L-Citrulline Before Workout: The Supplement That Boosts Nitric Oxide

Anyone who walks into a supplement store today and picks up a random pre-workout bottle will almost certainly find one recurring ingredient: L-Citrulline, or its common form, Citrulline Malate. It is marketed as the source of the gym 'pump', that feeling of fullness in the muscle after a set, and as an ingredient that will allow you to squeeze out a few more reps before failure. But unlike many ingredients in the supplement industry, there is a real biological mechanism behind the promise.

The story of citrulline is an excellent example of the gap between marketing and evidence. On one hand, there is a well-established physiological mechanism of vasodilation through nitric oxide. On the other hand, results in human studies are mixed: some are impressive, some found nothing. In this article, we will separate the biology from the hype, look at the real numbers, and understand who citrulline is truly suitable for, both for athletes and for people who don't set foot in a gym.

What is L-Citrulline?

L-Citrulline is a non-essential amino acid, meaning the body can produce it on its own, but also obtains it from food. Its name is derived from Citrullus, the scientific name for watermelon, the richest natural source of citrulline:

• Non-essential amino acid involved in the urea cycle, the process by which the body removes excess nitrogen.
• Main natural source: Watermelon, especially the white rind. To get an effective dose from food alone, you would need to eat several kilograms a day.
• Converted in the body to L-Arginine, another amino acid that is the direct building block of nitric oxide.
• Two main forms in supplements: Pure L-Citrulline, and Citrulline Malate, a combination with malic acid that has been extensively studied in the context of performance.

The most interesting point is actually indirect: Taking L-Citrulline raises blood levels of L-Arginine more effectively than taking L-Arginine itself. The reason is that arginine from food is partially broken down in the liver before reaching the blood, while citrulline bypasses this station. This is why the supplement industry has largely moved from arginine to citrulline.

The Connection to Training: The Nitric Oxide Mechanism

To understand why L-Citrulline has become such a star in pre-workout supplements, you need to know about Nitric Oxide (NO). This is a tiny signaling molecule that plays a central role in vasodilation. When nitric oxide levels rise, the walls of blood vessels relax, their diameter increases, and blood flow improves.

The chain works like this: Citrulline is converted to arginine, and arginine is converted to nitric oxide via an enzyme called eNOS. The result is more blood, oxygen, and nutrients reaching the working muscle. In the gym, this manifests as the 'pump' sensation, and theoretically, also the ability to perform more reps before the muscle fatigues.

But there is a second mechanism, less discussed but perhaps more important: Ammonia removal. Citrulline participates in the urea cycle, which clears ammonia from the blood. Ammonia is a metabolic byproduct that accumulates during intense effort and contributes to the feeling of fatigue. It is possible that part of citrulline's effect on endurance stems precisely from its ability to help the body get rid of this ammonia faster.

The Current Evidence

Study 1: Citrulline Malate and Bench Press from 2010

The most cited study in the field was published in the Journal of Strength and Conditioning Research in 2010 by Pérez-Guisado and Jakeman. 41 men performed a protocol of 16 sets of bench press in a double-blind crossover design, once with 8 grams of Citrulline Malate and once with a placebo. The results were dramatic: in the last set, those taking citrulline performed 52.92% more reps than the placebo group. Additionally, they reported a 40% reduction in muscle soreness at 24 and 48-hour measurements. This is the study that made Citrulline Malate a standard ingredient in every pre-workout supplement.

Study 2: Meta-Analysis on Blood Pressure from 2019

A systematic review and meta-analysis published in the Avicenna Journal of Phytomedicine in 2019 by Barkhidarian and colleagues aggregated 8 controlled clinical trials. The main finding: L-Citrulline lowered blood pressure, but a significant reduction in diastolic pressure was only observed at doses of 6 grams per day and above. Additional meta-analyses indicated a reduction in systolic pressure in the range of 4 to 7 mmHg, a modest but consistent effect that highlights the cardiovascular role of nitric oxide, beyond the gym.

Study 3: Studies That Found No Acute Improvement

It is important not to present only the positive side. An acute study from 2023 examined 18 resistance-trained individuals who took 8 grams of L-Citrulline one hour before training. The result: No significant improvement in isometric strength, muscular endurance, or muscle oxygenation. Another study on 43 trained participants reached the same conclusion regarding maximal neuromuscular performance. The gap between the studies suggests that the effect depends on the protocol, dosage, and nature of the training, and that the effect is not automatic.

What About Blood Pressure and Heart Health?

One of the important insights is that the benefit of L-Citrulline is not limited to the gym. The same mechanism of vasodilation through nitric oxide, which improves the 'pump' in weightlifters, may also contribute to general cardiovascular health.

Studies have examined citrulline in populations with mild hypertension, arterial stiffness, and endothelial dysfunction, conditions that worsen with age. Vasodilation and improved peripheral blood flow are precisely the processes that are impaired in vascular aging. In this sense, citrulline is a nice example of an ingredient that starts its career as a supplement for athletes and discovers it has broader relevance for metabolic and vascular health in older age.

Should You Start Taking L-Citrulline?

As always, the answer depends on expectations. If you expect a dramatic jump in maximal strength, you will likely be disappointed: the evidence for acute strength improvement is inconsistent. But if the goal is more reps in later sets, less fatigue, and less muscle soreness the next day, there is a reasonable evidence base, especially with Citrulline Malate.

In terms of safety, L-Citrulline is considered very safe. Studies have used doses up to 8 grams without significant side effects, and even without the gastrointestinal discomfort typical of high-dose L-Arginine. However, there are some important warnings:

• Those taking blood pressure medications or nitrate-type drugs (for chest pain) must consult a doctor, because citrulline can enhance the blood pressure-lowering effect.
• Those taking erectile dysfunction medications (PDE5 inhibitors) need caution for the same reason; both drugs act on the nitric oxide pathway.
• The effect becomes stronger with regular use, so some of the cardiovascular benefit appears only after a few weeks, not from a single dose.

In terms of price, L-Citrulline is a relatively cheap supplement; a monthly supply usually costs just a few tens of shekels, making it one of the training supplements with a reasonable cost-benefit ratio. Purchase L-Citrulline on iHerb.

What to Take Away from the Research?

1. Dosage: 3 to 6 grams of pure L-Citrulline, or 6 to 8 grams of Citrulline Malate, about 30 to 60 minutes before training. Most positive studies used the upper end of the range.
2. Don't expect magic on the first day. Part of the effect, especially on blood pressure and flow, accumulates over several weeks of consistent use.
3. If you are sensitive to post-workout muscle soreness, it's worth trying. The evidence for reducing delayed onset muscle soreness (DOMS) is among the strongest citrulline has.
4. Check the label. Make sure you are getting an effective amount of citrulline; some pre-workout supplements contain too low a dose to make a difference.
5. If you have high blood pressure or take heart medications, consult a doctor before starting, even if it's just a supplement.

For those who want to build a personalized supplement protocol based on training goals, age, and gender, it is recommended to use our personal supplement selector which ranks each supplement by its level of evidence.

The Broader Perspective

L-Citrulline demonstrates a principle that repeats itself time and again in the supplement world: A real mechanism does not guarantee a dramatic result. Nitric oxide truly dilates blood vessels, citrulline truly raises it, and this can truly help. But the body is a complex system, and the effect of a single ingredient is usually modest and context-dependent, not magic in a bottle.

The rating of citrulline as a 'yellow' supplement in our selector reflects precisely this balance: Safe, cheap, with a well-established mechanism and some impressive results, but also with conflicting studies that temper the enthusiasm. It is not creatine, which has decades of unequivocal evidence, but it is certainly not just another marketing powder. If you train seriously and are looking for a small, safe edge, citrulline is one of the most worthwhile experiments you can run on yourself.

References:
Pérez-Guisado & Jakeman, Citrulline Malate Enhances Athletic Anaerobic Performance and Relieves Muscle Soreness, Journal of Strength and Conditioning Research, 2010
Barkhidarian et al., Effects of L-citrulline supplementation on blood pressure: A systematic review and meta-analysis, Avicenna Journal of Phytomedicine, 2019