Two natural supplements dominate the conversation around nitric oxide and athletic performance: citrulline and beetroot. Both increase nitric oxide. Both improve blood flow. But they work through different pathways — and the research on their performance effects is more mixed than marketing suggests.
This guide breaks down the science and helps you decide which one fits your goals.
In This Guide
- What Is Citrulline?
- What Is Beetroot?
- The Two Nitric Oxide Pathways
- What the Research Actually Shows
- Side-by-Side Comparison
- Which Should You Choose?
- Can You Take Both?
- Summary
- Sources
What Is Citrulline?
Citrulline is a non-essential amino acid found naturally in watermelon. In the body, it is converted into arginine, which then increases nitric oxide production. Nitric oxide relaxes blood vessels, improving blood flow to working muscles.
Citrulline is popular among strength and power athletes. It supports muscle pumps, nutrient delivery, and the removal of ammonia — a metabolic waste product that accumulates during intense exercise. Citrulline is not metabolized in the intestine or liver, making it more efficient than arginine at raising plasma arginine levels.
What Is Beetroot?
Beetroot is rich in dietary nitrate, which the body converts into nitric oxide through a different pathway than citrulline. This conversion improves oxygen efficiency — your muscles use less oxygen to produce the same power.
Beetroot also contains betalains (antioxidants unique to beetroot), betaine (which supports cellular hydration), and polyphenols. This full matrix of compounds is why whole-beetroot extract outperforms isolated nitrate salts in performance studies.
A 2009 study in the Journal of Applied Physiology found that dietary nitrate reduced the oxygen cost of exercise by up to 5%. A 2017 systematic review confirmed improvements in time-to-exhaustion of 4–25%.
The Two Nitric Oxide Pathways
This is the key distinction. There are two main pathways for nitric oxide production:
- NOS-dependent pathway: Citrulline → arginine → nitric oxide (via nitric oxide synthase)
- NOS-independent pathway: Nitrate → nitrite → nitric oxide
They are distinct systems. This is why the two supplements can be stacked — they do not compete for the same pathway.
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What the Research Actually Shows
The Direct Comparison Study
A 2019 randomized controlled trial in the Journal of Strength and Conditioning Research directly compared citrulline malate (8g) and beetroot juice (400mg nitrate) during maximal leg extension exercise in 27 recreationally active men.
What happened:
- Beetroot significantly increased plasma nitrate/nitrite (233.2 vs 15.3 μmol/L for citrulline)
- Neither treatment enhanced performance, blood flow, metabolic efficiency, or hormonal response
This is the honest finding. Both increased nitric oxide markers. Neither improved the measured outcomes in this specific protocol.
The Citrulline Evidence
A 2023 systematic review and meta-analysis examined nine studies on citrulline and endurance performance in young healthy adults. The results:
- Time-to-exhaustion: SMD = 0.03 (no significant effect)
- Time-to-completion: SMD = −0.07 (no significant effect)
The conclusion: "The current evidence suggests no significant benefit of citrulline supplementation for endurance performance."
However, a 2026 meta-analysis of citrulline malate found a small improvement in overall exercise performance (g = 0.16, p = 0.01), with possible benefits for aerobic endurance and short anaerobic tasks.
The Beetroot Evidence
Beetroot has the stronger evidence base for endurance:
- 180+ studies, 2,672 participants in the largest umbrella review (Poon et al. 2025)
- Significant improvements in time-to-exhaustion (SMD = 0.33)
- Reduced oxygen cost of exercise (Bailey et al. 2009)
- IOC Level A classification — the highest tier for supplement evidence
Side-by-Side Comparison
Which Should You Choose?
Choose citrulline if:
- Your sport demands short, explosive efforts
- You want acute muscle pumps and improved blood flow
- You train at maximum intensity for brief periods
- You want to support ammonia clearance during high-volume training
Choose beetroot if:
- Your sport demands sustained endurance (cycling, running, triathlon, rowing)
- You want improved oxygen efficiency and delayed fatigue
- You want faster recovery between sessions
- You want a supplement backed by 180+ studies and classified as IOC Level A
The honest conclusion: Beetroot has the stronger evidence base for endurance. Citrulline has better evidence for specific populations (hypertensive postmenopausal women) and for ammonia clearance. The direct comparison study found neither improved performance in a single maximal leg extension protocol.
Can You Take Both?
Yes. They work through different nitric oxide pathways and can be stacked. Space them apart:
- Citrulline: 60–90 minutes before exercise
- Beetroot: 2–3 hours before exercise
The research on stacking is limited but the mechanisms do not interfere. Test both individually first, then test the combination in training.
Summary
| Question | Answer |
| Which is better for endurance? | Beetroot. The evidence base is stronger and more consistent. |
| Which is better for muscle pumps? | Citrulline. It's widely used for acute vasodilation. |
| Which has more research? | Beetroot. 180+ studies vs. a smaller but growing citrulline evidence base. |
| Do they work through the same pathway? | No. Citrulline uses the NOS-dependent pathway. Beetroot uses the NOS-independent pathway. |
| Can you take both? | Yes. They can be stacked with different timing. |
| Does the direct comparison study show a winner? | No. Neither improved performance in the Trexler 2019 leg extension protocol. |
This article is part of the Stamox Performance series.
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Sources
All studies referenced in this guide are publicly available on PubMed. We only cite peer-reviewed, published research.
Trexler, E.T., et al. (2019). Effects of Citrulline Malate and Beetroot Juice Supplementation on Blood Flow, Energy Metabolism, and Performance During Maximum Effort Leg Extension Exercise. Journal of Strength and Conditioning Research, 33(9), 2321-2329. View on PubMed →
Suzuki, T., et al. (2016). Oral L-citrulline supplementation enhances cycling time trial performance in healthy trained men. Journal of the International Society of Sports Nutrition, 13, 6. View the study →
Kang, Y., et al. (2024). L-Citrulline Supplementation Improves Arterial Blood Flow and Muscle Oxygenation during Handgrip Exercise in Hypertensive Postmenopausal Women. Nutrients, 16(12), 1935. View on PubMed →
Bailey, S.J., et al. (2009). Dietary nitrate supplementation reduces the O₂ cost of low-intensity exercise and enhances tolerance to high-intensity exercise in humans. Journal of Applied Physiology, 107(4), 1144-1155. View on PubMed →
Domínguez, R., et al. (2017). Effects of Beetroot Juice Supplementation on Cardiorespiratory Endurance in Athletes. A Systematic Review. Nutrients, 9(1), 43. View on PubMed →
Poon, E.T., et al. (2025). Dietary Nitrate Supplementation and Exercise Performance: An Umbrella Review of 20 Published Systematic Reviews with Meta-analyses. Sports Medicine, 55(5), 1213-1231. View on PubMed →
How We Source Our Information
Every claim on this blog is backed by peer-reviewed research and Stamox R&D data. We cite studies from PubMed, the International Olympic Committee's consensus statements, and the Australian Institute of Sport's supplement framework.
This article was last reviewed on October 2026.
