Can watermelon help keep your cucumber crisp?

Word is on the street that watermelon (Citrullus lanatus), God's gift to summertime family reunions, may produce a Viagra-like effect. As a commenter on reddit put it: "They cool you above and fire you up down below."

This effect is apparently made possible by an amino acid called citrulline, which is found in both the rind and flesh of the watermelon. It's also found in mammals, where it is present as a free molecule in the liver as an intermediate in the urea cycle (toxic ammonia in blood -> urea in kidney -> you pee it out) and occasionally found incorporated into proteins. As it is not coded by DNA, it must be produced via the post-translational (occurring after a protein is synthesized from amino acids) modification of arginine, a structurally related amino acid. This process is called citrullination, and it occurs primarily in proteins found in myelin or involved in keratinization (deposition of keratin in skin, hair, and nails). Insufficient citrullination of these proteins has been implicated in some autoimmune diseases including rheumatoid arthritis, psoriasis, and multiple sclerosis.

Anyway, back to this watermelon = possible stiffy business. Following the consumption of watermelon, the citrulline it contains is enzymatically converted into arginine, which in turn is broken down to release a gas called nitric oxide (NO). NO causes the smooth muscle within the walls of blood vessels to relax, leading to dilation of affected vessels and increased blood flow through them. Increased blood flow to the penis leads to an erection. Sounds good, right?

Now here's the thing. Lots of substances can dilate blood vessels, including alcohol and chocolate. We synthesize citrulline in our guts from glutamine, a much more common amino acid. Dairy products, meat, seafood, bread, oatmeal, nuts, seeds, and lentils all contain decent amounts of arginine, which is what citrulline is converted to before it becomes NO. Viagra is so good at doing what it does because it acts on a particular enzyme that opposes the action of NO and is found almost exclusively in the penis. Just because something dilates blood vessels and increases blood flow doesn't mean its going to produce a woody.

The way I figure it, either the researchers at Texas A&M wanted some publicity and so went out of their way to mention that watermelon could potentially be used to treat erectile dysfunction, or the media decided to concentrate on the sex and ignore the science. Possibly both.

It should be mentioned that even if this whole watermelon boner thing is bogus, drugs that relax blood vessels are important for the treatment of many cardiovascular issues such as angina and hypertension. Citrulline is currently used to treat inherited urea cycle disorders and may eventually find use as a means of boosting the citrullination of proteins implicated in autoimmune diseases.

- Curis E, Nicolis I, Moinard C, Osowska S, Zerrouk N, Bénazeth S, Cynober L. Almost all about citrulline in mammals. Amino Acids. 2005 Nov;29(3):177-205. Review.