Many mushrooms are also good sources of selenium, an antioxidant mineral, as well as copper, niacin, potassium and phosphorous. Additionally, mushrooms provide protein, vitamin C and iron. Because their cells walls are undigestible unless exposed to heat, you must cook mushrooms to get their nutritional benefits.ancer Prevention. White button mushrooms, such as crimini, commonly found in grocery stores and salad bars, help remove excess estrogen from circulation, making them helpful for preventing breast cancer.