More than 13 years ago, I wrote an item in my newsletter Applied Metabolics about the possible ergogenic effects of an amino acid called citrulline. The compound I discussed was a combination of citrulline and malate. Malate is found naturally in such foods as apples and plays an important role in the Krebs energy cycle in mitochondria, helping produce adenosine triphosphate, or ATP, the body’s most immediate source of energy for muscular contraction. Citrulline is produced during the metabolism of two other amino acids in the body, arginine and ornithine. It’s also found in some foods, with particularly high concentrations in watermelon. In fact, the name citrulline is derived from the Latin word for watermelon.
Three years ago I reported the results of a study whose subjects drank 24 ounces of watermelon juice and showed an 11 percent rise in blood arginine. In a more recent study those who drank 24 ounces of the juice had an 18 percent rise in blood arginine.
Arginine is a conditionally essential amino acid, meaning that it’s needed in human nutrition under certain conditions. In bodybuilding, it’s best known as a growth hormone–stimulating amino acid and for being the direct dietary substrate for the synthesis of nitric oxide in the body. The latter property of arginine explains why it’s the major ingredient in various sports supplements touted to increase NO production. It also boosts the production of growth factors called polyamines and is a major precursor of creatine in the body.
There are some problems with oral arginine intake, whether as part of a protein food or in supplement form. It’s picked up in the portal blood system in the liver, and most of it is degraded by an enzyme called arginase. Citrulline bypasses the liver roadblock for arginine and is converted back into arginine in the kidneys. More than 80 percent of an oral dose of citrulline goes to producing NO in the endothelium, or lining of blood vessels. In fact, citrulline boosts blood arginine higher than direct oral doses of arginine do. Citrulline malate is now commonly used in various sports supplements, which raises the question of whether it’s really beneficial.
We get a hint of citrulline’s effectiveness when we consider that in Europe it’s sold as a drug under the trade name “Stimol,” used to treat asthenia, which is a fancy word for weakness and loss of strength. The dose for that purpose is one gram, taken three times a day. European athletes didn’t take long to consider using it as a way of offsetting workout fatigue and improving exercise recovery. For that purpose, Stimol is taken in doses of four to 10 grams an hour prior to exercise. How can it boost training efficiency?
Citrulline malate participates in a body process called the urea cycle, which helps rid the body of ammonia and lactic acid—agents of muscle fatigue—generated as a result of intense exercise. The presence of malate is decisive because it pushes the conversion of lactic acid to pyruvic acid, which in turn can convert into glucose. Since citrulline malate bypasses the arginine absorption barriers, it can be readily converted into NO, which leads to greater glucose uptake into muscle. Increased NO also results in more production of cellular mitochondria, where energy is produced and fat is oxidized; improved muscle contraction ability; increased blood flow to muscles for delivery of oxygen and removal of waste products; and increased recovery from intense training through heightened activity of muscle satellite cells and myotrophic factors, both of which are vital for muscle repair and growth.
A study found that providing citrulline malate to rodents increased their resistance to fatigue. A subsequent human study found that supplying six grams of citrulline malate to subjects for 15 days resulted in a significant reduction in perceived fatigue, along with a 34 percent increase in ATP production during exercise. Citrulline malate also boosted the recovery of creatine phosphate in muscle by 20 percent after exercise, which suggests that it would be ideal to partner with a creatine supplement.
That study, however, emphasized aerobic exercise. Whether citrulline malate has any value as an ergogenic aid for anaerobic exercise characterized by high intensity and short rest intervals was the subject of a recent study.1 Forty-one men did two chest-training sessions involving a total of 16 sets. They took eight grams of citrulline malate in one session and a placebo before the other training session. After taking citrulline malate, the subjects showed an 18 percent increase in reps done during the fourth set of bench presses and 53 percent extra reps during the last set. Analysis showed that all of the subjects performed increased reps on the final set of bench presses after using citrulline malate. In addition, those using citrulline malate had a 40 percent lower rate of muscular soreness at 24 and 48 hours after the exercise session ended. The only side effect occurred in 14.63 percent of users, who reported stomach discomfort.
Another new study involved giving six grams of citrulline malate to cyclists two hours before they engaged in a 137-kilometer bicycle ride.2 The citrulline malate enhanced the use of other amino acids, especially branched-chain aminos, for purposes of maintaining energy during extended workouts. It also enhanced blood arginine and produced a greater growth hormone response than exercise alone.
Based on the research showing citrulline malate’s ergogenic effects, how should it be incorporated into a bodybuilding work-out? I would suggest experimenting with dosages to determine individual tolerance. Note that almost 15 percent of subjects experienced stomach discomfort after a dose of eight grams of citrulline malate in the study discussed above, while European athletes routinely use four to 10 grams. I would suggest starting with four grams taken an hour prior to training. Be aware that many products containing citrulline malate don’t reveal the precise ingredient measures in the formula. Some stand-alone citrulline malate products are available, however, and that’s what you should use if you want to experiment.
Since creatine and probably beta-alanine are synergistic with citrulline malate, you could take all three in a stack consisting of four grams of citrulline malate, five grams of creatine and four grams of beta-alanine. That combination would produce higher and more sustained ATP counts, along with significantly decreased ammonia and lactic acid. The net effect would be increased energy, faster recovery between sets and more muscle endurance and strength during training.
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1 Pérez-Guisado, J., et al. (2010). Citrulline malate enhances athletic anaerobic performance and relieves muscle soreness. J Strength Cond Res. 24(5): 1215-22.
2 Sureda, A., et al. (2010). L-citrulline malate influence over branched-chain amino acid utilization during exercise. Eur J Appl Physiol. 110(2): 341-51.