What is d main cause of belly fat.

Excess calories: When you consume more calories than you burn, the excess is deposited as fat, everywhere in the body, although the belly is a common place to store that fat.

Carbs stored as Fat: Why do animals store excess food as fat? Water rejects fat whereas proteins & especially carbohydrates (i.e. all simple & complex sugars) have lots of charged side groups & bind water molecules thereby adding a lot of extra body weight to carry around. Eg. the same energy stored as fat, if stored as carbohydrate, would more than quintuple storage weight because of the associated water molecules.