A very controversial subject among the community is whether or not artificial sweeteners and diet beverages cause weight gain. The answer is NO they do not, but they can contribute. Artificial sweeteners and diet beverages are nearly 0 calories. If a food or drink does not contain calories it cannot cause someone to gain weight. However, the affect these things have on the body can contribute to overeating.
When you drink things with artificial sweeteners you body wants to use it for energy (the calories are the energy source) but because there are no calories present the body makes you want more and more. Since the body is looking for calories (because of the diet drinks and artificial sweeteners) it causes you to feel hungry which can lead to over eating/ extra calories and then causes weight gain.