Artificial sweeteners (e.g. aspartame, splenda, saccharine), are low in calories. Most people believe that if you use them instead of sugar, they will help you to lose weight.
In fact they can have the opposite effect.
Our ancestors in paleolithic times were hunter gatherers. They had no access to artificial sweeteners. Their bodies adapted to the types of food that they did have. We still have pretty much the same genes that they did since there has not been time for significant evolution since then.
In their times, when something that tasted sweet was eaten, that something was almost certainly something sugary, e.g. honey. So their bodies immediately pumped insulin into their blood steam in order to be prepared to cope with an increase in blood sugar. That was good.
In our times, the same thing happens. And if the sweetness arises from sugar or honey, the increase in insulin is good. But if the sweetness arises from an artificial sweetener, the insulin is produced but no extra blood sugar appears in the bloodstream.
So the insulin can cause blood sugar to drop sufficiently to cause cravings for carbohydrate, within an hour or so. So you then eat extra carbohydrate food (e.g. bread, cakes, biscuits, sugary snacks). The extra calories from those cause you to put on weight.