The Food and Drug Administration issued a warning letter to Coca-Cola Co. earlier this month explaining to the company that its label for Diet Coke Plus violates the FDA’s guidelines for using the word “plus” and for claiming that it provides “vitamins and minerals.”

Coke launched Diet Coke Plus in March 2007 as a calorie-free soft drink and “a good source of vitamins B3, B6, and B12, and the minerals zinc and magnesium.”

According to the FDA, companies may use the word “plus” only if their foods contain at least 10 percent more of the reference daily intake reference value of a nutrient than a similar product. The FDA also does not consider appropriate to fortify snack foods such as carbonated beverages.

The warning letter was sent to Coke Dec. 10. It comes as a growing number of nutrient-enhanced beverages, from energy drinks to enhanced waters, have appeared on store shelves, putting pressure on the FDA to police more aggressively the claims many of the products make about the benefits they can deliver.

The FDA asked Coca-Cola to “take prompt action to correct these violations” and asked the company to respond within 15 days of receipt of the letter.

Scott Williamson, a spokesman for Coca-Cola, said the company will respond to the FDA in early January, but he also added that the company has no plans to change the label.

“We take seriously the issues raised by the FDA in its letter. This does not involve any health or safety issues, and we believe the label on Diet Coke Plus complies with FDA's policies and regulations,” he said.