Selenium is a mineral essential in small amounts, although it is toxic at high levels. Selenium assists in the function of some enzymes known as selenoproteins. Selenium is important for the immune system and healthy hair and nails. It is an important part of your body's antioxidant sytems to protect cells from damage. Selenium is present in many different foods, with most losses related to leaching into cooking water.
Throw your veg in the pan soon after chopping them up to avoid losing all the good stuff in them. Vegetables can lose nutrients once exposed to light and air.