Dark, green, leafy vegetables are rich sources of many vitamins including A, C, and E, and trace minerals as well as a vast number of flavonoids. Numerous studies have shown that, much like cruciferous vegetables, green leafy vegetables are both cardio-protective and anti-carcinogenic.
Among all vegetables, kale, spinach, and watercress have the highest concentrations of antioxidants. Eating green leafy vegetables on a regular basis provides you with a number of cardio-protective antioxidants. It is also one of the most effective dietary means to help reduce your risk of many cancers.
(Source: Every Heart Attack is Preventable, by Michael Mogadam, M.D.)