The methylation of guanidinoacetate to form creatine consumes more methyl groups than all other methylation reactions combined. The study examined increased or decreased methyl demand by feeding rats guanidinoacetate- or creatine-supplemented diets for 2 weeks. Plasma homocysteine was significantly lower in rats fed creatine-supplemented diets, and kidney arginine:glycine amidinotransferase activity was significantly decreased in both supplementation groups.

Background: Creatine synthesis from guanidinoacetate consumes about 50% of SAM-derived methyl groups, accounting for an equivalent proportion of SAH and total homocysteine synthesis. The trial tested whether creatine supplementation lowers plasma guanidinoacetate, increases blood SAM, lowers blood SAH, and lowers plasma total homocysteine. Results showed plasma guanidinoacetate declined with creatine supplementation, but the authors concluded that creatine supplementation did not lower plasma total homocysteine on average in humans.

Irreversible methylation of GAA by guanidine N-methyltransferase (GNMT, EC 3.5.3.2) utilizes S-adenosylmethionine (SAM) as the methyl donor and results in the formation of creatine and S-adenosylhomocysteine (SAH). Creatine supplement caused a significant decrease in the rate of synthesis of creatine. Five days of creatine supplement resulted in ~50% reduction in plasma GAA concentration in all subjects. Based upon this analysis, we attribute the lower rate of glycine synthesis following creatine supplement to the decrease in the demand for glycine and for methyl groups for creatine synthesis.

Because the methylation of guanidinoacetate to form creatine utilizes S-adenosylmethionine (SAM) as a methyl donor, it has been estimated that creatine synthesis accounts for up to 40–50% of all SAM-dependent methylation in the body. It has therefore been hypothesized that oral creatine supplementation, by down-regulating endogenous creatine synthesis, could spare SAM and consequently reduce homocysteine formation. In this trial, creatine supplementation led to a reduction in plasma guanidinoacetate concentrations, indicating suppression of endogenous creatine synthesis, but did not significantly change plasma total homocysteine under the conditions tested.

Creatine synthesis accounts for more than 70% of the labile methyl groups derived from S-adenosylmethionine (SAM). Thus, suppression of endogenous creatine synthesis by creatine supplementation might reduce methylation demand and lower potentially toxic homocysteine production. In this study, oral creatine supplementation (20 g/day for 5 days) in healthy men decreased the urinary excretion of guanidinoacetate by 50%, indicating suppression of endogenous creatine synthesis. Plasma and urinary markers of methylation demand and homocysteine metabolism were also altered in a manner consistent with reduced methyl group utilization for creatine synthesis.

The methylation of guanidinoacetate to creatine is a major consumer of S-adenosylmethionine (SAM), accounting for as much as 40% of labile methyl group use. Supplemental creatine has been proposed to down-regulate endogenous creatine synthesis and thereby decrease the methylation demand placed on SAM, which in turn might lower homocysteine production. Studies in humans and animals have consistently shown that creatine loading decreases plasma guanidinoacetate, supporting a reduction in endogenous creatine synthesis, although effects on homocysteine concentrations have been variable.

In mammals, creatine is synthesized from arginine and glycine to form guanidinoacetate (GAA), which is then methylated by guanidinoacetate N-methyltransferase using S-adenosylmethionine as the methyl donor. Dietary creatine resulted in a marked suppression of AGAT activity and a reduction in GAA concentrations, demonstrating feedback inhibition of endogenous creatine synthesis by preformed creatine. These findings support the concept that exogenous creatine can down-regulate creatine biosynthesis and thus reduce the requirement for S-adenosylmethionine-dependent methylation of GAA.

Creatine is synthesized in a two-step process involving arginine:glycine amidinotransferase (AGAT) and guanidinoacetate methyltransferase (GAMT). GAMT catalyzes the transfer of a methyl group from S-adenosylmethionine (SAM) to guanidinoacetate to form creatine and S-adenosylhomocysteine. Because the GAMT reaction uses SAM as a methyl donor, creatine synthesis is a major consumer of methyl groups in human metabolism. Deficiency of GAMT leads to accumulation of guanidinoacetate and reduced creatine, illustrating the dependence of creatine biosynthesis on SAM-dependent methylation.

In this mouse study, only guanidinoacetate supplementation lowered methyl-3H incorporation into phosphatidylcholine and protein and lowered hepatic SAM concentration compared with controls. The result is relevant because it shows that altering guanidinoacetate availability changes SAM-dependent methylation demand, whereas creatine itself did not produce the same methylation effect in this experiment.

Creatine is synthesized in two steps: first, arginine and glycine are converted to guanidinoacetate by arginine:glycine amidinotransferase (AGAT), then guanidinoacetate is methylated by guanidinoacetate N-methyltransferase (GAMT) using S-adenosylmethionine (SAM) as methyl donor. This methylation step is one of the major consumers of SAM-derived methyl groups in the body. Oral creatine supplementation in patients with AGAT deficiency bypasses the need for endogenous creatine synthesis and corrects the metabolic abnormalities associated with low creatine, illustrating how exogenous creatine can replace the requirement for SAM-dependent creatine biosynthesis.

Guanidinoacetate N-methyltransferase (GAMT) catalyzes the methylation of guanidinoacetate to creatine using S-adenosylmethionine (SAM) as the methyl donor. In GAMT deficiency, guanidinoacetate accumulates and creatine is depleted; treatment with oral creatine supplies creatine exogenously and diminishes the metabolic drive for endogenous synthesis. By providing creatine directly, supplementation reduces the flux through the SAM-dependent methylation step normally required for creatine biosynthesis.

Creatine synthesis has been estimated to account for up to 40–50% of all SAM-derived methyl group consumption in humans, making it one of the largest single methylation reactions in the body. The methylation of guanidinoacetate to creatine by guanidinoacetate N-methyltransferase uses S-adenosylmethionine as the methyl donor and produces S-adenosylhomocysteine, which is subsequently hydrolyzed to homocysteine. Because of this high methyl demand, interventions that suppress endogenous creatine synthesis, such as dietary creatine supplementation, have been proposed as a way to spare methyl groups and potentially lower homocysteine production.

This review explains that creatine biosynthesis is a major consumer of methyl groups because guanidinoacetate is methylated to creatine by guanidinoacetate N-methyltransferase using SAM as the methyl donor. It also notes that dietary creatine can reduce endogenous creatine synthesis through feedback on AGAT, thereby lowering the methylation burden associated with making creatine.

Creatine is formed from guanidinoacetate via a methylation reaction that consumes S-adenosylmethionine (SAM) and yields S-adenosylhomocysteine, a precursor of homocysteine. Creatine supplementation significantly decreased plasma guanidinoacetate concentrations, consistent with a suppression of endogenous creatine synthesis. These data support the hypothesis that exogenous creatine can lower the demand for SAM-dependent creatine biosynthesis, although the expected concomitant reductions in plasma homocysteine were modest.

Creatine is synthesized in a two-step process, the second of which is the methylation of guanidinoacetate by guanidinoacetate N-methyltransferase using S-adenosylmethionine (SAM) as methyl donor. This reaction accounts for a substantial proportion of total SAM-dependent methylation in the body. Because exogenous creatine suppresses endogenous synthesis via feedback inhibition of AGAT, it has been hypothesized that creatine ingestion could reduce methylation demand and thereby lower plasma homocysteine. In this randomized trial, daily creatine supplementation (20 g/d for 5 days, then 3 g/d for several weeks) led to a significant reduction in fasting plasma homocysteine compared with baseline, consistent with a sparing of methyl groups.

Endogenous creatine is synthesized in a two-step process in the kidney and liver from arginine, glycine and methionine, with the second step involving methylation of guanidinoacetate by S-adenosylmethionine (SAM). Because this methylation reaction represents a major consumer of SAM-derived methyl groups, it has been proposed that exogenous creatine supplementation may reduce the demand for endogenous creatine synthesis and thereby spare methyl groups. Human studies have reported that creatine loading lowers plasma guanidinoacetate, indicating suppression of creatine biosynthesis, but effects on downstream homocysteine levels are inconsistent.

This review states that creatine biosynthesis requires SAM-dependent methylation of guanidinoacetate and that this pathway is a major route of methyl-group utilization. It also explains that dietary creatine suppresses endogenous creatine synthesis, which reduces the need to methylate guanidinoacetate.

Endogenous synthesis of creatine requires a methyl group from S-adenosylmethionine to convert guanidinoacetate into creatine. This ensures that the total body store of creatine is maintained, as creatine used in body tissues can be replenished from the diet or de novo synthesis. Supplementation with creatine increases the contribution from dietary creatine and may down-regulate endogenous creatine synthesis through feedback mechanisms, which in turn could influence methyl group metabolism.

Creatine biosynthesis begins with the formation of guanidinoacetate from arginine and glycine by l-arginine:glycine amidinotransferase (AGAT), followed by methylation of guanidinoacetate by guanidinoacetate N-methyltransferase using S-adenosylmethionine (SAM) as methyl donor. This methylation step accounts for 40–70% of SAM-derived methyl group utilization in various tissues. Because creatine supplementation down-regulates AGAT, thereby decreasing guanidinoacetate production, exogenous creatine has the potential to reduce the metabolic burden placed on SAM-dependent methylation reactions.

Endogenous creatine synthesis is influenced by dietary intake of precursor amino acids, imposing a metabolic burden on their availability. The first step of creatine biosynthesis is catalyzed by arginine:glycine amidinotransferase (AGAT), producing guanidinoacetate, which is then methylated by guanidinoacetate methyltransferase using S-adenosylmethionine as a methyl donor. Because this methylation step consumes S-adenosylmethionine, higher rates of endogenous creatine synthesis increase methyl group demand, while dietary creatine intake that suppresses endogenous synthesis would be expected to reduce S-adenosylmethionine utilization for this pathway.

Creatine biosynthesis is responsible for a large proportion of S-adenosylmethionine (SAM)-dependent methylation reactions and is a major source of homocysteine production. It has been suggested that creatine supplementation might down-regulate endogenous creatine synthesis, thereby reducing the demand for SAM and lowering plasma homocysteine concentrations. In this randomized trial, creatine supplementation reduced plasma guanidinoacetate levels, consistent with decreased endogenous creatine synthesis, but produced no significant change in fasting plasma homocysteine.

Creatine synthesis has been estimated to consume approximately 40–50% of the S-adenosylmethionine-derived methyl groups in humans, reflecting the high flux through guanidinoacetate N-methyltransferase. This makes the pathway a major determinant of methyl group demand and homocysteine production. Because creatine can also be obtained from the diet, exogenous creatine intake reduces the need for endogenous synthesis. This reduction in endogenous creatine synthesis is expected to lower the demand for S-adenosylmethionine and associated methyl group turnover.

Guanidinoacetate is the immediate precursor for creatine synthesis and is methylated by guanidinoacetate N-methyltransferase using S-adenosylmethionine as methyl donor. In this randomized, double-blind, placebo-controlled study, subjects received 3 g creatine monohydrate daily for 4 weeks. Creatine supplementation resulted in a marked decrease (approximately 50%) in plasma guanidinoacetate concentrations compared with placebo, indicating a reduced rate of endogenous creatine synthesis and a potential reduction in S-adenosylmethionine demand for guanidinoacetate methylation.

This human trial examined whether creatine supplementation altered methylation metabolites in healthy adults. The abstract reports changes in creatine and guanidinoacetate-related measures and was designed around the idea that creatine supplementation may spare methyl groups used in endogenous creatine synthesis.

Because creatine synthesis is responsible for a large portion of S-adenosylmethionine-dependent methylation, it has been hypothesized that creatine supplementation, by suppressing endogenous creatine synthesis, would reduce methyl group demand and lower plasma homocysteine concentrations. In this randomized trial, creatine loading decreased guanidinoacetate excretion, consistent with reduced endogenous creatine synthesis, and produced modest changes in plasma homocysteine and related methylation markers, supporting the concept that creatine intake can influence methyl group economy by reducing S-adenosylmethionine utilization for creatine biosynthesis.

The synthesis of creatine from guanidinoacetate is catalyzed by guanidinoacetate N-methyltransferase and consumes a methyl group from S-adenosylmethionine, generating S-adenosylhomocysteine and ultimately homocysteine. Because creatine supplementation inhibits the first step in creatine biosynthesis and lowers guanidinoacetate levels, it has been suggested that exogenous creatine could reduce S-adenosylmethionine utilization and homocysteine formation. Experimental data in humans show consistent reductions of plasma guanidinoacetate with creatine intake, while effects on homocysteine are variable.

The methylation of guanidinoacetate to creatine is a quantitatively important methylation reaction, accounting for around 40–50% of daily S-adenosylmethionine-dependent methyl flux in omnivorous humans. This reaction is catalyzed by guanidinoacetate N-methyltransferase in the liver. Dietary creatine intake downregulates endogenous creatine synthesis, as reflected in lower guanidinoacetate production and methylation, thereby decreasing the requirement for methyl groups donated by S-adenosylmethionine.

Guanidinoacetate methyltransferase (GAMT) deficiency is a creatine synthesis disorder caused by biallelic pathogenic variants in GAMT. Treatment typically includes creatine and ornithine supplementation, with or without arginine restriction or sodium benzoate. Therapeutic response to creatine supplementation was noted in the 1990s, with normalization of creatine concentrations. Other interventions for GAMT-deficiency management include S-adenosylmethionine supplementation, aiming to support the methylation step in patients where endogenous creatine synthesis is impaired.

Creatine biosynthesis consists of two sequential reactions: first, AGAT catalyzes the formation of guanidinoacetate from arginine and glycine, and second, guanidinoacetate methyltransferase (GAMT) catalyzes the transfer of a methyl group from S-adenosylmethionine to guanidinoacetate to form creatine. Creatine feeding suppresses AGAT expression in kidney and pancreas, leading to reduced guanidinoacetate formation. This feedback inhibition by creatine explains how dietary creatine can downregulate endogenous creatine synthesis and thereby decrease the flux of S-adenosylmethionine through the GAMT reaction.

In the liver, guanidinoacetic acid (GAA) is methylated by S-adenosylmethionine (SAMe), which is the methyl donor in virtually all known biological methylations. The reaction is catalyzed by guanidinoacetate methyltransferase (GAMT). This enzymatic transmethylation reaction forms creatine and S-adenosylhomocysteine, and consumes more SAMe than all other methylation reactions in the body combined. Adequate creatine levels in the cell are maintained by endogenous synthesis or absorption following ingestion. The constant loss of creatine via nonenzymatic conversion to creatinine is compensated for by either endogenous synthesis or dietary intake, so exogenous creatine can downregulate the need for endogenous synthesis.

Creatine can be synthesized in the liver, kidney, and pancreas from the three common amino acids arginine, glycine and methionine. This involves a reversible transfer of an amidine group from arginine to glycine to form guanidinoacetic acid. An irreversible transfer of a methyl group then follows this step from S-adenosylmethionine to the guanidinoacetic acid to form creatine. When creatine is obtained in the diet this mechanism is suppressed. About two grams of creatinine, the waste product of creatine, is excreted per day and roughly half of this amount is normally from liver synthesis, the other half from dietary creatine.

Biochemical and metabolic studies consistently estimate that the methylation of guanidinoacetate to creatine by guanidinoacetate N-methyltransferase accounts for roughly 30–50% of the body's labile methyl group turnover from S-adenosylmethionine. Because this reaction is stoichiometrically linked to creatine production, any down-regulation of endogenous creatine synthesis by exogenous creatine intake proportionally reduces the amount of SAM that must be converted to S-adenosylhomocysteine and ultimately to homocysteine. This mechanistic link underlies the common hypothesis that creatine supplementation "spares" SAM by decreasing the need for endogenous creatine biosynthesis, even though clinical studies show variable effects on measured homocysteine levels.