What Is L-Citrulline and How Does It Work?
L-citrulline is a non-essential amino acid that participates in the urea cycle and is an important precursor to nitric oxide (NO) production. The body synthesises citrulline from two main sources: from arginine in immune cells and from glutamine in the intestinal epithelium. Citrulline is then converted back to L-arginine in the kidneys, thereby raising plasma arginine levels more effectively than direct arginine consumption .
Citrulline is found naturally most abundantly in watermelon (Citrullus lanatus), from which it takes its Latin name.
Deficiency Symptoms
Primary clinical L-citrulline deficiency is rare, occurring mainly in urea cycle enzyme disorders, which are genetic conditions. However, reduced intestinal citrulline production can have functional consequences:
- Reduced NO production, affecting vascular dilation and muscle blood flow during exercise
- Decreased exercise tolerance, particularly in endurance sport
- Muscle soreness and extended recovery time after intense exertion
- Immune function decline during intestinal illness or prolonged stress
At-Risk Groups
- Endurance athletes — intense training increases NO demand; citrulline needs rise (Pérez-Guisado & Jakeman, 2010)
- People with intestinal absorption problems — the intestinal enterocytes are the primary source of citrulline; those with inflammatory bowel disease may have reduced production
- Older adults — intestinal epithelium becomes less efficient with age
- People on low-protein diets — low glutamine intake limits citrulline synthesis
- Rare genetic urea cycle disorders (e.g. citrullinaemia)
How Is It Tested?
Plasma citrulline levels can be measured by specific amino acid panels. Plasma citrulline is a biomarker of intestinal mass and is sometimes used in clinical practice as an indirect measure of gut health. In Estonia, such tests are performed when clinically suspected; they are not available as a routine test.
Nordic and Estonian Context
Triathlons, marathon and orienteering training are popular in Estonia. High-volume endurance athletes need efficient NO synthesis for muscle oxygen delivery and pump support. Among products sold in Estonia, citrulline is known as a more effective alternative to arginine — through renal metabolism, citrulline raises arginine levels more reliably.
Supplement vs Diet: When to Choose Which?
Food sources contain citrulline in modest amounts. Watermelon is the best natural source, but supplement concentrations are significantly higher.
Supplements make sense:
- During intense training periods to support the pump and recovery
- For endurance athletes looking to improve exercise tolerance
- Citrulline malate may additionally support muscle pH and oxygen utilisation
MaxFit.ee offers good citrulline options: MST L-citrulline 1100mg 120caps, MST L-Citrulline 240caps and OstroVit Citrulline 4400 150caps are popular pure L-citrulline products among Estonian athletes.
References
Pérez-Guisado, J., & Jakeman, P. M. (2010). Citrulline malate enhances athletic anaerobic performance and relieves muscle soreness. Journal of Strength and Conditioning Research, 24(5), 1215-1222.
FAQ
Why is citrulline preferred over arginine?
Citrulline bypasses the liver's first-pass effect, during which arginine is largely metabolised. As a result, citrulline raises plasma arginine levels more effectively than direct arginine supplementation.
When should I take L-citrulline?
Most studies have used a dose taken 30–60 minutes before training. For extended recovery support, post-workout use is also appropriate.
Is citrulline malate better than pure citrulline?
Citrulline malate combines citrulline with a malic acid salt. Some studies suggest the malate component offers additional benefit for muscle acid buffering, but pure citrulline is equally effective for NO synthesis.
