Can you increase creatine levels by eating more arginine and glycine?

Marginally. While arginine and glycine are required precursors, endogenous creatine synthesis is tightly regulated by AGAT enzyme activity (which is feedback-inhibited by creatine itself). Eating more precursors provides diminishing returns. Direct creatine supplementation at 3-5g/day is far more effective than increasing precursor intake.

Which foods are high in glycine and arginine for creatine production?

Red meat, poultry, fish, and dairy are rich in both glycine and arginine. Gelatin and bone broth are exceptionally high in glycine. Nuts, seeds, and legumes provide arginine. However, these foods also contain preformed creatine (in the case of meat and fish), making supplementation more efficient.

Is glycine conditionally essential because of creatine synthesis?

Some researchers argue yes. The combined demands of creatine synthesis, glutathione production, and collagen turnover may exceed the body's capacity for glycine synthesis, especially during growth, pregnancy, or on plant-based diets. Creatine supplementation reduces this glycine demand significantly.

Creatine, Glycine, and Arginine: The Biosynthetic Precursors

The First Step: Arginine Meets Glycine

Creatine biosynthesis begins when two amino acids — arginine and glycine — react in the kidneys under the catalysis of AGAT (arginine:glycine amidinotransferase). This reaction transfers the amidino group from arginine to glycine, producing guanidinoacetate (GAA) and ornithine (T et al., 2011) .

Arginine + Glycine → GAA + Ornithine

This is the rate-limiting step of creatine synthesis, and its regulation has far-reaching implications for amino acid metabolism throughout the body.

1-2g

of creatine produced daily by the body from arginine, glycine, and methionine precursors

Wallimann et al., 2011

AGAT Enzyme: Structure and Regulation

AGAT is a mitochondrial enzyme expressed primarily in the kidneys, with lower levels in the pancreas and brain. Its regulation is particularly elegant:

Feedback inhibition by creatine:

Creatine itself inhibits AGAT gene transcription
When intracellular creatine levels rise (as with supplementation), AGAT expression decreases
This reduces GAA production, effectively shutting down the pathway when exogenous creatine is available
The downregulation takes approximately 2-4 weeks to reach full effect

Hormonal regulation:

Growth hormone and thyroid hormones upregulate AGAT activity
This connects creatine synthesis to the body’s overall growth and metabolic status

Substrate availability:

AGAT activity is influenced by the availability of both arginine and glycine
The Km (Michaelis constant) for arginine is approximately 2-3 mM, which is near physiological plasma concentrations
This means AGAT operates at sub-saturating substrate levels and is sensitive to dietary arginine fluctuations

Arginine: The Amidino Donor

Arginine contributes the amidino group (-C(=NH)NH2) to glycine during creatine synthesis. This is the same functional group that gives creatine its characteristic guanidino structure, which is essential for phosphocreatine formation.

The arginine consumed by creatine synthesis has significant implications:

Competition with nitric oxide synthesis: Arginine is also the substrate for nitric oxide synthase (NOS), which produces nitric oxide (NO) — a potent vasodilator. In theory, heavy demand on arginine for creatine synthesis could limit NO production. However, the body compartmentalizes these reactions:

Creatine synthesis occurs primarily in the kidneys
NO production occurs in endothelial cells, neurons, and immune cells
Each tissue maintains its own arginine pool to some degree

Nonetheless, creatine supplementation reduces arginine consumption for creatine synthesis, potentially freeing more arginine for NO production in other tissues.

Ornithine production: The AGAT reaction produces ornithine as a byproduct. Ornithine is an intermediate in the urea cycle, which converts toxic ammonia to urea for excretion. Reduced creatine synthesis (due to supplementation) means less ornithine production, but this is compensated by the body’s other ornithine sources (RB et al., 2017) .

Glycine: The Acceptor Amino Acid

Glycine serves as the structural backbone of creatine, accepting the amidino group from arginine. Despite being the simplest amino acid (with just a hydrogen atom as its side chain), glycine’s metabolic demands are enormous:

Creatine synthesis: 1-2g of glycine consumed daily
Glutathione synthesis: glycine is one of three amino acids forming glutathione (the body’s primary antioxidant)
Collagen synthesis: glycine is every third amino acid in collagen, the body’s most abundant protein
Bile acid conjugation: glycine conjugates bile acids for fat digestion
Porphyrin synthesis: glycine is incorporated into heme (hemoglobin)
Hippuric acid formation: glycine conjugates benzoate for urinary excretion

Some researchers estimate that total daily glycine demand may exceed 10g, while endogenous glycine synthesis capacity is approximately 3g/day. Dietary glycine intake on a typical mixed diet provides 3-5g/day. This creates a potential shortfall, leading to the proposal that glycine is conditionally essential.

By supplementing with creatine and eliminating 1-2g/day of glycine demand for creatine synthesis, the body gains significant relief for its other glycine-dependent processes.

Dietary Considerations for Precursor Amino Acids

The availability of arginine and glycine for creatine synthesis varies substantially with diet (DG et al., 2003) :

Omnivorous diets:

Provide preformed creatine from meat and fish (1-2g/day from a typical diet)
Rich in arginine and glycine from animal proteins
The combination of dietary creatine and abundant precursors means endogenous synthesis demand is moderate

Vegetarian and vegan diets:

Provide zero (vegan) or minimal (lacto-ovo vegetarian) dietary creatine
The body must synthesize all needed creatine from amino acid precursors
This places a greater burden on arginine, glycine, and methionine pools
Explains why vegetarians have lower baseline muscle creatine stores and show greater response to supplementation

High-protein diets:

Provide excess arginine and glycine beyond creatine synthesis needs
Also typically provide significant dietary creatine from meat sources
Endogenous synthesis is naturally downregulated due to dietary creatine intake

The GAA Intermediate: A Biomarker

Guanidinoacetate (GAA), the product of the AGAT reaction, has gained attention as both a biomarker and a potential supplement:

Elevated urinary GAA indicates active creatine synthesis
Very high plasma GAA with low creatine suggests GAMT deficiency (the rare genetic condition where GAA cannot be methylated to creatine)
GAA supplementation has been explored as an alternative to creatine, since it is methylated to creatine in the liver. However, GAA supplementation increases homocysteine (it consumes SAM), while creatine supplementation does not

Summary

The AGAT-catalyzed reaction between arginine and glycine is the first and rate-limiting step of creatine biosynthesis. This reaction consumes significant amounts of both amino acids and connects creatine metabolism to nitric oxide production, antioxidant defense, and connective tissue health. Creatine supplementation reduces the demand on these precursors, providing metabolic relief particularly valuable for vegetarians, the elderly, and individuals with high metabolic demands.