The high vitamin C content of lettuce makes it a heart-healthy vegetable. A serving of lettuce can provide us with up to 4 mg or 40 percent of our daily recommended vitamin C needs. In particular, vitamin C prevents cholesterol from becoming sticky and adhering to the walls of blood vessels. Therefore, if you suffer from heart disease or have a family history of heart disease, you should include one or two daily servings of lettuce in your diet, in order to keep your heart healthy.