Researchers disagree about whether or not diet soda affects someone's metabolism, but some scientists do believe that diet soda causes the metabolism to slow down because the body becomes confused and does not process sugar correctly. They believe that diet soda may affect hormone levels and can increase the likelihood of type 2 diabetes, metabolic syndromes and cardiovascular disease.
Other scientists disagree and think it might be an instance where two events are simply correlated, but one event does not cause the other. For example, some researchers theorize that people eat food high in calories or sugar because they "saved" calories by having a diet soda. They may inadvertently reward themselves for drinking diet soda by eating unhealthy foods.