For 13 years running, Americans have been drinking less Coke. Now Diet Coke sales are falling off a cliff. Globally, sales growth of soda is slowing amid concerns about sugar intake and obesity.

The trends are industrywide, but it is especially bad news for Coca-Cola Co., a company that derives almost 75% of its global sales volume from carbonated soft drinks.

"Sugar water with bubbles is not the future of the world. There's an existential issue,'' said Tom Pirko, an industry consultant at Bevmark LLC.

A growing number of industry analysts suggest Coca-Cola should spend less to advertise cola and more to diversify aggressively through acquisitions of companies, like energy-drink maker Monster Beverage Corp. Sales of Coke's nonsodas, including Minute Maid juice, Dasani water and Powerade sports drinks, rose 5% last year by volume.

But that isn't what Coke says it has in store. Instead, the Atlanta-based company plans to double down on its namesake brand. The company is boosting advertising, introducing new products, and using singer Taylor Swift as a pitchwoman. Chief Executive Muhtar Kent has said that last year, when Coke's U.S. soda volume dropped 2%, was an anomaly. Soda can return to healthy growth, even in the U.S., especially if it is a brand name like Coke, he said.