Daily consumption of watermelon or concentrated watermelon powder providing 2.7–4.0 g of L-citrulline for 6 weeks in adults with prehypertension or obesity is associated with reductions in central and peripheral blood pressure and arterial stiffness, although results vary across studies.

When someone eats watermelon or takes watermelon powder, the body absorbs a compound called L-citrulline, which the kidneys turn into L-arginine. L-arginine is used by blood vessel lining cells to make nitric oxide, a molecule that tells smooth muscle in artery walls to relax. This relaxation opens the arteries, lowers blood pressure, and makes arteries less stiff. At the same time, natural antioxidants in watermelon stop harmful molecules from destroying nitric oxide or damaging the enzyme that makes it, so more nitric oxide stays active to keep blood vessels open.