L-Citrulline vs. L-Arginine: What is a better supplement than L-arginine?

October 3, 2025 •

4 min read

Over 70% of orally supplemented L-arginine is broken down by the gut and liver before it can effectively enter the bloodstream, limiting its impact. This significant metabolic hurdle leads many to question, what is a better supplement than L-arginine? The answer for many is L-citrulline, an amino acid that sidesteps this issue for more consistent results.

Quick Summary

L-citrulline is a superior and better-tolerated alternative to L-arginine, boosting nitric oxide production more efficiently due to higher bioavailability. Its conversion to arginine in the kidneys bypasses digestive breakdown, leading to sustained and potent effects for athletic and cardiovascular health.

Key Points

L-Citrulline is superior to L-Arginine: Due to its high bioavailability, L-citrulline is more effective at increasing systemic L-arginine and nitric oxide levels.
L-Arginine suffers from poor absorption: The liver and gut break down a large percentage of oral L-arginine through first-pass metabolism, reducing its systemic impact.
L-Citrulline offers sustained effects: Converted to arginine in the kidneys, L-citrulline provides a more consistent and longer-lasting boost to nitric oxide.
L-Citrulline is better tolerated: Compared to high amounts of L-arginine, L-citrulline causes fewer gastrointestinal side effects like bloating and diarrhea.
L-Ornithine is a viable alternative for fatigue: It helps the body detoxify ammonia via the urea cycle, which can reduce fatigue and aid liver function.
Combining supplements can offer dual benefits: Some studies show combining L-citrulline and L-arginine can produce a powerful, dual-phase nitric oxide effect.
Watermelon is a rich food source: For those preferring dietary options, watermelon is an abundant source of naturally occurring L-citrulline.

The Problem with L-Arginine Absorption

L-arginine is an amino acid and a direct precursor to nitric oxide (NO), a molecule vital for vasodilation—the widening of blood vessels. While L-arginine sounds like the most direct path to boosting NO, its effectiveness as an oral supplement is limited. When consumed, L-arginine faces what is known as 'first-pass metabolism'. Enzymes, primarily arginase in the gut and liver, extensively break it down before it can reach the systemic circulation. This process dramatically reduces the amount of L-arginine that actually gets into the bloodstream to produce nitric oxide, making it far less efficient than anticipated. Furthermore, higher amounts of L-arginine are often required to overcome this poor absorption, which can lead to uncomfortable gastrointestinal side effects such as nausea, bloating, and diarrhea.

Why L-Citrulline Surpasses L-Arginine

In stark contrast, L-citrulline, another amino acid involved in the urea cycle, completely bypasses this metabolic bottleneck. Instead of being degraded in the liver, L-citrulline travels safely through the bloodstream to the kidneys, where it is then efficiently converted into L-arginine. This indirect pathway provides a more reliable and sustained release of L-arginine into the body, resulting in a more potent and longer-lasting boost in nitric oxide production.

The Bioavailability Advantage

Research consistently shows that L-citrulline is more effective at raising plasma L-arginine levels than L-arginine itself. This superior bioavailability is the primary reason for its effectiveness. The sustained presence of L-arginine in the bloodstream promotes better vasodilation, which in turn enhances oxygen and nutrient delivery to muscles during exercise. This improved blood flow can lead to numerous benefits, including:

Enhanced exercise endurance and power output, particularly during high-intensity and anaerobic activities.
Improved muscle pumps, a desired effect for many bodybuilders and athletes.
Accelerated muscle recovery and reduced post-workout soreness.
Consistent support for cardiovascular health and blood pressure regulation.

Extended Benefits and Better Tolerance

Unlike L-arginine's short-lived and often irritating effects, L-citrulline is well-tolerated and provides a more gradual, sustained impact. This makes it a far more practical option for those seeking consistent benefits without the gastrointestinal distress associated with high amounts of L-arginine supplementation.

Other Potential Alternatives and Synergies

While L-citrulline is the leading alternative, other supplements may offer complementary or distinct benefits, depending on the user's goals. L-ornithine, another amino acid in the urea cycle, helps detoxify ammonia, which contributes to fatigue. It is also converted into L-citrulline and then L-arginine, offering indirect support. Some research suggests L-ornithine may help reduce stress and improve sleep quality. For those seeking both a quick and lasting nitric oxide boost, a combination of L-arginine and L-citrulline has been shown to produce higher plasma arginine concentrations than either supplement alone. The L-arginine provides an immediate effect, while the L-citrulline sustains the levels over a longer period.

Comparison of Supplements


Feature	L-Citrulline	L-Arginine	L-Ornithine
Bioavailability	High (bypasses liver)	Low (extensive first-pass metabolism)	Moderate
Primary Function	Indirectly boosts nitric oxide production	Directly boosts nitric oxide production	Ammonia detoxification, precursor to citrulline
Effect Duration	Sustained and longer-lasting	Quick, but short-lived	Steady, supports overall energy cycle
Side Effects	Generally minimal and well-tolerated	Potential GI distress (nausea, bloating, diarrhea) at higher amounts	Generally well-tolerated
Best For	Endurance, muscle pumps, sustained performance	Immediate, short-term NO boost	Fatigue reduction, liver support, potentially stress management

Dietary Sources of L-Citrulline

For those who prefer to obtain nutrients from food, L-citrulline is naturally present in several dietary sources. Watermelon is particularly rich in this amino acid, which is how citrulline got its name (from Citrullus lanatus, the scientific name for watermelon). Other excellent food sources include:

Cucumbers
Pumpkins
Winter squash
Bitter melon
Certain legumes like chickpeas
Nuts, including peanuts and walnuts

Additionally, foods high in nitrates, such as beetroot, spinach, and other leafy greens, can also support nitric oxide production through a different metabolic pathway. However, achieving therapeutic levels through diet alone is often difficult, making supplementation a more practical choice for specific performance goals.

Conclusion

While L-arginine is the direct precursor to nitric oxide, its poor oral bioavailability due to extensive first-pass metabolism makes it a less efficient supplement. L-citrulline, which is converted to L-arginine in the kidneys, offers a superior and more sustained approach to boosting nitric oxide levels for improved athletic performance, muscle pumps, and cardiovascular health. It also comes with a lower risk of uncomfortable gastrointestinal side effects. For those with specific concerns like fatigue, L-ornithine presents another viable alternative. For maximized benefits, some studies even suggest combining L-arginine and L-citrulline for a dual-phase effect. Ultimately, the best choice depends on individual goals, but for most people, L-citrulline is the clear winner as a more reliable and efficient supplement. Before beginning any new supplement regimen, especially for long-term use, it is always recommended to consult with a healthcare professional to ensure it aligns with your health needs and avoids potential interactions.

Here is a reputable source on L-arginine for further reading.

Potential Interactions to Consider

When taking any supplements that affect blood flow, it is critical to be aware of potential interactions with medications, especially those for managing blood pressure. Combining these supplements with drugs for hypertension could cause blood pressure to drop too low. Similarly, if you are on blood thinners, the risk of bleeding may increase. Always discuss your supplement plans with a doctor, especially if you are taking prescription medication.

Frequently Asked Questions

L-citrulline bypasses first-pass metabolism in the gut and liver, allowing it to enter the bloodstream more efficiently and get converted into L-arginine in the kidneys, leading to higher plasma levels.

L-citrulline is generally well-tolerated and is less likely to cause the digestive issues such as bloating and diarrhea that are often associated with high amounts of L-arginine.

L-ornithine is a good substitute for supporting ammonia detoxification and reducing fatigue. While it can be converted to L-citrulline and L-arginine, it offers distinct benefits focused on the urea cycle.

While effective amounts of L-citrulline for athletic performance have been studied, it's important to consult with a healthcare provider for personalized recommendations.

Yes, some studies indicate that taking L-citrulline and L-arginine together can produce a higher and more sustained increase in plasma L-arginine levels compared to taking either alone.

Yes, L-citrulline is found in foods such as watermelon, cucumbers, pumpkins, and various legumes. Watermelon is particularly rich in this amino acid.

Yes, by boosting nitric oxide and improving blood flow, L-citrulline has been shown to enhance endurance, increase power output, and accelerate muscle recovery, especially during high-intensity exercise.