L Citrulline, like arginine, is an important amino acid since it can convert into nitric oxide. Glutamine is a precursor of ornithine, which can be converted to citrulline by the intestine; citrulline is transformed in the kidneys to arginine. Hepatic citrulline uptake limits the amount of gut-derived citrulline reaching the kidney.
   Citrulline is found in watermelon rind and less so in watermelon flesh, but I doubt consuming watermelon rind or eating watermelon fruit will have any significant sexual enhancing effect. If you have an interest in a potent herbal sexual formula, Passion Rx is the product to seriously consider rather than eating watermelon. Passion Rx is much more potent than citrulline or arginine in terms of libido and erectile function.

Citrulline supplement, 600 mg
Arginine supplement 750 mg
Passion Rx, effective sex enhancer for men and women
FTH pure L-Cotrulline is an essential component of the urea cycle, helps detoxify ammonia, and metabolizes to form Arginine, which is very Important for nitric oxide production for endothelial and cardiovascular support. Each capsule of pure L Citrulline contains 600 mg. of the highest quality citrulline available.

According to FTH, the manufacturer, L Citrulline:

Increases L-Arginine Formation. The production of nitric oxide, which is necessary for endothelial health and proper circulatory function.
Urea cycle function - Citrulline, as an essential component of the urea cycle, is necessary to help detoxify ammonia and convert it to Urea.
Ornithine Transcarbamoylase (OTC) Deficiency, which is a genetic urea cycle disorder causes a Citrulline deficiency.

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L Citrulline Supplement Facts
Amount Per Serving:
L Citrulline - 600 mg
* Citrulline daily value not established.

Citrulline side effects
Q. Would you please tell me if there any citrulline side effects?
   A. I have not seen any reports in the medical literature regarding citrulline side effects and have not had any consumers email with such side effects.

Watermelon has citrulline
Watermelon consumption increases plasma arginine concentrations in adults.
Nutrition. 2007 March. Collins JK, Wu G, Perkins-Veazie P, Spears K, Claypool PL, Baker RA, Clevidence BA. USDA-ARS, South Central Agricultural Research Laboratory, Lane, Oklahoma, USA.
Watermelon is a rich source of citrulline. No detailed studies have been conducted to evaluate plasma arginine response in humans after long-term feeding of citrulline from natural plant sources. This study investigated if watermelon juice consumption increases fasting concentrations of plasma arginine, ornithine, and citrulline in healthy adult humans. Subjects consumed a controlled diet and 0 (control), 780, or 1560 g of watermelon juice per day for 3 weeks in a crossover design. The treatments provided 1 and 2 g of citrulline per day. Compared with the baseline, fasting plasma arginine concentrations increased 12% after 3 wk of the lower-dose watermelon treatment; arginine and ornithine concentrations increased 22% and 18%, respectively, after 3 wk of the higher-dose watermelon treatment. Fasting citrulline concentrations did not increase relative to the control but remained stable throughout the study. The increased fasting plasma concentrations of arginine and ornithine and stable concentrations of plasma citrulline in response to watermelon juice consumption indicated that the citrulline from this plant origin was effectively converted into arginine.

Does watermelon have effects similar to Viagra?
In July 2008 the media made a big story of watermelon having Viagra-like effects since watermelon contains a substance called citrulline which converts into the amino acid arginine. Arginine helps dilate blood vessels. Because of this biochemical connection, Bhimu Patil, a researcher and director of Texas A&M's Fruit and Vegetable Improvement Center was quoted saying, "Arginine boosts nitric oxide, which relaxes blood vessels, the same basic effect that Viagra has, to treat erectile dysfunction and maybe even prevent it." Well, this is all the media needed to hype this story. But, does eating watermelon flesh or rind have any sexual enhancing effects? I love watermelon, it is one of my favorite foods. I actually love the combination of watermelon and feta cheese. I have been known to eat half a large watermelon at one sitting. I have not noticed any Viagra-like effects from eating watermelon or drinking watermelon juice. I have also tried citrulline supplements and arginine supplements and have not noticed much of an effect on erectile function. Even if citrulline and arginine dilate blood vessels, their effects are brief. If you really want to enhance libido, sensation and sexual stamina, Passion Rx with Yohimbe is one of your best options. 
   According to Bhimu Patil, more citrulline - about 60 percent - is found in watermelon rind than in the flesh.

Citrulline in foods
Citrulline is found in high concentrations in watermelon rind and flesh. Citrulline is also present in other curcubits, like cucumbers and cantaloupe, at very low levels, and in the milk protein casein. The highest concentrations of citrulline are found in walnut seedlings.

Information on Arginine, Citrulline, and Ornithine
The kidney plays a major role in arginine metabolism in 3 principal ways: arginine synthesis, creatine synthesis, and arginine reabsorption. Appreciable quantities of arginine are synthesized in the kidney from citrulline produced by the intestine. The renal enzymes of arginine synthesis, argininosuccinate synthetase and argininosuccinate lyase, occur in the cells of the proximal tubule. The rate of arginine synthesis depends on citrulline delivery and does not appear to be regulated by dietary arginine availability. Renal arginine synthesis in humans produces approximately 2 g arginine/d, which may be compared to an intake, from a Western diet, of approximately 4 to 5 g/d. Spontaneous, nonenzymatic breakdown of creatine and creatine phosphate to creatinine causes the excretion of 1 to 2 g creatinine/d and requires the replacement of an equivalent amount of creatine from the diet and by endogenous synthesis. The first enzyme of creatine biosynthesis, L-arginine:glycine amidinotransferase, occurs in the kidney and produces guanidinoacetate, which is released into the renal vein. The renal output of guanidinoacetate, however, is rather low, and we propose that the entire pathway of creatine synthesis may also occur in the liver. Renal arginine reabsorption salvages approximately 3 g arginine/d. At the apical membrane of proximal tubular cells, arginine shares a transporter with lysine, ornithine, and cystine. Defects in this heteromeric transporter cause cystinuria, which is also characterized by urinary loss of arginine, lysine, and ornithine. Arginine is transported out of the proximal tubular cells at the basolateral membrane by another heteromeric transporter, which also transports lysine and ornithine. Defects in this transporter cause lysinuric protein intolerance.

Can arginine and ornithine support gut functions?
Gut. 1994 Jan;35(1 Suppl):S42-5. Cynober L.
Laboratoire de Biochimie, Hopital Saint-Antoine, Paris, France.
Arginine and ornithine are precursors of nitric oxide and polyamines, respectively. These metabolites intimately participate in permeability and adaptive responses of the gut. The liver possesses high arginase activity as an intrinsic part of urea synthesis and would consume most of the portal supply of dietary arginine. The gut reduces this possibility by converting dietary arginine to citrulline, which effectively bypass the liver and is resynthesized to arginine in the kidney. Dietary ornithine supplementation, in the form of ornithine alpha-ketoglutarate (OKG) can be considered as an arginine precursor. Several supplement studies have shown both amino acids to promote growth hormone and insulin secretion with anabolic effects in postoperative patients. Their intermediary metabolites (for example, glutamine, proline) may also be of benefit in trauma metabolism. Specific effects of either amino acid on the gut are poorly reported. One recent animal study showed improved morphology after OKG administration, perhaps through increased polyamine secretion. Generation of nitric oxide from arginine has two facets. Excess production from high dose arginine potentiated the effects of experimentally induced sepsis, whereas low doses improved survival. These considerations suggest that the role of enteral diet supplementation with arginine or OKG should be urgently examined for any benefits it may have on mucosal barrier function.

See Amino Acids for additional information.

Citrulline is sometimes misspelled as citruline

Citrulline supplement questions
Q. I am a healthy 71 year old married man experiencing some reduction of erectile function. I have been taking two 750 mg L-Arginine capsules per day, one in the morning and one in the evening, and my perception is that this does provide some improvement. I can usually function sexually several times per month without resorting to Cialis or Viagra, but I'm not as reliable as I used to be most of my life. I tried using your Passion Rx product and used a 30 day supply, following the directions of taking one every other day. Frankly, I couldn't notice any effect at all with this product. I have recently read articles regarding the combination of L-citruline and L-Arginine indicating that this combination seems to have significantly more effect than L-Arginine alone. However, I don't find any guideline for dosages of these amino acid products. Do you have any advice regarding the combination of L-Arginine and L-Citruline?
   A. At present we don't have any research on anecdotes on the combination of arginine and citrulline. We have about an 80 % response rate to Passion Rx, we wish it was higher but for some reason some people don't respond while others notice significant benefit with even a third of a capsule. Some people who don't respond to Passion Rx respond to Prostate Power Rx, but this is not consistent either. Studies show Viagra and Cialis have about a 60 to 70 percent response rate.

Q. How is L-Citrulline Ethyl Ester HCl different clinically than citrulline?
   A. I have not seen any studies comparing L-Citrulline Ethyl Ester HCl to plain citrulline, so not much can be said.

Q. Do you know if Citrulline and l-Arginine are considered safe for a diabetic to use as a dietary supplement?
   A. We have no reason to believe that citrulline and arginine would not be safe when used in low dosages by a person with diabetes.

Q. Is citrulline taken daily or every other day. How does citrulline supplement affect erections and sexual desire? How does citrulline affect the penis in the flaccid state since there is more nitric oxide in the system.
   A. We are not impressed by citrulline as a supplement for erection enhancement or sexual desire improvement. The use of a citrulline supplement is not likely to influence penis erection.