A fungus rather than a vegetable, mushrooms have been used in Eastern medicine for thousands of years. These nutritional powerhouses are packed with B vitamins, nutrients, minerals and proteins. They are also extremely rich in antioxidants such as selenium, which protect cells from free-radical damage. Although all mushrooms provide a valuable immune system boost, the reishi, shitake and maitake varieties are especially beneficial. Shitake mushrooms also have powerful anti-carcinogenic properties. Commonly used as a cancer treatment in Japan, they are full of a compound called lentinan which is believed to slow or stop tumor growth.