The print ads which began appearing today in major US newspapers show a pair of laughing women, one holding a bottle of diet Coke in hand, alongside text that reads: "Quality products you can always feel good about."
The copy beneath the picture says, "The safety of aspartame is supported by more than 200 studies over the last 40 years."
The soda industry is facing declining US sales as some blame sugary drinks for fueling the obesity epidemic in America.
Scientists are not in agreement on whether aspartame, approved in the 1980s, is safe over the long-term, said Karen Congro, a nutritionist and director of the Wellness for Life Program at the Brooklyn Hospital Center.
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"Despite claims from Coke and other companies about the safety of aspartame, we still don't know about its long-term effects," she said in an email to AFP.
"There is a growing backlash against - not only diet soda - products with artificial sweeteners and other artificial ingredients."
"Relying on artificial sweeteners probably causes cravings for sweets and sugar, which can contribute to obesity and poor eating habits," she said.
"It's possible that the brain reacts to artificial sweeteners in diet soda and other products by sending out insulin, which can make people hungry - especially for sugar.
The US Food and Drug Administration says aspartame "may be safely used in food in accordance with good manufacturing practice as a sweetening agent and a flavor enhancer in foods.