If you do have vegetables left over, cover them tightly and refrigerate. Vitamin loss, especially C , increases with air, temperature, water and time. So the less of each, the better. Microwave cooking of food is not harmful, based on current research. In fact, microwaves preserve more of the vitamins and minerals in food because the food's exposure to heat and water is shorter than with conventional cooking methods.
Microwaves cause the water molecules in food to vibrate rapidly.
It is this vibration that gives off heat, which cooks the food. What initially scared consumers about using microwaves was the warning on the door about radiation if the door was open. All microwaves have a safety mechanism that prevents you from operating the oven with the door open. The only person who should not use a microwave oven is someone who has a heart pacemaker implanted. The microwave could disrupt the electrical impulses produced by the pacemaker.