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Exploring Important Medicinal Uses for Watermelon Rinds

Most people don't think much about watermelon rinds. Although some use rinds to make pickled, candied and even fried dishes, most folks discard them once they eat the melon's sweet fruit.

However, an Agricultural Research Service study has found the rinds contain citrulline, an amino acid that plays an important role in the human body's urea cycle, which removes nitrogen from the blood and helps convert it to urine. That's where citrulline helps create arginine, another amino acid--one in which some people are deficient.

Recent medical research by other scientists has examined whether the citrulline-arginine relationship can be exploited to create treatments for vascular tone problems associated with sickle-cell anemia.

Agnes Rimando, a research chemist at the ARS Natural Products Utilization Research Unit in Oxford, Miss., says the discovery may lead to production of rind-based extract or dietary-supplement products that address arginine- or sickle-cell-related deficiencies. She conducted the research in collaboration with plant physiologist Penelope Perkins-Veazie of ARS' South Central Agricultural Research Laboratory in Lane, Okla.

In the urea cycle, citrulline combines with another acid to create arginine. Also found within the watermelon's sweet and watery interior, citrulline is a "nonessential" amino acid, meaning it need not be ingested because the body produces it. Amino acids, the body's "building blocks," are produced when digestion breaks down protein.

Arginine boosts nitric oxide, which relaxes blood vessels and thus may help treat angina and other cardiovascular problems. This intrigues scientists studying blood circulation problems associated with sickle-cell anemia. Arginine has also been credited with boosting muscle growth, improving wound healing, combating fatigue, stimulating the immune system, curing impotence and fighting cancer.

Disorders in the urea cycle can lead to a lethal buildup of proteins such as ammonia in the bloodstream. Arginine or citrulline are often recommended to address these disorders.

Rimando says the discovery came early during a study, involving her and Perkins-Veazie's labs, that was aimed at determining the citrulline content of different watermelon varieties.

ARS is the chief scientific research agency of the U.S. Department of Agriculture.