Definition: "An ergogenic aid is any substance or phenomenon that enhances performance "

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Cyclists on L-citrulline use more amino acids

L-Citrulline has become such a commercial success in the power sports world that it's easy to forget that the first citrulline users were actually endurance athletes. L-Citrulline used to be an ingredient in Stimol, a preparation that runners swore by in the seventies and eighties. Well, maybe they were right, if a Spanish study on the effects of citrulline on cyclists is anything to go by.

But let's get things clear right away: no, the Spaniards' study doesn't prove that citrulline will make you cycle faster. What the Spaniards did discover is a couple of potentially ergogenic effects.

The researchers gave 8 young cyclists, aspiring professionals, 6 g L-citrulline. This dose was equivalent to the total amount of L-arginine that the men consumed daily through their food. L-Arginine, L-citrulline and L-ornithine all convert into each other in the body. Of the three, L-citrulline is the most stable. Two hours after taking the supplement, the athletes cycled 137 km.

Nine other cyclists were given a placebo, and made up the control group.

Power athletes are interested in L-citrulline as a supplement because enzymes use the amino acid as a raw material for nitrogen monoxide [NO]. This is the substance that stimulates muscle cells to grow. The researchers discovered this also happened in the cyclists. They measured more nitrite in the cyclists' blood immediately after they had finished cycling and 3 hours afterwards too. Nitrite is a marker for NO.

Cyclists on L-citrulline use more amino acids

NO widens the blood vessels, thereby easing the transport of oxygen and energy to the muscle cells. This is interesting for endurance athletes.

The supplement had no effect on the red blood cells. The amino acid did increase the concentration of growth hormone, the insulin level and the concentration of creatinine, however. Creatinine is a waste product of creatine. The researchers think that citrulline supplements help muscle cells to produce more creatine. Sounds plausible, because arginine is a building block for creatine.

Cyclists on L-citrulline use more amino acids

The researchers found 30 percent more urea in the blood of the citrulline group than in the control group. Urea is a waste product from the process of protein metabolism. This indicates that citrulline boosts protein metabolism.

Blood analysis showed that the concentration of BCAAs in the citrulline group had decreased after the 137-km ride. BCAAs are the amino acids that active muscle cells need most.

Cyclists on L-citrulline use more amino acids

The researchers are not sure how to interpret this effect: they are undecided between 2 possibilities. Maybe the muscle cells use the amino acids for their own recovery, so it would be an anabolic effect. That’s a positive sign. But maybe the muscle cells convert BCAAs into energy. This is more likely – and this is also a positive effect for cyclists.

If the latter theory is correct, then cyclists using L-citrulline probably need more BCAAs. But that's an easily solved problem.

Eur J Appl Physiol. 2010 Sep; 110(2): 341-51.