It is thought that cooking in the microwave causes food to lose nutrients. Some studies found this to be wrong. Microwaved food may retain vitamins BETTER than stovetop-cooked food. That's because the microwave zaps it quickly and without much water. One study found that spinach retained on 77 percent of the B-vitamin folate when cooked on a stove - but retained ALL of its folate when cooked in a microwave.
(Taken from Consumers Reports on Health)