How This Natural, Sugar-Free Soda Is Making Major Inroads With Consumers Coke and Pepsi, look out. Zevia has its eye on you.

It all changed for Paddy Spence during a trip to Whole Foods in March 2010. That's when the food-industry veteran, who had given up sugar cold turkey a decade earlier, discovered Zevia. The zero-calorie soda was sweetened with stevia, a plant-based natural sweetener he and his wife used instead of sugar.

Six months later Spence, who had worked at food company Kashi and founded natural-foods research firm SPINS, purchased a lot more than a six-pack: He brought together a group of investors and executives to buy the then-3-year-old company for an undisclosed sum.

By focusing on quenching Millennials' thirst for natural beverages, Spence has ushered in serious growth at the Culver City, Calif.-based firm. Today Zevia is the No. 14 low-calorie soda in supermarkets—which might not seem like much until you consider that it is the only independent brand in the top 20, according to Spence, who serves as CEO. Actual sales figures are kept closely guarded, but he says Zevia is the fastest-growing zero-calorie soda on the market and estimates that the brand will sell 100 million beverages in 2015.