Stevia followers are a diverse bunch, including health nuts and food-industry magnates. The draw? The sweetener is all-natural and naturally calorie-free. But "natural" doesn't necessarily mean safe, and scientists have long struggled to make sense of early evidence hinting that stevia could be toxic. A series of studies published last month in the journal Food and Chemical Toxicology put that question to the test for one type of stevia-based sweeteners.