Is diet soda bad for you? Specifically, does it actually help you lose weight? Long story short, no. In fact, a Purdue researcher says public health officials should tell people to avoid diet soda much like they do with regular, sugar-sweetened soda. Susan E. Swithers, PhD, a professor of psychological sciences and a behavioral neuroscientist at Purdue says warnings may need to be expanded to include limiting intake of all sweeteners, including no-calorie sweeteners.