L-Citrulline: What is the main ingredient in nitric oxide boosters?

December 10, 2025 •

4 min read

Nitric oxide is a gas molecule crucial for vasodilation, the widening of blood vessels, but many people still wonder what is the main ingredient in nitric oxide boosters. The answer lies not in actual nitric oxide, but in key precursor compounds like amino acids and nitrates that help the body produce more NO naturally.

Quick Summary

Most nitric oxide boosters contain precursor ingredients like L-citrulline and L-arginine, which help increase nitric oxide production for improved blood flow and exercise performance.

Key Points

L-Citrulline is Key: L-citrulline is considered the most effective primary ingredient because it is better absorbed than L-arginine and leads to more sustained nitric oxide production.
Arginine's Role: L-arginine is a direct precursor to nitric oxide, but its poor absorption limits its effectiveness when taken alone in supplement form.
Nitrates are Another Pathway: Dietary nitrates, especially from beetroot extract, offer an alternative, effective route for increasing nitric oxide levels in the body.
Boosters Contain Blends: High-quality nitric oxide boosters often combine precursors like L-citrulline and nitrates, along with synergistic ingredients like antioxidants, for maximum effect.
Enhanced Performance: The goal of these boosters is to promote vasodilation, which increases blood flow and can enhance exercise performance, muscle pumps, and cardiovascular health.
Check Safety: It is important to consult a healthcare provider before taking nitric oxide boosters, especially if you have pre-existing health conditions or are on medication.

What is a Nitric Oxide Booster?

Nitric oxide (NO) boosters are a class of dietary supplements designed to increase the body's production of nitric oxide, a signaling molecule that plays a key role in various physiological processes. NO's primary function is vasodilation—the relaxation and widening of blood vessels. This leads to improved blood flow, which in turn can enhance oxygen and nutrient delivery to muscles during exercise, support cardiovascular health by lowering blood pressure, and assist with other functions like erectile function. However, these supplements don't actually contain nitric oxide; instead, they provide precursor ingredients that the body converts into NO. While L-arginine was long considered the standard for this purpose, research has increasingly highlighted other, more effective precursors.

The Primary Precursor: L-Citrulline

While many people assume L-arginine is the main ingredient, L-citrulline is arguably the more effective primary precursor in most modern nitric oxide boosters. This is due to its superior bioavailability and more sustained effect on increasing blood arginine levels.

Unlike L-arginine, which is heavily metabolized and broken down in the gut and liver before it reaches the bloodstream (a process called "first-pass metabolism"), L-citrulline bypasses this process. It is efficiently absorbed and then converted into L-arginine in the kidneys. This indirect pathway results in higher and more consistent plasma L-arginine levels over a longer period, leading to greater nitric oxide production.

Citrulline Malate: A common form found in supplements, citrulline malate is L-citrulline combined with malic acid. The malic acid is believed to offer additional benefits related to energy production, although the primary NO-boosting effect comes from the L-citrulline.

The Direct Amino Acid: L-Arginine

L-arginine is an amino acid that serves as a direct substrate for the enzyme nitric oxide synthase (NOS), which creates nitric oxide. While a direct precursor, its effectiveness as an oral supplement is limited by the first-pass metabolism issue. Still, it is a very common ingredient in nitric oxide supplements and plays a crucial role in NO synthesis within the body. When combined with L-citrulline, L-arginine can create a synergistic effect, providing both an initial spike and sustained levels of NO.

The Nitrate Pathway: Beetroot Extract

Another popular and effective precursor pathway for boosting nitric oxide is through dietary nitrates. Vegetables like beetroot, spinach, and arugula are naturally rich in nitrates. When ingested, nitrate is converted to nitrite by bacteria in the mouth and then further converted to nitric oxide in the acidic environment of the stomach. This pathway is especially efficient when oxygen levels are low, such as during intense exercise.

How it works: Ingested nitrate is absorbed, enters the bloodstream, and is concentrated in the saliva. Oral bacteria convert this to nitrite, which is then swallowed and converted to NO. This process helps relax blood vessels and improve blood flow.

How L-Citrulline and Nitrates Work Together

Combining L-citrulline with nitrates can be a powerful strategy for increasing nitric oxide. L-citrulline uses the NOS-dependent pathway, while nitrates use the NOS-independent pathway. This dual-action approach ensures multiple avenues for boosting NO availability, which can lead to a more significant and robust effect on athletic performance and cardiovascular function.

Comparing the Main Nitric Oxide Booster Ingredients


Feature	L-Citrulline	L-Arginine	Beetroot Extract (Nitrates)
Bioavailability	High; bypasses first-pass metabolism.	Low as a supplement; most broken down in the gut/liver.	Dependent on oral bacteria and stomach acid; effectively converted.
Primary Mechanism	Converted to L-arginine in the kidneys for sustained NO production.	Directly converted to NO via the NOS enzyme.	Reduced to nitrite by oral bacteria, then to NO in the stomach.
Impact on Performance	Longer-lasting, sustained effects on exercise performance.	Potential for quicker, though less consistent, NO spike.	Proven to improve endurance and reduce oxygen cost of exercise.
Gastrointestinal Effects	Generally well-tolerated.	Higher doses can cause digestive discomfort.	Generally well-tolerated, may cause reddish urine/stools.

Other Supporting Ingredients in Boosters

Commercial nitric oxide boosters often contain more than just the main precursors. These additional ingredients work synergistically to support NO production and its benefits.

Antioxidants (e.g., Vitamin C, Polyphenols): These protect nitric oxide from breakdown by free radicals, increasing its overall availability in the body.
Coenzyme Q10 (CoQ10): Found in meats and fish, CoQ10 is believed to help preserve nitric oxide levels.
Glycine Propionyl-L-Carnitine (GPLC): This compound is thought to increase blood nitrate/nitrite levels, which are markers for NO.

Maximizing Your Nitric Oxide Levels Naturally

While supplements offer a concentrated dose, you can support your body's natural NO production through diet and lifestyle.

Eat Nitrate-Rich Vegetables: Incorporate leafy greens (spinach, arugula), beetroot, and carrots into your diet.
Consume Arginine-Rich Foods: Get L-arginine from nuts, seeds, red meat, and dairy.
Include Citrulline-Rich Foods: Watermelon is a particularly good source of citrulline.
Exercise Regularly: Physical activity itself is a powerful stimulus for NO production.

Are Nitric Oxide Boosters Safe?

For most healthy adults, nitric oxide boosters are generally safe when taken at appropriate doses. However, side effects can include gastrointestinal issues like stomach pain, bloating, or diarrhea, especially with high doses of L-arginine. Individuals with certain health conditions, including cirrhosis, low blood pressure, kidney disease, or a history of heart attack, should consult a doctor before use. Since these supplements are not regulated by the FDA, it is crucial to choose reputable brands. For more detailed information on NO synthesis, an insightful review can be found on the National Institutes of Health website.

Conclusion

In conclusion, while L-arginine is a direct precursor, L-citrulline has emerged as the more effective main ingredient in nitric oxide boosters due to its superior absorption and sustained conversion to arginine in the body. When combined with other precursors like nitrates from beetroot extract and supporting co-factors, these ingredients work through multiple pathways to effectively increase nitric oxide production. This enhanced NO availability supports better blood flow, which may lead to improved athletic performance, cardiovascular health, and exercise endurance. Choosing a booster with effective ingredients like L-citrulline or beetroot extract, and following professional medical advice, is key to maximizing benefits safely.

Frequently Asked Questions

L-Citrulline is often considered the most effective ingredient for increasing nitric oxide because it has superior bioavailability compared to L-arginine and is efficiently converted to arginine in the kidneys, leading to more sustained NO levels.

L-citrulline is better absorbed by the body than L-arginine, which is heavily broken down in the liver. By taking L-citrulline, the body's L-arginine levels are raised more effectively and for a longer duration than by taking L-arginine directly.

Yes, you can increase your body's nitric oxide levels naturally by consuming foods rich in nitrates and amino acids. Excellent sources include beetroot, leafy greens like spinach, watermelon, nuts, seeds, and protein sources like meat and poultry.

Beetroot extract is rich in dietary nitrates. The body converts these nitrates into nitric oxide through a different pathway from L-arginine, effectively increasing NO levels and supporting enhanced endurance and blood flow.

Possible side effects can include gastrointestinal issues such as stomach pain, bloating, and diarrhea, particularly with higher doses of L-arginine. Individuals with certain health conditions or those on medication should consult a healthcare provider before use.

The speed at which nitric oxide boosters work depends on the ingredients. For example, some studies have shown increased nitric oxide levels within 45 minutes of consuming beet juice, while the sustained release from L-citrulline can provide longer-lasting effects.

Individuals with conditions such as low blood pressure, liver cirrhosis, or kidney disease should be cautious. Anyone on blood pressure or diabetes medication should also consult a doctor due to potential interactions. People with a history of heart attacks should be wary of L-arginine.