Betsy Blaney, a writer from Associated Press, wrote "A slice of cool, fresh watermelon is a juicy way to top off a Fourth of July cookout and one that researchers say has effects similar to Viagra!" in her news report. 
Watermelons contain an ingredient called citrulline that can trigger production of a compound that helps relax the body's blood vessels. Citrulline reacts with the body's enzymes and is changed into arginine. Arginine boosts nitric oxide, which relaxes blood vessels, the same basic effect that Viagra has, to treat erectile dysfunction. 
The nitric oxide can also help with angina, high blood pressure and other cardiovascular problems. Thus, eating watermelon benefits the heart and the circulatory and immune systems. 
Study also have suggested that taking an arginine supplement for weeks or months would significantly increase the production of sperm and drastically improve erection. The effective doses range from 2 to 5 grams per day. 
Bhimu Patil, a researcher and director of Texas A&M said that watermelon rind contains 60% more ciltrulline than in the flesh. And, among different watermelons, the yellow-fleshed types contain the most citrulline.
However, you need to have about six cups of watermelon to get enough citrulline to boost the body's arginine level. In addition, watermelon is a diuretic, you may need to "visit bathroom" frequently after taking a large quantity of watermelon. Another issue is its amount of sugar, this may lead to side effect of cramping. [1,2]
Though its high content of sugar, some researchers believe that intake of watermelon may benefit those who are obese or suffer from type-2 diabetes. 
Citrulline is also present in nuts, such as almonds, walnuts, cashews, pine nuts and other curcubits, such as cucumber and cantaloupe. [1,2]
While, Ben Wasserman, reporter at Foodconsumer.org, suggested that instead of eating watermelon, those who want to have a boost in their bedroom performance may consider eating peanuts and other foods that contain high levels of arginine. In the meantime, they should also eat high amounts of vitamin C and if possible some omega-3 fat. 
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