Probably the prime benefit of Citrulline supplementation is showing itself
to be increasing erectile strength and hardness factor. In fact, in my forum some
of the recent comments from men with erectile dysfunction have been strongly positive. One
man with apparently rather severe erectile dysfunction commented that he had an erection of
sufficient quality for penetration, something that clearly had been missing for him for a
long time.  In fact, he mentioned that he had tried other supplement combinations without
any success, so L-Citrulline appeared to have the most horsepower for him.
L-Citrulline is somewhat of a newcomer to the erectile supplement market. Researchers have for
years concentrated on L-Arginine to raise nitric oxide and results had stalled. Arginine had had
lackluster results and in one study even appeared to be dangerous for some men with advanced heart
disease - see my link on The Pros and Cons of Arginine for more information.
However, just when "all hope seemed lost", L-Citrulline showed up as the real superstar in
recent latest. What is interesting is that L-Citrulline is a precursor of L-Arginine. So one
would not expect it to really do any better or worse than Arginine supplementation, right? Actually,
the opposite turned out to be case. Taking L-Arginine directly has issues, which we will discuss below,
that make it less than ideal. Arginine had to be taken in overly high dosages (5 grams +) to have an
effect. For example, one study found that about a third of men, probably those with endothelial
dysfunction as we will discuss below, had significantly improved erections with 5 grams of L-Arginine
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Unfortunately, 5 grams approaches the daily amount given (9 grams/day) in the above-mentioned study
of Arginine where six men died. Of course, Arginine can be used successfully when at much lower doses
(1.5 grams/day) when combined with other supplements such as Pycnogenol. But all of this goes to show
that L-Arginine has definite issues when used by itself.
One of the big issues with ingesting L-Arginine directly is that it is metabolized in the liver by
arginase. And in men with heart disease, arginase is “upregulated” or more active. This means that
more Arginine is converted to the amino acid ornithine, which, as mentioned previously, can potentially
lead to herpes outbreaks and other issues.
L-Citrulline, on the other hand, seems to produce equal or better results at much lower dosages. For
example, one 2011 study found that a mere 1.5 grams of Citrulline increased the hardness factor of a
significant number of men.  One of the reasons that so much less is required is that Citrulline
supplements completely bypass
the liver and the whole arginase conversion process. One study put Arginine
head-to-head with Citrulline and showed that Citrulline was the clear winner.
One beautiful thing is that you can easily get this dose through food, namely in watermelon. One
study found that an average dosage of watermelon, although the content can be quite variable, was 1.9 mg of
Citrulline per gram of weight.  This may not sound like a lot but you have to realize that a cup of
watermelon is about 154 grams or 290 mg of Citrulline. I don’t know about you, but I can easily down a
few cups, and – voila! – you’ve just consumed a clinical dose, making watermelon a true super erection
Yet another study  found that drinking enough watermelon juice to get 1 or 2 grams of L-Citrulline
daily led to 12% and 22% increases in plasma Arginine levels, respectively. This will translate to
increased nitric oxide levels, especially for most middle-aged and beyond men. CAUTION: It
also increased Ornithine levels by 18% as well.
What I have used is Citrulline in powder form from Source Naturals: Source Naturals L-Citrulline Powder, 100g.
This works very well for me and seems to be the most powerful erectile
dysfunction supplement for me. The powder, at least in my opinion,
actually tastes half way decent and is probably the cheapest option. I
have taken between 1-3 grams/day in 3 divided dosages and all of them seem to
help. Talk to your doc first of course.
And it is no wonder that Citrulline works so well for me : it is now thought of as an excellent solution for men with endothelial dysfunction.
The reason? It actually acts as an alternative source for arginine - rather than
the typical endothelial pathway - and thus provide an excellent way to increase
your baseline nitric oxide levels even if you arterial issues. In
addition, you lose some of your nitric oxide just from aging, so this is a way
Remember: even Viagra and Cialis really do not "create" nitric oxide but rather work on the enzyme that breaks it
down. Viagra and Cialis simply allow nitric oxide to stay in your system longer, because it is
broken down less quickly.
However, this is one of the reasons that Viagra, Cialis and Levitra do not work for many men with
endothelial dysfunction. In these men, the lining of their arteries and blood vessels is so
damaged that they do not put out enough nitric oxide for these PDE5 Inhibitors to act upon. L-Citrulline
can substantially help such men by actually providing more substrate for these medications to act upon.
CAUTION: Always check with your physician before trying any new supplement with another medication.
Example: One forum poster said that Citrulline gave him a migraine (since any vasodilator can do that). 
Or, better yet in my opinion, L-Citrulline can act as a source of nitric oxide for other
more natural NOS (the "nitric oxide enzyme") solutions such as Ginseng, Pycnogenol, exercise and
food. This, by the way, is one of strategies behind the nitric oxide boosting supplement that
has recently been all over the web called Neo-40. It is an interesting combination of L-Citrulline
and Hawthorn Berry Extract that does just this. One of the foremost nitric oxide researchers in the world,
Nathan Bryan, helped design it and so we know we are onto something here. A
couple of the guys on the Peak Testosterone Forum have taken it with decent results.
Citrulline also has excellent potential for improving general arterial health, which always benefits
erectile function as well. How does it do it? By slowing down the progression of arteriosclerosis.
That’s right, an animal study showed that by increasing nitric oxide and blood flow and reducing
superoxide (a potent free radical) gene expression, it could actually greatly help in the batlle
against hardening of the arteries. 
The short term benefits are hard to argue with. As I mentioned, one poster on a forum thread
said that L-Citrulline restored his morning erections. And yet another poster stated
"I have only been on the Citrulline for 2 days and it appears to be marvellous
stuff- practically the equivalent of having an explosion in my pants!"
DOSAGE: The highest dosage that I have seen in a study is 3 grams per day. However, I would recommend trying a lower dosage, 1-2 grams to start out with . Keep in mind that even a few cups of watermelon will likely help. There are also many excellent sources of L-Arginine in food, which tend to have a nice balance of both Arginine and Lysine (important for herpes control). For example, an egg white has almost equal levels of Arginine and Lysine (200 mg). Rice protein, on the other hand, has about a 3 to 1 ratio of Arginine to Lysine.
2) Urology, Jan 2011, 77(1):119-22, "Oral L-citrulline supplementation improves erection hardness in men with mild erectile dysfunction"
3) BJU Int, 1999 Feb, 83(3):269-73, "Effect of oral administration of high-dose nitric oxide donor L-arginine in men with organic erectile dysfunction: results of a double blind, randomized, placebo-controlled study"
4) Journal of Chromatography A, Jun 2005, 1078(1-2):196-200, "Determination of citrulline in rind"
5) Nutrition, Mar 2007, 23(3):261–266, "Watermelon consumption increases plasma arginine concentrations in adults"
6) PNAS, Sep 20 2005, 102(38):13681-13686, "L-citrulline and L-arginine supplementation retards the progression of high-cholesterol-diet-induced atherosclerosis in rabbits"
7) Br J Clin Pharma, 2008, 65:51-59, "Pharmacokinetic and pharmacodynamic
properties of oral L-citrulline and L-arginine: impact on nitric oxide