Coca-Cola Co. (KO) faces a purported class-action lawsuit initiated by a consumer-advocacy group and other litigants, who say that the beverage giant made deceptive claims about its line of VitaminWater beverages.

The consumer group, called the Center for Science in the Public Interest, said its litigation department is serving as co-counsel in the suit, which was filed Wednesday in California.

“The idea that you can improve your immunity by drinking one of these VitaminWater drinks is nonsense,” David Schardt, a senior nutritionist for the nonprofit group, said during a conference call with reporters.

The group said Coke markets VitaminWater as a healthful alternative to soda and said the company makes a wide range of assertions, including claims that the drinks variously reduce the risk of chronic disease and support immune function. The nonprofit’s nutritionists say the levels of sugar in the drinks promote obesity, diabetes, and other health problems. The other law firms involved in the case are Reese Richman LLP and Whatley Drake & Kallas LLC.

A Coke spokeswoman, Diana Garza Ciarlante, in an email called the suit ” ridiculous.”

“Glaceau VitaminWater is clearly and properly labeled and shows the amount of vitamins and calories in the product,” she said. “This is not about protecting the public interest. This is about increasing the readership of CSPI’s increasingly irrelevant newsletter.”

Coke described VitaminWater as a “great complement to our often less-than- perfect diet” and a good way to “get more of the vitamins and hydration we all need each day.”

Coke bought company that makes VitaminWater for $4.1 billion in 2007, making it the costliest takeover in the company’s history. Since its acquisition, the Atlanta giant has pushed to increase distribution within the U.S. and abroad, and the drink has become a formidable competitor to many other drinks such as rival PepsiCo Inc.’s (PEP) Gatorade.

This also isn’t the first time that the soft-drink industry is facing a pushback on its claims.

Most recently, the U.S. Food and Drug Administration called Coca-Cola’s Diet Coke Plus product “misbranded” and said the product makes a nutrient claim but doesn’t meet the criteria to do so.

Shares of Coca-Cola were trading recently at $42.55, off 7 cents.

UPDATE: Consumer Group Files Suit Vs Coke On VitaminWater