurine, which have their uptake into cells enhanced during periods of hypertonia in order to increase cellular swelling.
Cellular hydration status is tied into creatine influx and efflux by modulating the expression of the creatine transporter and thus cellular uptake. This is similar to other osmolytes in the human body.
Phosphocreatine, the higher energy form of creatine, can associate with and protect cell membranes. This was first observed in Drosophilia, which do not express the creatine kinase enzyme (and cannot use creatine for energy purposes) yet still received cellular protection from creatine.
In a later study, it was found that biologically relevant concentrations (10-30mM) of creatine bind synthetic membranes with lipid compositions mimicking the inner mitochondrial membrane or plasma membrane in a concentration-dependent manner. This also conferred a degree of protection, increasing membrane stability in response to challenge from a number of destabilizing agents. Phosphocreatine was more effective than creatine in this regard, although both were able to bind and stabilize membranes.
Cyclocreatine (an analogue of creatine) has been shown to protect microtubules in a cell and protect its structure, but it is not known whether these benefits can be expanded to creatine.
Due to the phosphate group, phosphocreatine can bind to cellular membranes. This seems to exert a protective effect by increasing membrane stability. This protective effect is not related to either cell hydration or the creatine kinase system, and its relevance in vivo is not clear at this time.
2.3. Methyl Donation
Creatine is involved indirectly in whole body methylation processes. This is due to creatine synthesis having a relatively large methyl cost, as the creatine precursor known as guanidinoacetate (GAA) requires a methyl donation from S-Adenosyl Methionine (SAMe) in order to produce creatine. This may require up to half of the methyl groups available in the human body.
Creatine supplementation will downregulate the body’s own production of creatine by suppressing the enzyme which mediates the above conversion (Guanidinoacetate methyltransferase or GAMT), and because of this it is thought that SAMe gets backlogged and is more available for other processes that require it.
SAMe is the primary methyl donor in the human body, and supplements that preserve SAMe (such as Trimethylglycine; TMG) promote a variety of benefits in the human body, like a reduction in homocysteine and reduced risk of fatty liver. Creatine has been implicated in both reducing homocysteine and preventing fatty liver in rodents, thought to be secondary to preserving SAMe.
Creatine synthesis requires a large amount of S-adenosyl methionine (SAMe), and downregulating creatine synthesis (via supplementation) indirectly preserves SAMe levels in the body. This is thought to indirectly promote the benefits of SAMe supplementation by reducing its consumption, acting in a similar manner to TMG.
Creatine supplementation may be able to enhance lifespan, secondary to increasing intracellular carnosine stores. Carnosine is the metabolic compound formed from Beta-Alanine supplementation, and in a mouse-model for premature aging (senescence-accelerated premature aging, SAMP8) creatine supplementation without any beta-alanine has been shown to increase cellular carnosine stores. That being said, the aforemented SAMP8 study noted an increase in carnosine levels at middle age, but not old age in the mice. A human study using 20g of creatine for one week in otherwise healthy persons failed to find an increase in intracellular carnosine stores.
Creatine been noted to increase intracellular carnosine stores in a mouse model for premature aging. While this is thought to have an anti-aging effect in mice, oral ingestion of creatine has not been shown to increase carnosine levels in humans, and there is currently no evidence to support an anti-aging effect.
In the stomach, creatine can degrade by about 13% due to the digestive hormone pepsin, as assessed by simulated digestion. Although creatinine is a known byproduct of creatine degradation, simulated gastric digestion did not increase creatinine levels, indicating that other breakdown products were formed. Creatinine was noted to increase in the presence of pancreatin, a mixture of pancreatic enzymes, however.
Stomach acid can degrade a small amount of creatinine, which does not appear to be too practically relevant, judging by the multitude of studies noting benefits with oral creatine monohydrate.
The overall bioavailability of creatine is quite good, ranging from 80% up to nearly 100% depending on the dose ingested, since higher acute doses are absorbed less efficiently.
The specific mechanism of intestinal uptake for creatine is not clear, although transporters have been identified in rat jujenum, and confirmed at the mRNA level in humans. The observation that creatine can be absorbed against a concentration gradient to a max ratio of 8:1 (8 times more creatine in the intestinal cell post absorption, relative to the lumen) supports transporter-mediated uptake, and the dependence on sodium and chloride implicate SLC6A8 (Creatine Transporter 1) as the operative transporter.
In standard dosages (5-10g creatine monohydrate) the bioavailability of creatine in humans is ~99% although this value is subject to change with different conjugates (forms) of creatine and dosage. Coingestion of cyclocreatine (an analogue) can reduce uptake by about half and coincubation of Taurine, Choline, glycine, or Beta-Alanine had minimal attenuation of absorption, which are likely not practically relevant. The inhibition noted with cyclocreatine may be due to receptor saturation.
There is also evidence that increased ingestion of creatine leads to an increased fecal creatine value, suggesting that the intestinal uptake can be saturated.
Intestinal uptake is most likely mediated by SLC6A8 or a related sodium-dependent transporter. Absorption does not appear to be hindered by other common supplements, although too much creatine at one time (greater than 10g) can saturate receptors leading to excretion.
Assuming absolutely no supplementation and standard dietary intake, basal (fasted) creatine concentrations in humans are in the range of 100-200µM , which is lower than that observed in rats (140-600µM).
Under fasting and nonsupplemental conditions, concentrations of creatine in the human body are in the micromolar range.
After the ingestion of 5g creatine in otherwise healthy humans, serum levels of creatine were elevated from fasting levels (50-100µM) to 600-800µM, within one hour after consumption. The receptor follows Michaelis-Menten kinetics with a Vmax obtained at concentrations higher than 0.3-0.4mmol/L, with prolonged serum concentrations above this amount exerting most of its saturation within two days.
2.5 hours after the ingestion of a 20g bolus of creatine, serum levels can increase up to over 2000µM.
Creatine in serum follows a dose-dependent relationship, with more oral creatine ingested causing more serum increases. The rate of accrual into muscle cells may be maximized at a serum concentration achievable with 5g of creatine supplementation.
4.3. Cellular Kinetics (Creatine Transporter)
The Creatine Transporter is a sodium and chloride dependent membrane-associated transporter that belongs to the Na+/Cl-dependent family of neurotransmitter transporters. In muscle cells and most other cell types, the isomer of the creatine transporter is known as SLC6A8 (solute carrier family 6, member 8). SLC6A8 is encoded by the gene present on the Xq28 region of the human X-chromosome and is expressed in most tissues. A related gene encoding a creatine transporter variant has also been identified at 16p11.1 that is expressed exclusively in the testes. These two transporters share 98% homology.
The creatine transporter is a sodium and chloride dependent transporter of the SLC family, also known as SLC6A8. It is the sole mechanism for the transport of creatine from the blood into cells.
Creatine transport has been shown to increase when muscle creatine stores are depleted. This was only noted to occur in muscle with particular fiber types (soleus and red gastrocnemius), however, while other fiber types such as white grastrocnemius did not show any clear trend. This indicates that transport in relation to total creatine levels varies across different muscle fiber types.
In muscle cells, the creatine transporter is predominantly localized to the sarcolemmal membrane. Western blot analysis of creatine transporter expression revealed the presence of two distinc protein bands, migrating at 55kDa and 70kDa on reducing SDS-PAGE gels. The 73kDa band has been reported to be the predominant band humans, with no differences based on gender. A more recent report demonstrated that the 55kDa creatine transporter variant is glycosylated, forming the 73 kDa protein. Therefore the 55 and 75kDa protein bands are actually respective immature, and mature/processed forms of the creatine transporter protein.
The creatine transporter exists in two forms, an immature form with a molecular weight of 55 kDa, and a mature form that is processed by glycosylation, increasing the molecular weight to 73 kDa.
In general, muscle content of creatine tends to be elevated to 15-20% above baseline (more than 20mM increase) in response to oral supplementation. People who get a sufficiently high influx of creatine are known as responders. A phenomena known as “creatine nonresponse” occurs when people have less than a 10mM influx of creatine into muscle after prolonged supplementation. Quasi-responders (10-20mM increase) also exist. Nonresponse is thought to explain instances where people do not benefit from creatine supplementation in trials, since some trials that find no significant effect do find one when only investigating those with high creatine responsiveness. There are clear differences between those who respond and those who do not in regards to performance. People who are creatine responsive tend to be younger, have higher muscle mass and type II muscle fiber content, but this has no correlation with dietary protein intake.
Creatine non-response is when muscular loading of creatine is under a certain threshold (10mmol/L), while “response” to creatine means having more muscular creatine loading (20mol/L or more). There also exists a “grey area” in-between, where some benefits are achieved but not as many as pure responders will experience. Response appears to be positively correlated with muscle mass and type II muscle fibers.
4.4. Positive Regulators (Cellular Uptake)
Creatine is only taken up by its transporter, and changes in the activity level of this transporter are wholly causative of changes in creatine uptake. The transporter is regulated by mostly cytosolic factors as well as some external factors that affect creatine transport activity,  including extracellular creatine. Agents affecting creatine transport are further divided into positive regulators (those that increase activity of the transporter) and negative regulators (those that suppress activity).
The creatine transporter (CrT) is positively regulated by proteins known to be involved in sensing and responding to the cellular energy state, including the mammalian target of rapamycin (mTOR). Upon activation mTOR stimulates SGK1 and SGK3 to act upon PIKfyve and subsequently PI(3,5)P2 to increase CrT activity. Beyond mTOR, SGK1 also is stimulated by intracellular calcium and a lack of oxygen (ischemia). Because transient ischemia is associated with increased reactive oxygen species (ROS) production after blood flow is restored (reperfusion) it has been hypothesized that muscle contraction may increase creatine uptake through a similar ROS-mediated mechanism.
Stress-inducible Kinases (SGK1, SGK3) increase the activity of the creatine transporter, and these proteins are increased by any intracellular stress (such as a lack of oxygen or calcium release from inside the cell). Creatine transport activity is also activated by mTOR, an important nutrient sensor and “master-regulator” of protein synthesis.
Some other cytokines and hormones may increase the receptor activity. These include growth hormone (GH) which acts upon the growth hormone receptor (GHR) to stimulate c-Src which directly increases the activity of the CrT via phosphorylation. This is known to occur with the 55kDa version of c-Src but not the 70kDa version and requires CD59 alongside c-Src.
There is a nuclear receptor known as TIS1 (orphan receptor, since there are no known endogeouns targets at this time) which positively influences transcription of new creatine transporters and, in C2C12 myotubes, seems to be responsive to cAMP or adenyl cyclase stimulation from Forskolin (from Coleus forskohlii) with peak activation at 20µM.
Both growth hormone and TIS1 increase the activity of the creatine transporter in a manner different than the cellular stresses, since growth hormone directly activates it via another pathway (c-Src) while TIS1 is involved in making more of the receptors overall. TIS1 seems to respond to intracellular cAMP levels
Finally, starvation (nutrient deprivation for four days) appears to increase activity of the creatine transporter secondary to decreasing serine phosphorylation (SGK target) with no influence on tyrosine phosphorylation (c-Src target). Starvation-induced increases in creatine influx do not necessarily mean more phosphocreatine, however, due to a depleted cellular energy state.
Starvation increases creatine uptake into cells, but without appreciable conversion into phosphocreatine. Because phosphocreatine is the energetically useful form of creatine in the cell, starvation is not a viable means to increase the efficacy of creatine supplmentation.
In vitro, insulin promotes creatine uptake mouse and human muscle cells. In the human cells, insulin infusion was effective at 55-105mU, but not 5-30mU.
In regards to practical interventions,concurrent glycogen loading has been noted to increase creatine stores by 37-46% regardless of whether the tissue was exercised prior to loading phase. It is important to note, however, that creatine levels in response to the creatine loading protocol were compared in one glycogen-depleted leg to the contralateral control leg which was not exercised. This does not rule out a possible systemic exercise-driven increase in creatine uptake, and the increase in creatine noted above was larger than typically seen with a loading protocol (usually in the 20-25% range). Consistent with an exercise-effect, others have reported that exercise itself increases creatine uptake into muscle, reporting 68% greater creatine uptake in an exercised limb relative to (14%)without exercise.
Exercise itself appears to stimulate creatine uptake into muscle, although reports have been mixed. Given the positive effect of metabolic stress on CrT activity, It is also possible that the more metabolically intense the exercise is on the tissue level, the more creatine uptake is increased.
4.5. Negative Regulators (Cellular Uptake)
Negative regulators of the creatine transporter (CrT) are those that, when activated, reduce the activity of the CrT and overall creatine uptake into cells. As noted above, CrT activity is positively regulated by mTOR. Consistent with the well-known role of AMPK as a suppressor mTOR signaling, CrT activity has also been shown to be inhibited in response to AMPK activation in kidney epithelial cells. Since AMPK suppresses mTOR via upstream TSC2 activation, the negative regulation of AMPK on CrT activity in these cells appears to occur through an indirect mechanism. Although indirect, activation of AMPK has been noted to reduce the Vmax of the CrT without altering creatine binding, and is involved in internalizing the receptors. This pathway seems to max out at around 30% suppression, with no combination of mTOR antagonists and AMPK inducers further suppressing creatine uptake.
In contrast to kidney epithelial cells, others have reported that creatine transport is increased by AMPK in the heart, indicating that CrT is likely regulated in a cell-and tissue specific manner in response to local energy demands. Regulation of CrT by AMPK in a tissue-specific manner has not been explored.
Activity of the creatine transporter (CrT) protein is controlled by AMPK in an apparent cell and/or tissue-specific manner. More researh is needed to determine the effect of AMPK on CrT activity in various tissues, which could be relevant to nutrition and supplementation strategies to optimize creatine stores in skeletal muscle.
Extracellular creatine (creatine outside of a cell) appears to influence creatine uptake into a cell. It seems that prolonged and excessive levels of creatine actually suppress uptake (a form of negative regulation to prevent excessive influx).In vitro studies in rat muscle cells have shown that including 1mM creatine into cell culture medium substantially reduces creatine uptake into cells. The inhibitory effect was partially negated by protein synthesis inhibitors, suggesting that high levels of creatine induce the expression of a protein that suppresses creatine transporter activity. Similar findings were reported in a later study in cultured mouse myoblasts, which noted a 2.4-fold increase in intracellular creatine levels in the presence of the protein synthesis inhibitor cyclohexamide.
High extracellular creatine concentrations induce the expression of a factor that inhibits the creatine transporter (CrT). To date, neither the identity of- nor mechanism for- this putative CrT-suppressing factor has come to light. Future studies that are able to identify this creatine transport-suppressing factor and how it works may provide valuable insight into possible supplementation strategies that might be used to increase creatine uptake into muscle cells.
More recent studies on the regulation of CrT creatine transport activity have identified the protein kinase (Janus-Activating Kinase 2) JAK2, which suppresses the rate of creatine uptake via CrT without affecting creatine binding. JAK2 is a regulatory protein involved in stabilizing the cellular membrane and controlling water concentrations in response to osmotic stress. Similar to c-Src (a positive creatine transport regulator), Jak2 can also be activated by growth hormone signaling. The growth hormone receptor seems to activate these two factors independently, as gh-mediated activation of c-Src does not require JAK2. Given that c-Src is a positive regulator of CrT, JAK2 is a negative regulator, and the fact that downstream signals from both are induced by growth hormone, it is tempting to speculate that JAK2 activation downstream of the gh receptor may function as a homeostatic response to limit c-src induced creatine uptake. This has not been studied, however, and the effects of gh-induced JAK2 signaling on CrT activity have not been examined.
JAK2 (Janus-Activating Kinase 2) is a novel protein that has been shown to suppresses the activity of the creatine transporter CrT in vitro. The effects of JAK2 on CrT are not well-understood in vivo, however. Given the fact that growth hormone activates both c-src (increases CrT activity) and JAK2- which has been found to decrease CrT activity, it is plausible that JAK2 may function as a negative-feedback regulator of creatine uptake. Future research is needed to better understand the role of JAK2 on CrT activity in vivo.
4.6. Neurological Distribution
Creatine is vital for proper neural functioning, and true creatine deficiency results in mental retardation. Deficiency can occur by either hindered synthesis (lack of enzymes to make creatine, can be treated with supplementation) or by a lack of transport into the brain (untreatable with standard creatine).
Entry into neural tissues in general is mediated by the secondary Creatine Transporter (CrT-2) known as SLC6A10, which is the same transporter that is active in a male’s testicles. CrT-2 belongs to the family of SLC6 transporters that act to move solutes across the membrane by coupling transport with sodium and chloride. Genetic deletions in the 16p11.2 region, which encodes both SLC6A8 and SLC6A10 can result in severe mental retardation in humans and is one of the causes of “Creatine Deficiency Syndrome”. Creatine Deficiency Syndrome is not only caused by lack creatine transporter expression, however, as creatine synthesis is also critical for neural function.. Retardation caused by defective creatine synthesis can be reversed with creatine supplementation and dietary changes.
In regards to the blood brain barrier (BBB), which is a tightly woven mesh of non-fenestrated microcapillary endothelial cells (MCECs) that prevents passive diffusion of many water-soluble or large compounds into the brain, creatine can be taken into the brain via the SLC6A8 transporter. In contrast the creatine precursor (guanidinoacetate, or GAA) only appears to enter this transporter during creatine deficiency. More creatine is taken up than effluxed, and more GAA is effluxed rather than taken up, suggesting that creatine utilization in the brain from blood-borne sources is the major source of neural creatine. However, “capable of passage” differs from “unregulated passage” and creatine appears to have tightly regulated entry into the brain in vivo. After injecting rats with a large dose of creatine, creatine levels increased and plateaued at 70uM above baseline levels (where baseline levels are about 10mM, this equates to an 0.7% increase when superloaded). These kinetics may be a reason for the relative lack of neural effects of creatine supplementation in creatine sufficient populations.
Creatine is vital for the functioning of the brain, which has mechanisms to take up creatine, as well as regulate its intake. Although the diet appears to be the major source of creatine (and thus lack of dietary intake could cause a non-clinical deficiency) excess levels of creatine do not appear to “super-load” the brain similar to muscle tissue. Due to kinetics, creatine appears to be more “preventative” or acts to restore a deficiency in the brain. This is in contrast to creatine effects in muscle cells, where it can affect performance substantially on an acute timescale.
In addition to the BBB, SLC6A8 is also expressed on neurons and oligodendrocytes, but is relatively absent from astrocytes, including the astrocytic feet which line 98% of the BBB. Creatine can still be transported into astrocytes (as well as cerebellar granule cells) via SLC6A8, as incubation with an SLC6A8 inhibitor prevents accumulation in vitro. It seems to be less active in a whole brain model, relative to other brain cells.
That being said, many brain cells express both AGAT and GAMT, two enzymes that mediate creatine synthesis. Neural cells have the capacity to synthesize their own creatine.
Without supplementation, approximately 14.6mmol (2g) of creatinine, creatine’s urinary metabolite, is lost on a daily basis in a standard 70kg male aged 20-39. The value is slightly lower in females and the elderly due to a presence of less muscle mass. This amount is considered necessary to obtain in either food or supplemental form to avoid creatine deficiency. Requirements may be increased in people with higher than normal lean mass. Creatine excretion rates on a daily basis are correlated with muscle mass, and the value of 2g a day is derived from the aforementioned male population with about 120g creatine storage capacity. Specifically, the rate of daily creatine losses is about 1.6%-1.7%, and mean losses for women are approximately 80% that of men due to less average lean mass. For weight-matched elderly men (70kg, 70-79 years of age) the rate of loss of 7.8mmol/day is about half (53%) that of younger men.
Creatine appears to have a “Daily Requirement” like a vitamin to maintain sufficient levels, at or around 2 grams assuming a “normal” 70-kg male body.
Creatine levels in the blood tend to return to baseline (after a loading with or without the maintenance phase) after 28 days without creatine supplementation. This number may vary slightly from one individual to another, and for some may exceed 30 days. Assuming an elimination rate of creatinine (creatine’s metabolite) at 14.6mmol per day, six weeks of cessation is approaching the upper limit for serum creatine to completely return to baseline.
Despite this decrease to baseline levels, muscle creatine and phosphocreatine levels may still be elevating and provide ergogenic effects.
Creatine can be elevated above baseline after supplementation of more than 2 grams, and depending on the degree of loading it may elevate bodily creatine stores for up to 30 days.
Creatine retention (assessed by urinary analysis) tends to be very high on the first loading dose (65±11%) and declines throughout the loading phase (23±27%). This is likely due to increased muscular uptake when creatine stores are relatively low, which has been noted vegetarians. So creatine absorption is very high initially, but decreases througout the loading phase, as muscle creatine stores increase.
Coingestion of creatine with carbohydrate is known to increase glycogen accrual in skeletal muscle (possibly resulting in increased cell volume) although the creatine content in muscles does not appear to be significantly increased.
Creatine maintenance is known as the period following loading (if the user chooses to load) and its duration may be indefinite. The goal of maintenance is to find the lowest daily dose required to optimize creatine stores and benefits of supplementation while reducing potential side-effects of loading (intestinal and gastric distress).
Sedentary people who undergo a loading period (2g of creatine daily for up to six weeks) are able to retain much of the creatine loading into skeletal muscles. Studies following this protocol note that (total free creatine) a 30.6% increase with loading is attenuated to 12.9%.
This partial preservation of creatine stores with 2g may be wholly irrelevant in athletes, as assessed by elite swimmers where 2g as maintenance (no loading phase) failed to alter creatine content in muscle whatsoever.
A maintenance phase of 2g daily appears to technically preserve creatine content in skeletal muscle of responders either inherently or after a loading phase, but in sedentary people or those with light activity, creatine content still progressively declines (although it still higher than baseline levels after six weeks) and glycogen increases seem to normalize. 2g may be wholly insufficient for athletes, and a 5g maintenance protocol may be more prudent.
When looking collectively at the benefits accrued during loading and maintenance, it appears that changes in body mass are additive during this phase (increased during loading, further increased during maintenance).
Despite a possible decreasing creatine content in the muscles when maintenance is deemed suboptimal, the overall retention of weight and lean mass is merely additive over time. This is thought to be due to increases in skeletal muscle production (increase in body weight) compensating for the progressive declines in water and glycogen content (decreases in body weight).
When total free creatine declines (from 30.6% to 12.9%), the increase in glycogen seen during loading appears to normalize at a faster rate, so 2g of maintenance may not be sufficient to preserve glycogen either.
4.10. Mineral Bioaccumulation
Creatine at a concentration of 3mM does not appear to bind to nor modify the oxidant effects of iron in vitro.
5.1. Glutaminergic Neurotransmission
In vitro, creatine (0.125mM or higher) can reduce excitotoxicity from glutamate, which is thought to be secondary to preserving intracellular creatine phosphate levels.
Glutamate-induced excitotoxicity is caused by excessive intracellular calcium levels resulting from ATP depletion. Since high levels of calcium inside the cell are toxic, ATP preserves membrane integrity, in part by promoting calcium homeostasis. When ATP is depleted, the sodium-potassium ATPase pump (Na+,K+-ATPase) stops working, leading to sodium accumulation in the cell. This reduces the activity of the sodium-calcium exchange pump which, along with lack of ATP, reduces calcium efflux through the Na+,K+-ATPase. Thus, ATP depletion leads to intracellular calcium overload, loss of membrane potential, and excitotoxic cell death. Therefore by helping to preserve ATP levels, creatine is protective against excitotoxicity. This protective effect was noted after either creatine preloading or addition up to 2 hours after excitotoxicity. Protection from glutamate-induced toxicity also extends to glial cells and is additive with COX2 inhibition.
Creatine has been confirmed to be neuroprotective against excitotoxicity at a dietary level of 1% in rats (with no protective effect against AMPA or kainate receptors).
Creatine appears to be neuroprotective against glutamate-induced excitotoxicity. By helping to maintain intracellular ATP levels, creatine prevents the toxic accumulation of calcium inside cells, a driver of excitoxicity.
Creatine has been noted to increase the amplitude (0.5-5mM) and frequency (25mM only) of NMDA receptors although concentrations of 0.5-25mM also reduced signaling intensity. This was accredited to creatine causing an increase in ligand binding of glutamate with an EC50 of 67µM and maximal activity at 1mM creatine (158±16% of baseline). Creatine appears to modulate the polyamine binding site of the NMDA receptor as it is abolished by arcaine and potentiated by spermidine. This binding site is known to modify NMDA receptor affinity.
Activation of NMDA receptors is known to stimulate Na+,K+-ATPase activity secondary to calcineurin, which which has been confirmed with creatine in hippocampal cells (0.1-1mM trended, but 10mM was significant). This is blocked by NMDA antagonists. This increase in Na+,K+-ATPase activity is also attenauted with activation of either PKC or PKA which are antagonistic with calcineurin.
Creatine appears to positively regulate the polyamine binding site of NMDA receptors, thereby increasing signaling through this receptor and the effects of agonists such as glutamate or D-Aspartic Acid. This is a potential mechanism for cognitive enhancement.
In a prolonged study on mice, it was found that there was a two-fold upregulation of the transporter protein SLC1A6, which mediates glutamate uptake into cells. This may underlie the reduction of brain glutamate levels by creatine seen in Huntington’s Disease.
This is thought to be relevant since, in a study on subjects with amyotrophic lateral sclerosis (ALS), 15g of creatine daily was found to result in a significant reduction in combined glutamate and glutamine levels in the brain (not seen after 5-10g daily).
Creatine may also promote uptake glutamate into cells. How this influences signaling and neuroprotection is not yet clear.
5.2. GABAergic Neurotransmission
In isolated striatal cells (expressing creatine kinase), seven day incubation of 5mM creatine (maximal effective dose) appears to increase the density of GABAergic neurons and DARPP-32 (biomarker for spiny neurons) with only a minor overall trend for all cells and showed increased GABA uptake into these cells, as well as providing protection against oxygen and glucose deprivation.
5.3. Serotonergic Neurotransmission
One rat study that compared male and female rats and used a forced swim test (as a measure of serotonergic activity of anti-depressants) found that a sexual dimorphism existed, and females exerted a serotonin mediated anti-depressant response while male rats did not. It appears that these anti-depressive effects are mediated via the 5-HT1A subset of serotonin receptors, as the antidepressant effects can be abolished by 5-HT1A inhibitors.
In females, the combination of SSRIs (to increase serotonin levels in the synapse between neurons) and creatine shows promise in augmenting the anti-depressive effects of SSRI therapy. Another pilot study conducted on depression and females showed efficacy of creatine supplementation. The one study measuring male subjects noted an increase in mood and minimal anti-depressive effects, but it is not know whether this is due to gender differences or the model studies (Post-Traumatic Stress Disorder).
There is insufficient evidence to refute the notion that creatine supplementation only exerts anti-depressive effects in females. The evidence to suggest that creatine is an anti-depressant (via serotonergic mechanisms) appears to be much stronger for women than men
In humans, studies that investigate links between serotonin and creatine supplementation find that 21 trained males, given creatine via 22.8g creatine monohydrate (20g creatine equivalent) with 35g glucose, relative to a placebo of 160g glucose, was found to reduce the perception of fatigue in hot endurance training, possibly secondary to serotonergic modulation, specifically attentuating the increase of serotonin seen with exercise (normally seen to hinder exercise capacity in the heat) while possibly increasing dopaminergic activity (conversely seen to benefit activity in the heat).
Conversely, the suppression of serotonin spikes seen in males may enhance physical performance during periods when the body would normally overheat. The idea that this does not work in females cannot be refuted at this time
5.4. Dopaminergic Neurotransmission
Creatine may preserve dopamine synthesis in the striatum of mice (while protecting against dopaminergic depletion) when fed to mice at 2% of the diet for one week prior to MPTP toxicity. This is possibly secondary to increasing tyrosine hydroxylase activity, the rate-limiting step of dopamine biosynthesis. Two percent creatine was as protective as 0.005% rofecoxib (a COX2 inhibitor), but the two were additive in their protective effects (highly synergistic in regards to DOPAC by normalizing it, but not synergistic in preserving HVA).
The neuroprotective effects of creatine appear to exist in regards to dopamine biosynthesis, and the suppression of dopamine synthesis seen with some neurological toxins appears to be partially attenuated with dietary intake of creatine. The protective effect is weak to moderate in animal research, but appears to be additive with anti-inflammatories
5.5. Cholinergic Neurotransmission
Oral intake of 5-15g of creatine daily, over 1-15 days, has failed to modify neural concentrations of choline in subjects with amyotrophic lateral sclerosis (ALS) despite brain creatine increasing at 15g.
Creatine, through its ability to act as an energy reserve, attenuates neuron death induced by the MPTP toxin that can produce Parkinson’s Disease-like effects in research animals, reduces glutamate-induced excitotoxicity, attenuates rotenone-induced toxicity,L-DOPA induced dyskinesia, 3-nitropropinoic acid, and preserves growth rate of neurons during exposure to corticosteroids (like cortisol) which can reduce neuron growth rates. Interestingly, the energetic effect also applies to Alzheimer’s Disease, where creatine phosphate per se attenuates pathogenesis in vitro, yet creatine per se did not.
These effects are secondary to creatine being a source of phosphate groups and acting as an energy reserve. The longer a cell has energy, the longer it can preserve the integrity of the cell membrane by preserving integrity of Na+/K+-ATPase and Ca2+-ATPase enzymes. Preserving ATP allows creatine to act via a nongenomic response (not requiring the nuclear DNA to transcribe anything), and appears to work secondary to MAPK and PI3K pathways.
A protective effect on neurons by creatine, secondary to its ability to donate phosphate groups, exists and appears to be quite general in its protective effects
When assessing the antioxidant effects of creatine, it does not appear to sequester superoxide and may not be a direct antioxidant. Additionally, creatine failed to protect neurons from H2O2 incubation to induce cell death via pro-oxidative means. These results are in contrast to previously recorded results suggesting creatine as a direct anti-oxidant.
Some reports exist that creatine may be a direct anti-oxidant, but these have failed to be replicated. Creatine most likely does not possess anti-oxidant potential
The concentration of creatine that increases mitochondrial respiration in skeletal muscle (20mM) and this concentration also appears to work similarly in hippocampal cells. This promotes endogenous PSD-95 clusters and subsequently synaptic neurogenesis (thought to simply be secondary to promoting mitochondrial function).
Mitochondrial function per se appears to promote neuronal growth and proliferation, and at least in vitro, creatine is known to do the same and promote growth
5.8. Oxygenation and Blood Flow
One of the studies noting a reduction in fatigue in healthy subjects given creatine (8g) for five days prior to a mathematical test noted a relative decrease in oxygenation hemoglobin in the brain and an increase in deoxygenated, which normally indicates a reduction in cerebral oxygenation. The authors made note of how cytoplasmic phosphocreatine can increase oxygen uptake into cells (noted in vitro in a concentration dependent manner between 0-25mM) and suggested that either cells were taking up more oxygen from hemoglobin, or that increased mitochondrial efficiency resulted in less of a need for oxygen.
Creatine has been sought after for its effects on depression, due to the significant changes occurring in brain morphology and neuronal structure associated with depression and low brain bioenergetic turnover in depression, perhaps related to abnormal mitochondrial functioning, which reduces available energy for the brain. The general association of low or otherwise impaired phosphate energy systems (of which creatine forms the energetic basis of) with depression, has been noted previously. Due to associations with cellular death and impaired bioenergetics with depression, creatine was subsequently investigated.
Oral ingestion of 1-1000mg/kg bodyweight of creatine in mice was able to exert an anti-depressive effect, which was blocked by dopamine receptor antagonists. A low dose of creatine (0.1mg/kg) was able to enhance the dopaminergic effects of dopamine receptor activators, suggesting supplemental creatine can positively influence dopamine signalling and neurotransmission.
Mechanically, creatine may exert anti-depressant effects via mixed dopaminergic and serotonergic mechanisms. The exact mechanisms are not clear at this time
Anti-depressive effects have been noted in humans, where 5g of creatine monohydrate daily for 8 weeks was able to augment the efficay of SSRI anti-depressants. Benefits were seen at week two and were maintained until the end of the 8 week trial. These effects were noted before in a preliminary study of depressed adolescents (with no placebo group) showing a 55% reduction in depressive symptoms at 4g daily when brain phosphocreatine levels increased. Other prelimnary human studies suggest creatine might lessen unipolar depression and one study on Post-Traumatic Stress Disorder (PTSD) noted improved mood as assessed by the Hamilton Depression Rating Scale.
It is possible that females could benefit more than males due to a combined lower creatine kinase activity as well as having altered purine metabolism during depression, but no human comparative studies have been conducted yet. One rat study noted that creatine monohydrate at 2-4% of feed had 4% creatine able to exert anti-depressive and anxiolytic effects in female rats only.
Intervention studies with creatine supplementation and depression show promise, but only one well conducted study (used alongside SSRI pharmaceuticals) has been done, while other studies have flaws. Promising, but no conclusions can be made at this time
5.10. Brain Injury
Most causes of brain injury (calcium influx, excitotoxicity, lipid peroxidation, reactive oxygen intermediates or ROIs) all tend to ultimately work secondary to damaging the mitochondrial membrane and reducing its potential (which ultimately causes cellular apoptosis). Traumatic brain injuries are thought to work vicariously through ROIs by depleting ATP concentrations. Creatine appears to preserve mitochondrial membrane permeability in response to traumatic brain injury (1% of the rat’s diet for four weeks) which is a mechanism commonly attributed to its ATP buffering ability.
Brain injuries tend to cause continued damage to cells, secondary to ATP depletion, and creatine appears to preserve mitochondrial membrane permeability in response to brain injury, which is thought to be due to its ability to preserve ATP
In rats and mice given creatine injections (3g/kg) for up to five days prior to traumatic brain injury, supplementation was able to reduce brain injury by 3-36% (time dependent, with five days being more protective than one or three), and dietary intake of creatine at 1% over four weeks halved subsequent injuries.
Daily intake of creatine in rats appears to be capable of halving the effects of brain injuries
In children and adults with tramautic brain injury (TBI), six months of creatine supplementation of 400mg/kg bodyweight appears to significantly reduce the frequency of headaches (from 93.8% to 11.1%), fatigue (from 82.4% to 11.1%), and dizziness (from 88.9% to 43.8%), relative to unblinded control.
Preliminary evidence suggests that headaches and dizziness associated with brain injury can be attenuated with oral supplementation of creatine
5.11. Addiction and Drug Abuse
Due to a combination of its neuroprotective effects and dopaminergic modulatory effects, creatine has been hypothesized in at least one review article to be of benefit to drug rehabilitation. This study used parallels between drug abuse (usually methamphetamines) and traumatic brain injury and made note of creatine being able to reduce symptoms of brain trauma such as headaches, fatigue, and dizziness in clinical settings in two pilot studies. No studies currently exist that look at creatine supplementation and drug rehabilitation.
5.12. Memory and Learning
Acute administration of creatine (intra-cranial) appears to enhance learning from a previous stimuli, vicariously through the NDMA receptor and was enhanced via coincubation of spermidine, which amplifies NMDA currents.
In rats, an enhancement of spatial learning appears to be apparent and mediated via the NMDA receptor; a similar mechanism to preliminary studies in D-Aspartic Acid
Studies conducted in vegetarians tend to show cognitive enhancement in youth, possibly due to a creatine deficiency, as compared to omnivores. Vegetarian diets have lower levels of circulating creatine prior to supplementation, but attain similar circulating levels as omnivores when both supplement. Building on this latter point, supplementation of creatine monohydrate in a loading protocol (20g daily in orange juice) in omnivores does not alter levels of creatine in white matter tissue in the brain (test subjects: competitive athletes). On most of the parameters that vegetarians experience benefits, omnivores fail to experience statistically significant benefits, except possibly when sleep deprived, where the cognitive improvements rival that seen in vegetarians. Elderly people who are omnivorous may also experience increases in cognition to a similar level, in regards to long-term memory as well as forward number and spatial recall, although the study in question failed to find any significant benefit on backwards recall or random number generation, the latter of which is a test for executive working memory.
Creatine has been demonstrated to increase cognition (memory, learning, and performance) in people with no dietary creatine intake (vegetarians and vegans). These benefits also appears to extend to the sleep deprived and elderly people without any saliant cognitive decline
In a rested state, young omnivores may experience an increase in reaction speed.
One study that did not control for dietary or lifestyle choices, but was conducted in young healthy adults, noted that 8g of creatine supplementation daily (spread out in multiple doses) for 5 days was able to reduce fatigue during a mathematical test (Uchida-Kraepelin test).
Creatine has limited potential in increasing cognition in otherwise healthy young omnivores, but it does possess a general pro-cognitive effect
5.13. Sedation and Sleep
5g of creatine four times daily for a week (loading) before sleep deprivation for 12-36 hours was able to preserve cognition during complex tasks of executive function at 36 hours only, without significant influence on immediate recall or mood. A similar protocol replicated the failure to improve memory and attention, but noted less reports of fatigue (24 hours) and less decline of vigor (24 hours) although other mood parameters were not measured.
6.1. Cardiac Tissue
The creatine kinase (CK) enzyme in rat heart tissue appears to have a KM around 6mM of creatine as substrate[reference|url=http://www.