Coke and Pepsi are chasing the sweet spot: a soda with no calories, no artificial sweeteners and no funny aftertaste.

The world's top soft drink companies hope that's the elusive trifecta that will silence health concerns about soda and reverse the decline in consumption of carbonated drinks. But such a formula could be years away.

That's because the ingredient that makes soda taste good is also what packs on the pounds: high-fructose corn syrup. Artificial sweeteners like asparatame that are used in diet drinks don't have any calories but are seen as processed and fake. Natural sweeteners that come from plants present the most promising alternative, but companies haven't yet figured out how to mask their metallic aftertaste.

Despite the complexities, soft drink makers push on in their search.