Mustard greens are an excellent source of vitamin C, vitamin K, vitamin E, folate, beta-carotene, and manganese. Along with other cruciferous vegetables, mustard greens have been linked to cancer prevention due to their high antioxidant content. Specifically, mustard green consumption has been associated with the prevention of bladder cancer, breast cancer, colon cancer, lung cancer, prostate cancer, and ovarian cancer. (Read more about the health benefits of mustard greens on World's Healthiest Foods.)