Today on a Moment of Science we put to rest once and for all the myth that microwaving food somehow damages or irradiates it. There is no evidence that microwave food is less nutritious than any other kind of food, even if it doesn't always come out crispy.
Microwaves are low energy electromagnetic waves that have so little energy they can't trigger chemical reactions in the molecules they encounter. And as long as there's nothing in your microwave that can cause a spark, they also don't leave behind any traces. When you heat food in the microwave, what you're actually doing is running an electrical current through the water molecules in the food. This causes the water molecules to rotate and collide, and these collisions generate friction that produces a lot of heat. The reason microwaves are so speedy is because food is made up mostly of water. In contrast, when you use an oven or a grill, you have to wait for the food to heat from the outside in.
Consequently, the only chemical and physical changes that happen to your food when you microwave it are the results of the heating process. For example, at high temperatures, sugars and proteins might interact and vitamins might be destroyed. The thing to keep in mind is that these changes happen regardless of the method you use to heat food. What's more, because microwaves heat up food so quickly, some of these changes might be less drastic than in foods that are heated up over a long time.