How to keep our body heat? I mean to avoid our body from negative degree? . Any food items like vegetable or fruits?

Eat fat. You get more calories from fat which translates into more energy for maintaining body heat on cold exposure.
Mmmmm. The body temperature is depending on the basal metabolism and there is natural drinks suck as chocolate, cocoa, coffee and hot peppers, they have capabilities of increased the metabolism by increasing heart rate and rise the blood pressure due to the alcaloids such as caffeine and caspaicine.