Question

how is foodconsidered a fuel for our body

Answer 1

It's a bit like having two tanks of petrol, the one your car is using and a reserve tank in case you run out of fuel. So, if your body runs out of glucose it switches to the reserve tank, glycogen. Once there is enough glycogen stored in your liver and muscles the rest turns into fat.

Answer 2

the reserve is enough glycogen stored in you liver and muscles the rest turns into fat