Fans of Vitaminwater are demanding that parent company Coca-Cola drop a new formula that uses stevia, a low-calorie sweetener known for its metallic aftertaste.

Coca-Cola Co. changed the formula for its full-calorie Vitaminwater in May, and the new bottles have been hitting shelves nationwide ever since.

Previously, the drinks were sweetened with a mix of crystalline fructose and sugar. Now they are sweetened with a mix of sugar and stevia, a natural sweetener companies use to reduce the sugar content in drinks.

The change has prompted fans of the drink to inundate Vitaminwater’s Facebook page with complaints about the taste, and demands that the company bring back the old formula.

A spokeswoman for Coca-Cola, Danielle Dubois, couldn’t immediately say why the company decided to make the change or if it would consider reverting back to the previous formula. The drinks still have the same caloric content, at 120 calories per bottle

Last year, Vitaminwater’s sales volume in the U.S. was down 18 percent, according to the industry tracker Beverage Digest. The decline seems to be in part because the enthusiasm for enhanced water from a year ago has cooled, said John Sicher, editor and publisher of Beverage Digest.

Sicher noted that Coca-Cola may have been trying to innovate with Vitaminwater to return it to stronger performance.

(AP)