CANDICE CHOINEW YORK (AP) â¿¿ Coke and Pepsi are chasing after 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 aspartame 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. "I can't say when it will be here, but it's in the reasonable future," said Al Carey, who heads the beverage unit for the Americas at PepsiCo Inc., the world's No. 2 soda maker. There's good reason that soft drink makers are so eager to tweak their formulas. Once a beloved American treat, sodas are now being blamed for the nation's bulging waistlines â¿¿ two-thirds of the country's adults are overweight or obese, according to the Centers for Disease Control and Prevention. That, coupled with the growing variety of flavored waters and sports drinks, has sent per capita soda consumption down 17 percent to about 1.3 cans a day since its peak in 1998, according to data from Beverage Digest, an industry tracker. In New York City, a ban on the sale of sugary drinks bigger than 16 ounces in restaurants, theaters and stadiums could take effect as early as March. The mayor of Cambridge, Mass., proposed a similar ban last month. And in Richmond, Calif., voters will decide in November whether to pass the nation's first penny-per-ounce tax on soda and other sugary drinks such as fruit juices and teas.