It's clear that consumers are ditching Diet Coke. The question now is whether investors should ditch the stock.
According to the trade publication Beverage Digest, Diet Coke sales fell 6.6 percent in 2014, ceding second place in the soda rankings to Pepsi (which saw volume fall 1.4 percent). First place remained Coke, which saw volume dip 1.1 percent. Soda sales overall declined for the 10th straight year.
As Coca-Cola faces what appear to be significant secular headwinds (not to mention adverse currency impacts from the soaring dollar) the stock is flat over the past two years, badly underperforming equities as a whole.
"Coca-Cola investors should be worried," said Dave Seaburg, head of equity sales trading at Cowen & Co. "I don't see [soda sales] coming back, and the indicators are saying it's not necessarily going to come back. ... I think the stock should be on the sidelines."
Analyst say the specific problem with Diet Coke is widespread public concern over the artificial sweetener aspartame as well as a general trend toward "naturalness" in food.
Indeed, if Coca-Cola can capitalize off of that trend, it may not be in such bad shape, contends JPMorgan analyst John Faucher.
"Diets are going to continue to decline, but they should be able to offset most of that decline as long as they have truly healthy beverages—that's what these consumers want," Faucher said Friday on CNBC's "Power Lunch."