Why is Diet Pepsi saying goodbye to aspartame?

PepsiCo says it's dropping aspartame from Diet Pepsi in response to customer feedback and replacing it with sucralose, another artificial sweetener commonly known as Splenda.

The decision to swap sweeteners comes as Americans keep turning away from popular diet sodas. Rival Coca-Cola said this week that sales volume for Diet Coke, which also uses aspartame, fell 5 percent in North America in the first three months of the year.

Atlanta-based Coca-Cola said in a statement that it has no plans to change the sweetener in Diet Coke, which is the country's top-selling diet cola.

The Food and Drug Administration says aspartame, known by the brand names Equal and NutraSweet, is "one of the most exhaustively studied substances in the human food supply, with more than 100 studies supporting its safety."

More recently, a government advisory committee for the U.S. Department of Agriculture's dietary guidelines said aspartame appears to be safe in the amounts consumed by Americans. But it added that there is still uncertainty about whether the sweetener increases risk for some blood cancers in men.

Executives at Coke and Pepsi blame the declines on perceptions that the sweetener isn't safe.

John Sicher, publisher of industry tracker Beverage Digest, noted that attitudes about aspartame can be very negative. Using an online tool called Topsy that measures Twitter sentiment on a scale of 0 to 100, he noted "aspartame" got a 22 ranking, below a 38 ranking for "Congress."

By comparison, "love" had a ranking of 96 and "Christmas" had a ranking of 88.

The negative attitudes about aspartame don't seem to extend to sucralose. Sparkling Ice, a zero-calorie drink created in the 1990s, is sweetened with sucralose and has been enjoying strong growth in recent years.

"Aspartame is the No. 1 reason consumers are dropping diet soda," said Seth Kaufman, vice president of Pepsi.

In tests, Kaufman said people still recognized the reformulated drink to be Diet Pepsi, but that it might have a "slightly different mouthfeel."

Diet Pepsi will also still have acesulfame potassium, or ace-K, which PepsiCo said it added to the drink in late 2012 to help prevent its taste from degrading over time.

PepsiCo says reformulated Diet Pepsi will start hitting shelves in August, with cans stating that the drink is "Now Aspartame Free." The change only applies to the U.S. market and will affect all varieties of Diet Pepsi, such as Caffeine Free Diet Pepsi and Wild Cherry Diet Pepsi. It will not apply to other PepsiCo drinks, such as DietMountain Dew.

Volumes for Diet Pepsi and Diet Coke began falling in 2005 and 2006, respectively, according Beverage Digest. Volumes have continued falling since then, and accelerated in the last two years.

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AP Writer Mary Clare Jalonick contributed from Washington, D.C.