Rated as the World's Healthiest Vegetable. Spinach is a highly nutritious vegetable that is rich in vitamins A, K & C, iron, calcium and antioxidants. It is low in calories and is high in oxalic acid. Minerals such as potassium, phosphorous and zinc are also present. It helps battle arthritis, heart disease and certain types of cancers such as "colon" and "prostate" cancer.