Screaming Penguin - Source Does Matter

Sweet tastes promote the release of insulin, which causes glucose (blood sugar) to be stored as fat. This is an adaptive response, because for millions of years sweet tastes have meant that blood glucose levels are about to rise, and when there is excess sugar it ought to be stored for times of fasting, when food is not readily available. Artificial sweeteners have the same effect on insulin: sweet diet drinks will increase insulin and thus the storage of fat. In this case, though, no sugar is provided by the beverage, so the drinker stores away glucose already present in the blood, glucose that is needed for energy. The result is a decrease in blood sugar, and a corresponding increase in hunger. The drinker eats more, and gains weight.