The first time I read this thread, I thought, Come on, this is ridiculous, there are no horrible effects from microwaving and microwaving isn't deforming molecules to change them to some mutated form that will kill you.
I still feel that way, however, I was interested in this and decided to do some reading and was really surprised by some of the things I found.
None of this information, by the way, is from any radical site that will remain unnamed, lol
I'm actually a chemist so I started reading some journal articles about microwave effects and there are two kinds. The thermal effects, which are the result of heating and the same effects you would get from cooking in other ways, and the non thermal effects which are unique to microwaving.
In the 80's and 90's, there were quite a few studies done that suggested the possibility of non thermal effects on food but more recent studies have disproved these results and conclude that there are no non thermal effects at all.
However,when I looked at some chemistry journal articles that are studies on the use of microwaves in applied chemistry, there are non thermal effects of microwaves on the reactants that are observed when the medium is non polar.
I'm now wondering if there could be some non thermal microwave effects on food that contain fats since they are non polar. If you melt butter in the microwave, is the effect on the butter molecules significant to food safety/quality because it is chemically different than when it is melted on the stove?
If the food is water soluble, there should be no non thermal effects and the heating reaction is the same as when you use the oven, stove, fire or whatever.
Although food cooked in the microwave does lose vitamin B, food cooked on the stove or in the oven loses many more vitamins that microwaving preserves. Also, there is a risk of carcinogen formation when food is cooked too well done on the stove, grill or oven that does not occur with microwaving.
I haven't found any studies that are available for no cost that look at these non thermal effects on foods that contain fats or other non polar solvents but it's interesting and I'm definitely going to continue to look into this.
From what I read though, overall, cooking with any heat source of course changes the molecules of the food and cooking on the stove/oven/fire, etc...has definitely been shown to cause carcinogen formation in some situations and pretty significant loss of nutrients.
Cooking in the microwave shows much less loss of nutrients and for the most part, seems to have thermal effects on molecules that are no different than cooking with other heat sources.
But now there is a question in my mind of whether the non thermal effects of microwaves have a significant effect on food when there are non polar solvents and what the ratio of polar/non polar solvents does to the formation of non thermal effects.
To answer the original OP's question, lol, I always reheat foods in the microwave for my children and the only thing I've ever worried about are the hot spots.
When I reheated breastmilk, I did it in warm water because the proteins are denatured faster in the areas where there are hot spots that form while microwaving. But if you heated it too high on the stove, that heat would have the same effect as the microwave heating.
ETA here is a link to one of the articles I read that talks about using the non thermal effects on reactions.