L-Citrulline

Overview:

L-Citrulline is a naturally occurring amino acid found in certain foods and produced by the body. It plays a crucial role in the urea cycle, which is responsible for the elimination of ammonia, a waste product in this cycle. As a result of this cycle, nitric oxide is produced, which gives this ingredient its vasodilating (pump inducing) properties. L-Citrulline and raw nitrites such as beetroot juice, have been found to have various effects on power, endurance, and vasodilation.

Simple:

When reviewing studies that analyze the effectiveness of this ingredient, we primarily find that this ingredient is better than its successor, L-Arginine, on many metrics. This is primarily due to L-Arginine’s poor oral bioavailability. L-Citrulline has been found to increase muscular endurance, strength, power, and delay fatigue during high-volume exercises, increase vasodilation (widening of blood vessels), and lower perceived exhaustion. This ingredient is commonly seen in the form of L-Citrulline Malate (CM) and raw citrulline in blends, but we are learning the effects of malic acid are minimal. In terms of the effective dosing of the amino acid, we see greater than 3 grams being a strong dose (of the raw form), but up to around 15 grams being tolerated. It seems the upper limit of effectiveness appears to be around 8-10 grams, but more research is needed surrounding the perfect dosing of this ingredient.

Complex: 

Going into depth on some of the research done to achieve these findings, one study done on trained cyclists showed that supplementation of 6 grams of citrulline 2 hours before a 40 km time trial resulted in 5.2% increase in time with a p value of 0.08. However, this study also found no change in the cycling sprint time task between the placebo and citrulline dosed group. Also this supplementation created an increase in heart rate and power output among the cyclists. This particular study shows the effect of citrulline on endurance and blood flow rate [1]

L-Citrulline and L-Arginine are both involved in the uric acid cycle involving the theoretical increase in nitric oxide (NO) which should improve blood flow. Among a review done with 38 articles comparing these two supplements, although arginine in the biochemical pathways would seem to have more of an effect on NO concentrations specifically, it was apparent based on this systematic review that the L-Citrulline had a better effect on perceived exhaustion, induced a decrease in lactate concentrations, and helped individuals perform better during time trials of different sorts [2].

L-Arginine has also generally not been recommended for strength training use due to its lack of evidence supporting vasodilation, blood flow, muscle protein synthesis, and general power and endurance performance [3]. Additionally, citrulline has some more promising data. First of all, Citrulline Malate, which is a commonly used form of L-Citrulline, lacks evidence on the use of malic acid, which was originally thought to help decrease lactic acid and improve ATP production, but the evidence is still not convincing. Pure L-Citrulline however has some studies that found that supplementing with this did increase vasodilation markers, but some more evidence is needed. In addition, supplementation has been shown to enhance aspects of strength and power and delay fatigue during high-volume resistance exercise [3]. It has been found that this ingredient also decreases soreness from participants days after exercising, specifically from 24-72 hours after exercise [5].  Dosing as little as 3g of L-Citrulline seems to be efficacious. Although more research needs to be done, it has been indicated that supplementing both L-Arginine and L-Citrulline can have positive effects on power and plasma levels of these aminos, better than if separate, such as with the more bioavailable version of Arginine, Nitrosigine. [3].

Lastly, another study showing the effects of vasodilation and blood flow volume from citrulline and arginine found that citrulline had a greater effect on these parameters than the ladder [4].

Works Cited:

1. Stanelle ST, McLaughlin KL, Crouse SF. One Week of L-Citrulline Supplementation Improves Performance in Trained Cyclists. J Strength Cond Res. 2020 Mar;34(3):647-652. doi: 10.1519/JSC.0000000000003418. PMID: 31860534.
2. Huerta Ojeda Á, Domínguez de Hanna A, Barahona-Fuentes G. Efecto de la suplementación de L-arginina y L-citrulina sobre el rendimiento físico: una revisión sistemática [The effect of supplementation with L-arginine and L-citrulline on physical performance: a systematic review]. Nutr Hosp. 2019 Dec 26;36(6):1389-1402. Spanish. doi: 10.20960/nh.02478. PMID: 31682459.
3. Gonzalez AM, Townsend JR, Pinzone AG, Hoffman JR. Supplementation with Nitric Oxide Precursors for Strength Performance: A Review of the Current Literature. Nutrients. 2023 Jan 28;15(3):660. doi: 10.3390/nu15030660. PMID: 36771366; PMCID: PMC9921013.
4. Moon JR, Vogel RM, Falcone PH, Mosman MM, Tribby AC, Hughes CM, Griffin JD, Tabor SB, LeFever DJ, McChaughey SB, Kim MP, Joy JM. A comparison of citrulline and arginine for increasing exercise-induced vasodilation and blood flow. J Int Soc Sports Nutr. 2015 Sep 21;12(Suppl 1):P6. doi: 10.1186/1550-2783-12-S1-P6. PMCID: PMC4595542.
5. Rhim HC, Kim SJ, Park J, Jang KM. Effect of citrulline on post-exercise rating of perceived exertion, muscle soreness, and blood lactate levels: A systematic review and meta-analysis. J Sport Health Sci. 2020 Dec;9(6):553-561. doi: 10.1016/j.jshs.2020.02.003. Epub 2020 Feb 8. PMID: 33308806; PMCID: PMC7749242.