Every time your heart contracts, it sends a pressure wave through your arteries. In a flexible artery, that wave travels smoothly and dissipates. In a stiff artery, the wave bounces back harder — and your heart has to work against that reflected force with every single beat. During a hard run, a heavy lift, or a sprint finish, this arterial stiffening effect is amplified. Evidence from a series of controlled trials suggests that L-citrulline supplementation may meaningfully blunt that response.

Why Arterial Stiffness Matters to Every Athlete

Arterial stiffness is not simply a concern for older adults or those with cardiovascular disease. It is an independent predictor of cardiovascular morbidity and mortality — and it is a dynamic variable that changes moment to moment in response to exercise, stress, and sympathetic nervous system activity. Pulse wave velocity (PWV) measures how fast a pressure wave travels along the aorta: stiffer arteries transmit the wave faster, and that speed predicts long-term cardiovascular outcomes independent of blood pressure alone. The augmentation index (AIx) measures the contribution of reflected pressure waves to central blood pressure, a marker of how much of each heartbeat's work is wasted pushing against a wave coming back from the periphery.

For athletes, the clinically important moment is not at rest — it is during effort. When working muscles create an oxygen demand that outpaces delivery, a reflex called the metaboreflex kicks in. Chemosensitive muscle afferents signal the sympathetic nervous system to raise blood pressure by increasing heart rate and peripheral vasoconstriction, directing more blood to working muscle. This is a normal and necessary response — but in athletes with elevated baseline arterial stiffness, the metaboreflex drives aortic pressure and wave reflection sharply upward. The heart must now eject blood against a stiffer aortic system while simultaneously meeting the increased demand of intense work. The cardiovascular cost of each beat rises. At the margins of performance, that extra work load is not trivial.

The question the research has started to answer is whether L-citrulline can reduce this stress-evoked stiffening — not simply at rest, where the evidence is mixed, but precisely at the moments of greatest cardiovascular demand. The answer, for specific populations at least, is looking encouraging.

L-Citrulline: The Smarter Route to Nitric Oxide

L-citrulline is a non-essential amino acid found naturally in watermelon and produced as a byproduct of nitric oxide (NO) synthesis in endothelial cells. Its principal role in vascular biology is as a precursor to L-arginine — the sole substrate for endothelial nitric oxide synthase (eNOS), the enzyme that produces NO to relax vascular smooth muscle. What makes citrulline strategically interesting is the route it takes to become arginine. Oral L-arginine is largely destroyed by enzymes in the gut and liver before it reaches the bloodstream — a process called first-pass metabolism that captures roughly 80% of an oral dose. L-citrulline bypasses this bottleneck entirely: it is absorbed intact, converted to arginine in the kidneys, and generates sustained plasma arginine levels that oral arginine cannot match dose-for-dose.

Inside the endothelial cell, the advantage compounds further. An enzyme called argininosuccinate synthase (ASS1) recycles citrulline — the byproduct of every NO synthesis event — back into arginine at the very site of eNOS activity. This local recycling loop means that citrulline supplementation replenishes the NOS-proximal arginine pool directly, independently of how much arginine happens to be circulating in the blood. This is why citrulline can modulate vascular tone even when blood arginine levels appear adequate — it is restocking a compartment that counts, not a systemic pool that may not.

How Citrulline Acts on Arterial Stiffness: Four Mechanisms

What the Clinical Research Shows

The evidence base for citrulline and arterial stiffness spans four controlled trials with positive findings, one meta-analysis, and two studies reporting null or inconsistent results. A critical and honest observation runs across all the positive findings: the benefits appear primarily under conditions of physiological stress — exercise, metaboreflex activation, cold pressor challenge — rather than at rest. This is not a limitation to hide. For athletes, stress-condition vascular performance is precisely what determines how the cardiovascular system copes with hard training and competition. Resting arterial stiffness measurements, while useful clinically, are less relevant to what happens on the track or in the gym.

What This Means for Training and Recovery

The metaboreflex scenario described in Figueroa et al. (2016) — sustained isometric contraction followed by blood flow restriction to working muscle — is not a contrived laboratory condition. It closely replicates the vascular environment during strength training with sustained muscular tension, sprint intervals with restricted recovery, or any sustained effort above lactate threshold. In those moments, the sympathetic system drives arterial stiffening to maintain perfusion pressure. If citrulline blunts that stiffening response, the heart ejects against a less resistant aortic system with every beat, reducing myocardial oxygen demand and potentially sustaining output quality for longer into a hard set or race.

The Dillon et al. (2024) finding adds a particularly meaningful dimension. By separating forward from backward aortic pressure waves, the study quantified two distinct vascular contributions: the forward wave represents the force your heart generates against aortic impedance during ejection; the backward wave represents the reflected pressure returning from arterial branch points in the periphery. Think of it as the difference between the water hammer in a rigid plumbing system versus the smooth flow in flexible hose — stiff arteries amplify the return wave, forcing the heart to contend with its own echo. Citrulline attenuating both waves simultaneously means it is reducing central stiffness at the aorta and improving compliance further out in the arterial tree. That is a meaningful dual-site vascular benefit for athletes managing cardiovascular output during high-intensity work.

Population matters substantially here. The most consistent benefits have been found in overweight adults, postmenopausal women, and those with type 2 diabetes — individuals whose endothelial function is already compromised and whose NO synthesis capacity is constrained by elevated ADMA, oxidative stress, or hormonal changes. If you are an otherwise healthy young athlete with a well-functioning endothelium, the evidence does not strongly support an arterial stiffness benefit. If you are an older athlete, a recreational exerciser managing cardiovascular risk, or someone returning to training after a period of deconditioning — populations where endothelial function is more likely to be impaired — the evidence is considerably more relevant to your situation.

The Evidence-Based Protocol

What the Research Doesn’t Yet Tell Us

The arterial stiffness literature for citrulline carries several important limitations that deserve honest acknowledgment. The most significant is that positive findings are concentrated under stress provocation rather than at rest — and while this aligns with the proposed mechanism and is arguably the more relevant condition for exercising individuals, it also means we cannot claim that citrulline reduces structural arterial stiffness in the way a medication or long-term lifestyle change might. The Bahari et al. (2026) systematic review found inconsistent arterial stiffness outcomes across studies in postmenopausal women — the population with arguably the most evidence — which means even within the best-evidenced group, the effect is not uniform. Additionally, a disproportionate share of positive vascular findings comes from a single research group (Figueroa and colleagues), and independent laboratory replication would substantially strengthen confidence in the results. Long-term data on whether 3 or 6 months of supplementation produces lasting reductions in resting arterial stiffness do not yet exist, and no study has measured hard cardiovascular endpoints. What we have is compelling mechanistic plausibility supported by short-term RCT evidence in specific populations — which is a solid foundation for informed use, but not a basis for overstating the certainty of the effect.

For healthy, well-trained athletes with no cardiovascular risk factors, the honest answer is that the current evidence does not support expecting arterial stiffness benefits. The Tryfonos (2023) null finding in healthy older males is important context. Where citrulline's vascular case is strongest is in the athletes-who-are-also-human category — those managing body weight, blood pressure, blood sugar, or the natural endothelial decline of aging — and for that population, the evidence is meaningfully positive, particularly during the moments of cardiovascular stress that intense training creates.