L-citrulline is an amino acid that is naturally in our bodies that helps blood vessels expand to improve blood & nutrient supply to our muscles. This can have a variety of positive effects that I'll discuss below.
In 1998 Drs. Ignarro, Furchgott and Murad earned the Nobel Prize for their research on how nitric oxide (NO) can support cardiovascular health. NO does not last very long in our bodies, so we don't have a steady supply. In addition, in our early 20's, we begin to produce less and less of it so by the time we are 40 we may only produce 1/2 of what we used to produce. To reap the NO benefits, we need to keep as much as possible in our system.
Since nitric oxide cannot be supplemented in the body directly, we have to take precursors (substances that are transformed into something else by our body) to nitric oxide to increase the amount we have in our blood.
The two primary precursors are the amino acids l-arginine and l-citrulline. The reason that I suggest supplementing with pure citrulline instead of arginine is that, as the diagram shows, citrulline is 100% absorbed into your body and turned into arginine in the kidney. Arginine, in turn, is transformed into NO. Over 1/2 of arginine that is ingested is lost in the kidneys. Therefore, you need to take over twice as much arginine to get the same NO impact as citrulline.
As I mentioned early, the physiological effect of NO as produced by citrulline supplementation, is the dilation of our blood vessels. This improves the volume of blood that can reach our muscles which carries more macro nutrients and oxygen to our muscles. This promotes more effective muscle growth.
In addition, the dilated blood vessels have been shown to have some lowering effect on high blood pressure as well as helping our bodies purge toxins. Both of these stand to reason, don't they? Dilated blood vessels allow the blood to flow more easily, lessening the amount of work our heart has to do. In addition, all that extra flow will help to "flush" your system of toxins. Both of these effects will be better for your overall general health.
From a dosage standpoint, there is not a lot of formal testing available. However, taking 4-6 grams spread out over the course of the day is a very common amount. I take 4 grams per day mixed with my protein powder. It has helped my muscles feel more full and I seem to be less sore the day after my workouts.
I definitely recommend adding citrulline to your supplement list.