Your body reacts to the sweetness and produces insulin, which when not paired with actual sugars, turns into fat. This is known as "Insulin Resistance" (http://en.wikipedia.org/wiki/Insulin_resistance)
Additionally, items which digest to glucose trigger insulin production. Aspartame has repeatedly been shown to not. Most other low calorie sweetners, additionally, have not as well.
Saccharin had one study that showed a correlation, however, several other studies did not.
However, protein does: http://www.ncbi.nlm.nih.gov/pubmed/1946186