By NATALIE DIGATE MUTH, M.D., M.P.H., R.D. Americans now can get their calorie-free sugar fix from an all-natural alternative to artificial sweeteners. Once limited to the health-food market as an unapproved herb, the plant-derived sweetener known as stevia is now widely available and rapidly replacing artificial sweeteners in consumer products. Thirty times sweeter than sugar and with no effect on blood sugar and little aftertaste, stevia sales are predicted to reach about $700 million in the next few years, according to the agribusiness finance giant Rabobank.