This week’s supplement that we will be reviewing is citrulline. L-citrulline is one of three amino acids in the Urea cycle along with L-Arginine and L-Ornithine. L-citrulline turns into L-Arginine after it is absorbed and sent to the kidneys and then turns into nitric oxide. There are two forms of citrulline that are used in supplement form which are L-citrulline and citrulline malate. L-citrulline is the free form version of the amino acid citrulline while citrulline malate is the amino acid citrulline attached to a molecule of malic acid. Citrulline is primarily used to reduce exercise-induced fatigue and improve post-exercise recovery and citrulline malate appears to be the more commonly used supplement used as an ergogenic aid.
One question you may have is if supplementing with citrulline increases arginine levels (which in turn is converted into nitric oxide) why not just supplement with arginine itself? That is a great question and I’d be happy to answer. When comparing citrulline and arginine supplementation, arginine increases plasma arginine levels but research has shown supplementation with L-citrulline increases plasma L-arginine more effectively. (1) It is speculated since L-citrulline is not subject to pre-systemic elimination and this is why it could be a more efficient way to elevate levels of L-arginine.
When looking for support for citrulline supplementation one of the main studies pointed to was one that claimed citrulline malate promotes aerobic energy production in human exercising muscle. (2) While the results of this study did show a 34% increase in the rate of oxidative ATP production during exercise and a 20% increase in the rate of phosphocreatine recovery after exercise, the study was done on non-athletes who complained of fatigue who had no documented disease and the biggest knock I have is the “exercise” done was finger flexion. I’m not about to deem a supplement effective because they felt less tired flexing their fingers. Regenerating ATP from finger flexion and from deadlifting 300 lbs doesn’t exactly translate the same way in my book. That said there is a difference, so it’s not necessarily smart to completely overlook it either.
In what is probably the best support for citrulline malate, research published in the Journal of Strength and Conditioning Research in 2010 looked at the possibility of citrulline malate enhancing athletic anaerobic performance and relieving muscle soreness. (3) In this study, 41 men performed 2 consecutive pectoral training session protocols of 16 sets. 8 grams of citrulline malate was used in 1 of the sessions and a placebo in the other. Their resistance was tested using the repetitions to fatigue test based on their 80% 1 rep max in the 8 sets of bench press during the training session. The results showed when supplementing with citrulline malate subjects achieved 52.92% more repetitions and the 100% response in the last set. There was also a decrease of 40% in muscle soreness at 24 and 48 hours post-training.
Something to keep in mind is the 8 grams of citrulline malate subjects were taking is 4 times the suggested serving size, showing it’s possible that you need very high doses for it to be effective. Another thing the study failed to mention was how experienced these subjects were before coming into the study. There is some research that hypothesizes citrulline supplementation may be effective for those who are untrained but not when subjects are highly trained. (4) However, if you search around for support for citrulline most articles will point to the two aforementioned studies. There are some others that show some support for its use, but nothing that does much for me. (5, 6, 7)
There was a journal published in the European Food Safety Authority in 2012 which gave a scientific opinion on the substantiation of the health claim of faster recovery from muscle fatigue after exercise from citrulline malate. (8) In this review they looked at a total of 33 references which was comprised of 18 human studies, 4 animal studies, 7 in vitro studies and 4 reviews (the reviews were not used because they contained no primary data which could be used for the scientific substantiation of the claim.) After sifting through all the research they were not able to conclude the claim of muscle recovery. Of note they did go on to talk about the bench press study and said “The Panel considers that owing to the methodological limitations of the study no conclusions can be drawn from this study for the scientific substantiation of the claim.”
I’m not ready to say citrulline is effective or not. There is a small amount of research that shows benefits but only one strong human study that directly demonstrated an enhancement of training performance. Personally I’d rather see a large amount of documentation proving its worth like you see with beta-alanine which has dozens of studies showing its effectiveness and creatine which literally has hundreds of studies showing positive results. At this point I’d say citrulline is at a crossroads where further research will either replicate positive results or show it’s just another dud. Whether you want to use it right now with limited research is up to you. Limited evidence doesn’t mean it doesn’t work, but it also doesn’t mean it does. Personally I’d rather have a clear understanding of what I’m putting in my body.
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