NEW YORK (CBS NEWS) – Drinking diet soda is a way to help you lose weight, right? Not according to several federal lawsuits. Separate cases against Coca-Cola, PepsiCo and Dr Pepper Snapple Group — the three largest US makers of carbonated beverages — allege that their marketing of diet sodas deceives consumers into thinking that the beverages will help them lose or manage weight because scientific studies have shown the opposite is true.

The lawsuits filed October 16 in federal court by the same plaintiffs’ attorneys use nearly identical language. They center on the use of aspartame, a sweetener first used in 1965 that has been controversial for decades. The suits allege that the companies’ use of the word “diet” in the beverages’ “false misleading and unlawful” marketing could make a “reasonable consumer” think the drinks are a diet aid.

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“What’s been going on is clearly deceptive advertising,” said Abraham Melamed, one of the attorneys behind the case. “In our opinion, it’s one of the biggest consumer scams in the last 50 years, and it has to stop. There’s a strong sense of urgency because there are hundreds of thousands, maybe millions, of consumers out there that are being deceived on a daily basis.”

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Atlanta-based Coca-Cola denounced the lawsuit against it as “completely meritless” and vowed to “vigorously defend against it,” according to Kate Hartman, a company spokeswoman.

“Diet Coke is a great-tasting, zero calorie beverage that is and always has been properly labeled and marketed in compliance with all applicable regulations,” she wrote in an email. PepsiCo, based in Purchase, New York, and Plano-based Dr Pepper Snapple declined to comment.

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