Most cyclists know about beetroot juice and dietary nitrate. Far fewer realise there is a second, more direct route to nitric oxide — one that bypasses the gut bacteria and delivers arginine right where the enzyme needs it.
L-citrulline malate is not a stimulant. It does not feel like anything when you take it. But the physiology it drives in your working muscle is genuinely remarkable.
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Nitric oxide (NO) is produced inside endothelial cells by the enzyme eNOS (endothelial nitric oxide synthase), using L-arginine as its substrate. More NO means vasodilation, lower peripheral resistance, and better oxygen delivery to contracting fibres. The logical supplement conclusion — take L-arginine — turns out to be wrong.
Oral L-arginine is highly degraded by arginase in the intestinal wall and liver during first-pass metabolism. Bioavailability in the systemic circulation is poor. Clinical doses sufficient to raise plasma arginine to the levels eNOS requires produce significant GI distress — diarrhoea, cramping, bloating — long before they become ergogenic.
L-citrulline sidesteps this completely. Unlike arginine, citrulline is not a substrate for arginase. It passes through the gut wall intact and is taken up by the kidneys, where it re-enters the urea cycle and is efficiently converted back to arginine — raising plasma arginine levels more reliably than supplemental arginine itself.
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The seminal human performance study came from Pérez-Guisado and Jakeman (2010, Journal of Strength and Conditioning Research). Forty-one male cyclists received either 8g citrulline malate or placebo 60 minutes before a 4km time trial. The citrulline group reduced time-to-completion by an average of 1.5% and reported significantly lower post-exercise muscle soreness at 24 and 48 hours — a dual performance and recovery benefit.
A subsequent Japanese study by Suzuki and colleagues (2016) confirmed that 2.4g citrulline per day for 7 days raised plasma arginine by 23% and significantly improved cycling mean power output over 4-minute maximal efforts compared to placebo.
The malate component is not merely a carrier. Malic acid is an intermediate in the Krebs cycle. Co-ingestion of citrulline with malate may augment ATP resynthesis during high-intensity efforts by feeding carbons directly into the citric acid cycle, though isolating this mechanism from the NO effect is experimentally challenging.
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Practically, the evidence supports:
- Dose: 6–8g citrulline malate (or 3–4g pure L-citrulline) 60 minutes pre-exercise
- Timing: Acute single dose is effective; multi-day loading does not appear to add benefit for plasma arginine elevation
- Sport specificity: Most evidence is from cycling time trials and repeated-sprint protocols (30–10 minutes); less evidence in ultra-endurance (>3 hours) where intensity drops below the threshold where NO becomes rate-limiting
- Interaction: Unlike dietary nitrate, citrulline does not require oral bacteria for conversion — mouthwash does not blunt its effect
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Citrulline also appears to have an anti-catabolic effect at the muscle level. Breuillé and colleagues demonstrated citrulline supplementation reduced urinary 3-methylhistidine (a marker of myofibrillar protein breakdown) during caloric restriction, suggesting improved nitrogen retention independent of the vascular mechanism.
For cyclists training in a caloric deficit — common during the race season — this combination of NO-mediated blood flow and lean mass preservation makes citrulline unusually versatile.
For athletes calculating the calorie demands of high-output rides and optimising on-bike fuelling around these efforts, the free tool at winsport.uk/tools/cycling/cycling-calorie-calculator estimates your energy expenditure based on power output, duration and body mass — giving you the caloric framework into which a citrulline strategy fits.
Have you experimented with pre-ride citrulline, and did you notice a difference on time-trial efforts versus endurance pace work?