Arginine is essential for the synthesis of nitric oxide, proline and creatine , which on their own right are required for various biological processes. For example, nitric oxide helps relax vascular vessels , which results in better blood circulation and energy and oxygen transport, and improves immune function. Proline is essential for joint function and wound healing. Creatine plays important roles in neurologic and muscular functions.

The nutrient in watermelon responsible for the increased plasma arginine level is called citrulline. On average,  a gram of watermelon flesh contains 1.5 mg citrulline.

Citrulline, an amino acid, is a precursor for the synthesis of arginine. An intake of 1 kg of watermelon flesh provides enough citrulline for the synthesis of 40% of the mean daily arginine intake ( 3.8 g) by an American adult.

In a study,  subjects (healthy, non-smoking, 12 men and 11 women, 36–69 years old), were treated with controlled diet and 0 (control), 780 (equivalent to 1 g citrulluine), or 1560 (equivalent to 2 g of citruline) grams of watermelon juice per day. The treatments lasted for three weeks. Compared with the control treatment, fasting plasma arginine concentrations increased 12% and 22% after 3 weeks of the lower-dose watermelon and the higher-dose watermelon treatments, respectively.

Furthermore, the higher dose watermelon juice increased plasma ornithine level by 18%.

The study was published in the Journal of Nutrition, March 2007 issue.

Watermelon related:
Nutritional facts on watermelon seed
Nutritional value and health benefits of watermelon
Eating watermelon increases arginine level

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J. Collins, G. Wu, P. Perkins-Veazie, K. Spears, P. Claypool, R. Baker, B. Clevidence. Watermelon consumption increases plasma arginine concentrations in adults. Nutrition, Volume 23, Issue 3, 2007, Pages 261-266 .