L-citrulline – an amino acid that supports performance and the cardiovascular system
What exactly is L-citrulline and why does it feature in almost every pre-workout formula? Find out what the research says about its potential impact on nitric oxide production, exercise performance and muscle pump.
L-citrulline is a non-protein amino acid which is not a component of structural proteins but performs an important metabolic function in the body. It is involved in the urea cycle, where it helps remove ammonia – a by-product of protein metabolism. In the context of sports supplementation, however, the most important fact is that L-citrulline is effectively converted in the body into L-arginine, and subsequently into nitric oxide (NO). It is precisely this mechanism that means research into citrulline focuses on physical performance, peripheral circulation and the so-called ‘muscle pump’. Interestingly, oral supplementation with L-citrulline raises blood arginine levels more effectively than supplementation with arginine alone – and this is one of the main reasons for its growing popularity.
What is L-citrulline and how does it work in the body
L-citrulline occurs naturally in watermelon – its name derives from the Latin name for this fruit (Citrullus vulgaris). The most extensively studied form of supplementation is L-citrulline malate, i.e. a compound of L-citrulline and malic acid. This combination is sometimes chosen because of the potential additional effect of malic acid on energy metabolism, although not all studies confirm a clear advantage of this form over pure L-citrulline.
The mechanism of action of L-citrulline is well understood from a biochemical perspective. After being absorbed from the gastrointestinal tract, it reaches the kidneys, where it is converted into L-arginine. Arginine is a substrate for nitric oxide synthase (NOS) – the enzyme responsible for the production of NO. Nitric oxide acts as a signalling molecule that dilates blood vessels, which theoretically results in improved blood supply to working muscles.
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What the research says about L-citrulline and performance
Research into L-citrulline in a sporting context focuses mainly on its impact on performance, delaying fatigue and post-exercise recovery. Some clinical trials suggest that a single pre-workout dose of citrulline may have a beneficial effect on the number of repetitions performed to muscle failure, particularly in upper-body resistance exercises. Other studies suggest that regular supplementation may influence the level of perceived fatigue during endurance exercise.
However, it is important to keep things in perspective. L-citrulline does not have any health claims approved by the European Food Safety Authority (EFSA) – the scientific data are promising, but do not yet meet EFSA’s rigorous criteria for the approval of specific claims. Citrulline should therefore be regarded as an ingredient offering potential support for those who train, rather than as a guaranteed solution. Those interested in other amino acids and their use in sport will find interesting information in the articles on taurine – properties and effects and L-DOPA – what it is and how it works.
L-citrulline and the cardiovascular system – what studies have shown
A second area of research into L-citrulline concerns the cardiovascular system. Clinical studies suggest that regular supplementation may have a beneficial effect on vascular endothelial function and blood pressure, particularly in people with high blood pressure. The mechanism is, of course, explained by an increase in nitric oxide production, which is a natural vasodilator.
However, this is an area where particular caution is required. L-citrulline is not a medicine and should not replace pharmacotherapy or consultation with a doctor. If you are considering taking it to support your cardiovascular system, speak to your doctor first – particularly if you are taking long-term blood pressure-lowering medication or medication for heart conditions.
[warning: If you are taking medication for high blood pressure, vasodilators (e.g. nitrates) or medication for erectile dysfunction, consult your doctor before taking L-citrulline supplements. Its mechanism of action may overlap with that of these medicines and increase the risk of your blood pressure becoming too low.]
L-citrulline dosage – how much, when and in what form
Scientific studies have typically used doses ranging from 6–8 g of L-citrulline malate (corresponding to approximately 4–5 g of pure L-citrulline), taken 30–60 minutes before training. This dose has been found to be appropriate for a single-dose effect on performance. For daily supplementation aimed at improving vascular function, lower doses of around 3–6 g per day, divided into 1–2 doses, have been used.
L-citrulline malate in powder form is convenient to dose – it is easy to measure out a precise portion and dissolve it in water or a pre-workout drink. The taste is fairly neutral, slightly tart, which makes it easy to combine with other supplements in a pre-workout shake.
[tip: It is best to take L-citrulline on an empty stomach, around 30–60 minutes before training – this is the regimen used in most studies on its effect on performance. It can be combined with electrolytes from our Sport and Recovery category.]
Who should consider L-citrulline
L-citrulline is a supplement primarily aimed at those doing strength and endurance training who are looking for potential support for performance and a noticeable muscle pump. It may be an interesting choice for those who already take basic training supplements (creatine, protein, electrolytes) and are looking for another well-researched addition to their supplement stack.
However, citrulline is not an essential supplement. The basis of an active person’s diet should be an adequate intake of protein, carbohydrates, fats and electrolytes – only then is it worth considering supplements with potential additional benefits. Those interested in gaining a better understanding of pre- and post-workout supplementation will find valuable information in the article on creatine monohydrate.
[note: Vitaler’s Citrulline is pure L-citrulline malate powder – free from artificial sweeteners, colourings and fillers. The convenient powder form allows for precise dosing and easy mixing with other pre-workout supplements.]
Safety and contraindications
L-citrulline is an amino acid that is well tolerated by most adults. In clinical trials, doses of up to 10–15 g per day were used without any significant adverse effects. The most commonly observed side effects are mild gastrointestinal discomfort at high doses, which usually subsides once the dose is reduced.
L-citrulline supplementation is not recommended for pregnant or breastfeeding women (insufficient safety data available) or for people under the age of 18. People with kidney or liver disease should consult a doctor before starting supplementation.
Summary: L-citrulline is an amino acid with a well-documented mechanism of action – it is converted into arginine, which plays a role in the production of nitric oxide, a vasodilator. Research suggests it may have a beneficial effect on exercise performance and vascular function, although it does not have any health claims approved by the EFSA. Typically taken at a dose of 6–8 g of malate 30–60 minutes before training, it can be a sensible addition to the supplement regimen of physically active individuals.