TL;DR — Brosnan et al. 2007
Brosnan and Brosnan published a thorough review on the inter-organ metabolism of creatine, detailing how the body synthesizes this critical molecule through a coordinated process involving the kidneys, liver, and pancreas.
The paper quantified the metabolic cost of creatine synthesis, showing that it consumes approximately 40% of all S-adenosylmethionine (SAM) methyl groups.
This finding highlighted the significant metabolic burden of endogenous creatine production and provided a biochemical rationale for dietary creatine intake and supplementation.
Background
Creatine is not merely a supplement — it is an essential metabolite that the body produces endogenously.
Understanding how the body makes creatine provides critical context for why supplementation is beneficial, particularly for populations with low dietary creatine intake.
The creatine kinase system, reviewed comprehensively by Wallimann et al. (2011), is central to cellular energy homeostasis (Wallimann et al., 2011) .
However, the upstream synthesis pathway received comparatively less attention in the sports nutrition literature despite its importance.
The Two-Step Synthesis Pathway
Step 1: Kidneys produce GAA
The first step occurs primarily in the kidneys, where the enzyme AGAT (arginine:glycine amidinotransferase) catalyzes the transfer of an amidino group from arginine to glycine, producing guanidinoacetate (GAA) and ornithine.
Step 2: Liver methylates GAA to creatine
GAA is released into the bloodstream and taken up by the liver, where GAMT (guanidinoacetate N-methyltransferase) transfers a methyl group from S-adenosylmethionine (SAM) to GAA, producing creatine.
The newly formed creatine is then released into the blood and taken up primarily by skeletal muscle and the brain via the creatine transporter (SLC6A8).
Key Insights
1. Creatine synthesis is metabolically expensive
The methylation step consumes approximately 40% of all SAM-derived methyl groups in the body. This is a remarkable metabolic investment, larger than any other single methylation reaction.
It means creatine synthesis competes with other critical methylation processes including DNA methylation and neurotransmitter synthesis.
2. Dietary creatine reduces the synthetic burden
When creatine is obtained from diet (meat, fish) or supplements, endogenous synthesis downregulates. This spares methyl groups, arginine, and glycine for other metabolic needs.
This provides a biochemical rationale for supplementation beyond simply increasing muscle creatine stores.
3. Vegetarians bear the full synthetic cost
Without dietary creatine, vegetarians must synthesize their entire daily requirement endogenously.
This explains the lower creatine stores consistently observed in vegetarians and their robust response to supplementation, as demonstrated by Harris et al. (1992) (Harris et al., 1992) .
Practical Implications
- Creatine is semi-essential: While the body can make it, dietary intake significantly reduces the metabolic burden of synthesis
- Vegetarians especially benefit: Supplementation offsets the increased demand on methylation pathways in those without dietary creatine
- Methyl group sparing: Creatine supplementation may indirectly benefit other methylation-dependent processes
- The standard 3-5 g/day dose is rational: This approximates the combined endogenous and dietary creatine turnover rate (Brosnan & Brosnan, 2007)
Malaysian Relevance
Malaysia has a diverse dietary landscape including significant vegetarian populations within the Indian-Malaysian and Buddhist communities.
Understanding that vegetarians bear a higher metabolic cost for creatine synthesis reinforces the value of supplementation for these groups.
The biochemistry also explains why meat-eating Malaysians may have adequate baseline creatine but can still benefit from supplementation for athletic and cognitive performance.
Full Citation
Brosnan JT, Brosnan ME. Creatine: endogenous metabolite, dietary, and therapeutic supplement. Annual Review of Nutrition.
2007;27:241-261. doi:10.1146/annurev.nutr.27.061406.093621
Where This Fits in the Evidence
Most creatine pages explain what supplementation does; Brosnan and Brosnan explain why the body cannot easily make enough on its own. By mapping the two-step kidney-to-liver pathway and quantifying that creatine synthesis claims roughly 40% of the body’s SAM methyl groups, the review reframes supplementation as offloading an expensive biochemical task rather than simply topping up muscle stores. That is the mechanistic foundation beneath findings reported elsewhere in this library — particularly why vegetarians, who bear the full synthetic burden, respond so strongly. As a biochemistry review it describes the system rather than testing an intervention, so it complements the clinical trials rather than standing in for them. The broader evidence base is collected in our research library.
Sources & References
This article is based on the review by Brosnan and Brosnan published in Annual Review of Nutrition (2007) and contextualized with Wallimann et al. (2011), Harris et al. (1992), and Kreider et al. (2017).
All citations reference PubMed-indexed publications.