Generally, heating food can denature proteins nutrients and basically make the nutrients non-functional for your body. however, there are different cooking methods that will help you retain more nutrients.
I've been told by a past physics teacher that microwaves are especially evil because they cook food at high frequencies and you can't distribute the heat by stirring. High frequencies are more deadly to nutrients than traditional cooking i suppose.
Think of it like this: what gets hotter? heating food in the microwave and hearing it go POP POP POP and the food has mini explosions and splashes all over the microwave interior or when you cook food on the stove where you can actually stir and evenly distribute heat amongst the food?
Quote from my physics teacher: "Why do people put cheese in the microwave?! it's crazy!"
so for reheating veggies... i think thats killing the nutrients twice... haha.. even worse in the microwave
thats my opinion, hope that helps