Watermelon
Almost entirely water, with citrulline in it. The blood pressure story comes from an extract and not from a slice.
How to use it
As fruit on a hot day, or as water you chew. With a little salt and lime over it, it becomes a dish.
The best version
Heavy for its size, with a yellow patch where it lay on the ground. Knock it: a ripe one sounds hollow.
How to prepare it
Eat it cold, in cubes. The white rind is edible and full of citrulline, though almost nobody eats it.
What it does
In 14 people with obesity and raised blood pressure, six weeks of watermelon extract at 6 grams of citrulline and arginine a day lowered systolic pressure by 15.1 mmHg at the arm and 11.5 mmHg at the ankle. Diastolic pressure fell by 7.6 mmHg.
The red colour is lycopene, one of the strongest antioxidants in food. Review work links it to less oxidative stress and less inflammation, and to protection of bone, eye, kidney, liver, lung, heart and nervous system.
Watch out
That review is mostly laboratory and animal work, and most of it was done with tomato and with lycopene as a supplement. It does not show that watermelon does this in you.
That was an extract in a capsule, in fourteen people who were already too high. In 21 healthy postmenopausal women who drank two 360 mL servings of watermelon juice a day for four weeks, nothing about their blood vessels changed.