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Fashion company J.W. Anderson will produce limited edition bottles for Diet Coke later this month.
Coca-Cola said global Diet Coke volume sales dropped 7% in its second quarter.
Sales of Diet Coke are dropping so precipitously that Pepsi-Cola surpassed Diet Coke to become the number two best-selling soda brand in the US earlier this year.
Sandy Douglas, Coca-Cola North America president, said on the company's earnings call on Wednesday that the "diet and frozen" parts of the food and beverage industry have been struggling for "a number of quarters, getting into years now" as the US consumer strongly moves into the fresh segment.
Douglas added: "It's a good dietary change for the country, but the impact on the category, and particularly the categories appealing to the diet-oriented positioning is pretty negative."
However, Douglas said Coca-Cola has recently seen its Diet Coke consumer base stabilize - "there's an incredible number of loyal drinkers that love Diet Coke."
That said, he noted that its largest competitor (Diet Pepsi) is changing its formula in August, removing the man-made aspartame sweetener that has been plagued with rumors it causes cancer. Aspartame in Diet Pepsi will be replaced with a blend of sucralose and acesulfame potassium.
Douglas thinks this will create "a lot of buzz in the category" and he hopes it will reinforce some of the messages it has been trying to push around sweeteners and the choice to opt for lower or no-calorie drinks too.
Coca-Cola CEO and chairman Muhtar Kent admitted there is a "challenge" with Diet Coke, although he said it was broadly a US-centric issue.
But this chart, using data from Euromonitor (which we found on Quartz) shows the decline of Diet Coke is a global problem. It may just be that North American sales are dragging down Diet Coke worldwide, but nevertheless, the numbers paint a depressing picture for the company.
Euromonitor/Quartz/Business Insider
Coke did say on the call that sales of Coke Zero are on the rise, but this is from a much smaller base. Diet Coke is the third best-selling soft drink in the US with an 8.5% share of the sector, according to Beverage Digest, while Coke Zero is in tenth place with a 1.8% share.
Earlier this month Diet Coke announced a collaboration with J.W. Anderson, which will see the London-based fashion brand creating limited edition bottles of the soda.