Nitric Oxide Boosters: Nitrates, Citrulline and Blood Flow

"Nitric oxide boosters" are a whole category of pre-workout ingredients built around one idea: increase the signaling molecule nitric oxide (NO), relax and widen blood vessels, and improve blood flow to working muscle. Better blood flow could mean more oxygen and nutrient delivery, better waste clearance, and the visible "pump." The concept is sound, but the ingredients differ a lot in how well they deliver.

Two pathways to nitric oxide

The body makes nitric oxide through two main routes, and different supplements target different ones:

• The L-arginine-NO pathway: The enzyme nitric oxide synthase converts the amino acid arginine into NO. Citrulline and arginine act here.
• The nitrate-nitrite-NO pathway: Dietary nitrate is reduced (partly by bacteria on the tongue) to nitrite and then to NO. Beetroot and leafy greens act here, and this pathway is especially active in low-oxygen, high-intensity conditions.

Because these are separate pathways, targeting both is a plausible strategy.

Dietary nitrate (beetroot): the strongest evidence

Dietary nitrate, usually from beetroot juice, has the most robust research of any NO-related supplement. Effective doses are around 300-600 mg of nitrate (roughly 5-8 mmol), taken 2-3 hours before exercise, with some protocols loading over several days.

What the evidence supports:

• Improved endurance performance and exercise economy (using less oxygen at a given intensity).
• Benefits that appear most reliable in recreational and sub-elite athletes; highly trained athletes often respond less.
• Some benefit for repeated high-intensity efforts.

A practical note: strong antibacterial mouthwash can blunt nitrate's effect by killing the oral bacteria that start its conversion.

Citrulline: reliable for blood flow

As covered in dedicated articles, L-citrulline (6-8 g) or citrulline malate (~8 g) raises blood arginine more effectively than arginine itself and supports the arginine-NO pathway. Its best-supported benefits are reduced soreness and modestly increased training volume, with a plausible role in the pump. It is a sensible choice for resistance training.

Arginine: the disappointing precursor

It seems logical that supplementing arginine, the direct NO precursor, would work best. In practice it usually does not. Oral arginine is heavily broken down in the gut and liver before reaching the bloodstream (poor bioavailability), so it raises circulating arginine far less than citrulline does. Most studies show limited or no performance benefit from oral arginine at practical doses. This is why formulators increasingly favor citrulline.

Where the evidence is weaker

• The "pump" and muscle growth: A bigger pump is largely a temporary, cosmetic blood-and-fluid effect. Evidence that NO boosters directly increase long-term muscle mass is limited.
• Strength and power: Effects are small and inconsistent, especially for maximal lifts.
• Elite endurance athletes: Nitrate's benefits shrink as training status rises.
• Exotic "NO" extracts: Many proprietary vasodilator ingredients lack solid human data or are under-dosed.

Putting it together

If your goal is endurance, dietary nitrate from beetroot has the best evidence, taken 2-3 hours before. If your goal is resistance training, citrulline is the more relevant choice. Arginine is generally not worth prioritizing. Whole-food sources (beetroot, spinach, arugula and other leafy greens for nitrate) provide these compounds too, alongside other nutrients.

The oral bacteria connection

One quirk of the nitrate pathway deserves special mention because it trips people up. The first step in turning dietary nitrate into active nitric oxide relies on bacteria living on your tongue, which reduce nitrate to nitrite. If you use a strong antibacterial mouthwash, you can wipe out these bacteria and substantially blunt the benefit of beetroot juice. Athletes using nitrate for endurance are therefore often advised to avoid antiseptic mouthwash and chewing gum around the time they are loading. This detail also illustrates why the nitrate route is genuinely distinct from the arginine route: it depends on a biological step that supplements acting on the arginine pathway do not.

Whole foods vs. supplements

You do not need a specialized product to raise nitrate intake. Beetroot, spinach, arugula, celery and other leafy greens are naturally rich in nitrate and come packaged with fiber, vitamins and polyphenols. Concentrated beetroot "shots" simply make it easier to hit a consistent, measured dose before a session. For citrulline, watermelon is a natural source, though reaching a 6-8 g dose from food alone is impractical, which is where supplementation has a clearer role.

Safety

Dietary nitrate and citrulline are well tolerated in healthy adults across the doses used in research. Beetroot can harmlessly turn urine or stool reddish, which is not a cause for concern. People on blood-pressure medication or nitrate-based heart medications (such as those used for angina) should consult a clinician before using these supplements, since they affect vascular tone and could interact. As always, competitors in tested sports should favor third-party-certified products.

Key takeaways

• Nitric oxide widens blood vessels; supplements target either the arginine or the nitrate pathway.
• Dietary nitrate (beetroot), about 300-600 mg 2-3 hours pre-exercise, has the strongest evidence, mainly for endurance.
• Citrulline (6-8 g) reliably supports the arginine-NO pathway and suits resistance training.
• Oral arginine is poorly absorbed and usually underperforms, despite being the direct precursor.
• The visible pump is largely cosmetic; effects on strength and muscle growth are limited, and trained endurance athletes respond less to nitrate.

References

1. Jones A.M. Dietary nitrate supplementation and exercise performance. Sports Medicine, 2014 — Sports Medicine
2. Domínguez R. et al. Effects of beetroot juice supplementation on cardiorespiratory endurance in athletes. A systematic review. Nutrients, 2017 — Nutrients
3. Schwedhelm E. et al. Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism. British Journal of Clinical Pharmacology, 2008 — British Journal of Clinical Pharmacology
4. Maughan R.J. et al. IOC consensus statement: dietary supplements and the high-performance athlete. British Journal of Sports Medicine, 2018 — British Journal of Sports Medicine