At the Whole Foods Market in Silver Spring, Md., the self-serve coffee counter offers four types of milk and nearly every imaginable alternative to granulated sugar. There's unrefined sugar, evaporated cane juice, agave nectar—and a no-calorie sugar substitute called Truvia.

The green packets are tucked behind the cash register; if you want it, you have to ask. That's because they have a way of disappearing. "People take a lot more than they need," says Liz Burkhart, a Whole Foods spokeswoman.

Truvia's...