Understanding Citrulline: The Non-Essential Amino Acid
Citrulline is an amino acid that your body produces internally, classifying it as 'non-essential' because you do not need to consume it to survive. The name comes from Citrullus vulgaris, the Latin term for watermelon, the fruit from which it was first isolated. While other amino acids are primarily known as the building blocks of protein, citrulline serves different key roles. Its main functions are related to the body’s metabolic processes and the production of a crucial signaling molecule called nitric oxide.
How Citrulline Powers the Body: The Nitric Oxide Pathway
Unlike L-arginine, which is rapidly broken down by the gut and liver, citrulline is efficiently absorbed into the bloodstream. It is then converted into L-arginine in the kidneys, where it is used to produce nitric oxide (NO). This unique conversion pathway gives citrulline a significant advantage in boosting nitric oxide levels. NO is a potent vasodilator, meaning it helps relax and widen blood vessels. This effect leads to improved blood flow throughout the body, delivering more oxygen and nutrients to muscles and tissues.
Performance and Health Benefits
The ability of citrulline to increase nitric oxide has led to its popularity as a dietary supplement for various health benefits. For athletes and fitness enthusiasts, the improved blood flow can translate to better performance during high-intensity exercise.
- Enhanced Athletic Performance: For weightlifters, studies have shown that citrulline malate supplementation can increase the number of repetitions performed and reduce post-exercise muscle soreness. While evidence is mixed for pure aerobic endurance, some research suggests it can improve oxygen use in muscles, which could boost endurance.
- Cardiovascular Health Support: By promoting vasodilation, citrulline can help lower blood pressure in individuals with pre-hypertension or hypertension. Long-term use may help improve overall vascular function and reduce arterial stiffness, particularly in individuals with certain health risks.
- Relief for Erectile Dysfunction (ED): Limited blood flow can be a factor in ED. Some studies suggest that the boost in nitric oxide from citrulline can aid blood vessel relaxation, potentially helping with mild ED.
- Immune System Modulation: NO is also involved in immune function. Citrulline's role in NO production may have a positive impact on the immune system.
- Removal of Metabolic Waste: As a key component of the urea cycle, citrulline aids in the detoxification process by helping the body remove ammonia, a waste product of protein metabolism.
Citrulline versus Arginine
When comparing citrulline and L-arginine supplements, the difference in how the body processes them is critical. Although both ultimately contribute to nitric oxide production, citrulline's superior bioavailability makes it a more effective tool for raising plasma arginine levels.
| Feature | Citrulline | Arginine |
|---|---|---|
| Absorption Rate | Highly absorbed in the intestines, bypassing first-pass metabolism. | Poorly absorbed; undergoes significant breakdown by gut and liver enzymes. |
| Effect on Blood Arginine | More effective at consistently and reliably increasing blood arginine levels. | Less effective at increasing blood arginine levels due to degradation. |
| Gastrointestinal Issues | Generally well-tolerated, with side effects mainly occurring at very high doses. | High doses can cause stomach discomfort, nausea, and diarrhea. |
| Mechanism for NO | Indirectly boosts NO by being converted to arginine in the kidneys. | Directly converts to NO, but less reaches the bloodstream for conversion. |
| Supplement Forms | L-Citrulline, Citrulline Malate. | L-Arginine, Arginine Alpha-Ketoglutarate (AKG). |
Dietary Sources and Supplementation Options
While citrulline is produced internally, increasing your intake through food or supplements is an option. Watermelon is a particularly rich source.
Foods Rich in Citrulline:
- Watermelon
- Cucumbers
- Pumpkin
- Bitter melon
- Muskmelon
- Some nuts
Beyond whole foods, citrulline is available in supplement form, most commonly as L-citrulline or citrulline malate. The malate version combines citrulline with malic acid, which is believed to help increase ATP (adenosine triphosphate) production and improve energy metabolism. It is always best to consult with a healthcare provider before starting any new supplement regimen.
Potential Side Effects and Safety Considerations
Citrulline is generally recognized as safe for oral use, and side effects are uncommon at standard doses. However, at high doses, some individuals may experience mild gastrointestinal distress, such as stomach pain, nausea, or diarrhea.
Caution should be exercised if you are taking medications for high blood pressure or erectile dysfunction (e.g., phosphodiesterase-5 inhibitors), as citrulline can also lower blood pressure. The combination could cause blood pressure to drop too low. There is also insufficient safety data for pregnant or breastfeeding women, and those scheduled for surgery should stop taking citrulline at least two weeks prior.
Conclusion
As a versatile amino acid, citrulline plays a key role in the body’s metabolic health and nitric oxide production. Its ability to boost nitric oxide more effectively than arginine offers clear advantages, especially for enhancing athletic performance and supporting cardiovascular health through improved blood flow. Whether obtained through a diet rich in foods like watermelon or through supplementation, understanding what is citrulline and its benefits can be a valuable step in optimizing your nutrition diet and performance goals. For further in-depth scientific research on its impact on cardiometabolic health, the National Institutes of Health provides comprehensive studies, such as this review on l-Citrulline Supplementation: Impact on Cardiometabolic Health.
