All parts of the watermelon, including the rind, flesh and seeds, contain citrulline, a non-essential amino acid that converts into the amino acid L-arginine when eaten. Your body can manufacture citrulline, which is why it’s called a non-essential amino acid; you don’t have to eat it. Citrulline converts ammonia, a waste product in your body, to urea for removal in the urine. Watermelon seeds are edible, but if you don’t chew them, they pass through the intestinal tract without breaking down
If you’d rather get your citrulline from supplements than seeds, you can purchase citrulline supplements, which purportedly enhance sports performance, although there’s no clinical proof of these benefits, according to NYU Langone Medical Center. Alternative practitioners also recommend citrulline to treat impotence. The normal supplemental dose is 6 to 18 g in the form of citrulline malate.
Citrulline might act as an antioxidant, as well as a vasodilator, according to the USDA. Citrulline’s ability to increase L-arginine supplies in the body might offer benefits in treating high blood pressure, sickle-cell anemia and elevated glucose levels, according to the United States Department of Agriculture. A USDA study reported in the March 2007 issue of “Nutrition” tested the effects of watermelon juice in amounts of 1 or 2 g per day. The study found that arginine concentrations increased by 12 percent at the lower dose and 22 percent at the 2 g dose. Fasting citrulline levels did not rise.
Citrulline in large amounts can cause harm if you have certain health conditions, even if you take it in watermelon form rather than in supplements, as a study by the Meyer Children's Hospital in the 2005 “Journal of Inherited Metabolic Disease” reported. Researchers gave healthy volunteers large amounts of watermelon and then tested their citrulline and arginine levels. All developed very high citrulline levels and, to a lesser extent, elevated arginine. While high levels of citrulline might not affect most people, it could harm people with citrullinemia, a genetic disorder that affects the urea cycle.
If you don’t like watermelon seeds, you can eat the rind, as it contains the most citrulline. If gnawing on the rind doesn’t appeal to you either, just eat the flesh, which also supplies plenty of citrulline. The yellow and orange varieties of watermelon contain more citrulline than the red, if you’re looking for maximum levels.