Sweet taste is not stored energy
Your body stores fat from energy it can use, not from the sensation of sweetness. A zero-calorie sweetener provides little or no usable energy, so there is no meaningful supply of fuel for the body to convert into stored fat. Depending on the sweetener, it may pass through largely unused or be processed in a way that yields too little energy to matter.
That is different from sugar. Sugar supplies calories because the body can break it down and use it for fuel. If energy intake repeatedly exceeds what the body uses, the surplus can be stored—often as body fat. The sweet taste itself does not cause that surplus.
Where later weight gain comes from
If someone uses a zero-calorie sweetener and later eats more food, the energy comes from that food. The body does not retroactively turn the sweetener into calories. Foods containing fat, carbohydrate, or protein can all contribute usable energy; what matters for storage is the overall balance between energy consumed and energy used over time.
So a sweetener may replace some calories when it takes the place of sugar, but it cannot cancel calories elsewhere in a meal. The practical distinction is simple: sweetness describes flavor, while calories describe usable energy.
