A consumer-advocacy group is calling today for federal regulatory agencies to investigate the use of the word diet by diet-soda manufacturers, calling the adjective “deceptive, false and misleading” and citing research that finds diet soda may actually lead to weight gain instead of weight loss.

“This looks like a classic case of false advertising,” says Gary Ruskin, co-founder and executive director of U.S. Right to Know, a nonprofit organization based in Oakland, Calif., which wrote two letters — one to the Federal Trade Commission (FTC) and another to the Food and Drug Administration (FDA) — demanding a sweeping investigation into the use of the word diet in advertising by companies that use artificial sweeteners. U.S. Right to Know claims that the use of the word violates federal law against false advertising, branding and labeling of food products. “We’re doing this to make sure that people don’t get sicker from these products and gain weight when they want to be losing weight,” Ruskin says.

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Some research suggests that diet soda may contribute to weight gain instead of weight loss, possibly by decoupling the link between sweet taste and caloric consequences, thus leading to overeating.

“Previous research, including human clinical trials, supports that diet beverages are an effective tool as part of an overall weight management plan,” said the American Beverage Association, the trade association representing the beverage industry, in a statement provided to TIME. “Numerous studies have repeatedly demonstrated the benefits of diet beverages — as well as low-calorie sweeteners, which are in thousands of foods and beverages — in helping to reduce calorie intake. Furthermore, low- and no-calorie sweeteners have repeatedly been deemed safe by decades of scientific research as well as regulatory agencies around the globe — including the U.S. Food and Drug Administration.”

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Write to Mandy Oaklander at mandy.oaklander@time.com.