How is creatine metabolized in the body?

Creatine is synthesized in the liver and kidneys from amino acids (arginine, glycine, methionine). It is transported to muscles via the blood and stored as free creatine or phosphocreatine. Daily, approximately 1.7% of the total creatine pool is non-enzymatically converted to creatinine, which is filtered by the kidneys and excreted in urine.

Is creatinine from creatine harmful to the kidneys?

No. Creatinine is a harmless waste product that is freely filtered by healthy kidneys. Creatine supplementation does raise creatinine levels in blood and urine, which can be misinterpreted as kidney dysfunction. However, studies up to 5 years confirm no adverse effects on kidney function in healthy individuals.

How much creatine does the body produce naturally?

The body produces approximately 1-2 grams of creatine per day through endogenous synthesis. This occurs in a two-step process: the kidney enzyme AGAT produces guanidinoacetate (GAA), which is then methylated by the liver enzyme GAMT to form creatine. This endogenous production accounts for about half of daily creatine needs.

Does creatine supplementation stop natural production?

Creatine supplementation may modestly downregulate endogenous synthesis via feedback inhibition of the AGAT enzyme. However, this is reversible — normal production resumes within days to weeks after stopping supplementation. There is no permanent effect on the body's ability to produce creatine.

Creatine Metabolism Explained: From Synthesis to Excretion

The Complete Creatine Metabolic Pathway

Creatine metabolism encompasses a full cycle: synthesis from amino acids, transport to target tissues, functional use in energy metabolism, and eventual breakdown and excretion. Understanding this complete pathway provides insight into why supplementation is effective and why it is safe (RB et al., 2017) .

Step 1: Endogenous Synthesis

Your body produces approximately 1-2 grams of creatine per day through a two-step enzymatic process involving the kidneys and liver:

Kidney Step — AGAT Reaction

The enzyme AGAT (arginine:glycine amidinotransferase) in the kidneys combines two amino acids — arginine and glycine — to produce guanidinoacetate (GAA):

Arginine + Glycine → GAA + Ornithine

This is the rate-limiting step in creatine synthesis and is subject to feedback inhibition by creatine itself.

Liver Step — GAMT Reaction

GAA is transported to the liver, where the enzyme GAMT (guanidinoacetate N-methyltransferase) adds a methyl group from S-adenosylmethionine (SAMe) to produce creatine:

GAA + SAMe → Creatine + SAH

This methylation reaction consumes a significant portion of the body’s methyl groups — approximately 40% of all SAMe-derived methyl groups are used for creatine synthesis.

~40%

of SAMe methyl groups used for endogenous creatine synthesis

Wallimann et al. 2011

Step 2: Transport and Uptake

Newly synthesized creatine enters the bloodstream and is distributed throughout the body. The sodium- and chloride-dependent creatine transporter (CrT/SLC6A8) actively pumps creatine into target cells, primarily skeletal muscle (95%), with smaller amounts going to the brain, heart, and kidneys.

Dietary creatine (from meat and fish, approximately 1-2g/day in omnivores) follows the same pathway — absorbed in the small intestine, circulated in blood, and transported into cells via CrT (RC et al., 1992) .

Step 3: Intracellular Metabolism

Once inside the cell, creatine participates in the creatine kinase (CK) reaction cycle:

Phosphorylation

The enzyme creatine kinase phosphorylates free creatine using ATP:

Creatine + ATP ⇌ Phosphocreatine + ADP

This reversible reaction is the core of creatine’s energy function. Approximately 60% of intracellular creatine exists as phosphocreatine (PCr) at rest, serving as the rapid energy reserve.

Energy Utilization

During high-energy-demand situations (muscle contraction, neural activity), the reverse reaction regenerates ATP:

Phosphocreatine + ADP → Creatine + ATP

This reaction occurs in milliseconds, providing the fastest means of ATP regeneration in the cell (T et al., 2011) .

Step 4: Creatinine Formation and Excretion

Creatine and phosphocreatine undergo irreversible, non-enzymatic degradation to creatinine at a rate of approximately 1.7% of the total pool per day. This is a spontaneous chemical reaction — not mediated by enzymes — that occurs through cyclization and dehydration.

The Numbers

For a 70kg person with a total creatine pool of ~120g:

Daily creatinine production: ~2g/day (1.7% of 120g)
With supplementation (pool ~150g): ~2.5g/day creatinine production

Renal Excretion

Creatinine is freely filtered by the kidneys and excreted in urine. It is not reabsorbed and not secreted in significant amounts. This is why serum creatinine is used clinically as a marker of kidney function — its production is relatively constant and its clearance depends on glomerular filtration rate (GFR).

~1.7%

of total creatine pool converted to creatinine daily

Biochemistry references

The Creatinine Caveat

Creatine supplementation increases serum creatinine levels, which can be misinterpreted as impaired kidney function. However, multiple long-term studies have confirmed that the elevated creatinine from supplementation does not indicate kidney damage (JR & M, 2000) .

If you are undergoing blood tests while taking creatine, inform your healthcare provider about your supplementation so they can interpret creatinine levels correctly.

The Metabolic Balance

Under normal conditions, creatine metabolism reaches a steady state:

Source	Daily Amount
Endogenous synthesis	~1-2g
Dietary intake (omnivore)	~1-2g
Total input	~2-4g
Daily creatinine loss	~2g
Net balance	Steady state

With supplementation (adding 3-5g/day):

Total input increases to ~5-9g/day
Creatinine excretion increases proportionally
Muscle creatine rises until saturation (~160 mmol/kg)
Excess beyond saturation is excreted

Feedback Regulation

Creatine metabolism is self-regulating through several feedback mechanisms:

AGAT inhibition — High creatine levels inhibit the AGAT enzyme, reducing endogenous synthesis. This is reversible upon stopping supplementation.
CrT downregulation — At muscle saturation, the creatine transporter reduces its activity, preventing over-accumulation
Increased creatinine excretion — More creatine means more creatinine, maintaining metabolic balance

These feedback mechanisms ensure that creatine supplementation is self-limiting and cannot cause dangerous accumulation.

Malaysian Context

For Malaysian consumers:

Blood tests — If your doctor orders kidney function tests, mention your creatine supplementation. Malaysian clinicians may not routinely consider supplement use when interpreting creatinine levels.
Hydration — Adequate water intake (2-3 liters daily in Malaysian climate) supports healthy creatinine excretion
Diet considerations — Malaysian diets with moderate animal protein provide some dietary creatine, but supplementation fills the gap effectively
Safety reassurance — Long-term studies up to 5 years confirm creatine does not impair kidney function in healthy individuals

Key Takeaways

Creatine is synthesized from arginine, glycine, and methionine in the kidneys and liver
Endogenous production provides 1-2g/day; diet provides another 1-2g/day in omnivores
Creatine is phosphorylated to PCr inside cells for rapid ATP regeneration
Approximately 1.7% of total creatine is converted to creatinine daily and excreted by the kidneys
Elevated creatinine from supplementation does not indicate kidney damage
The system is self-regulating through AGAT feedback inhibition and CrT downregulation

Sources & References

This article cites the ISSN Position Stand (Kreider et al., 2017), Wallimann et al. (2011), Poortmans & Francaux (2000), and Harris et al. (1992). Full citations are available in our Research Library.