Does warming food in microwave destroy the vitamins or minerals? secondly does it causes any other harmful impact ie; cause cancer?thanks

Microwaves denature: (destroys) the enzymes in food. Thus, you get little nutrition from microwaved food. This may account, to some degree, for the weakness you mention in your history. As far as other side effects of microwaved foods, it may be too early to tell. A toaster oven is quick enough and easy. Peace and good health.

It's fine: Minerals -- not at all. They can only be destroyed in a nuclear reactor. Vitamins -- maybe a little, notably vitamin C, but you probably get plenty. Causing cancer? That's internet scare hoax stuff. Taking proper care of your food is key to avoid food-borne illness. Best wishes.