WASHINGTON- Federal Trade Commission officials have rejected a consumer group’s call for an investigation into whether Coca-Cola, Pepsico and other manufacturers have falsely advertised that artificially sweetened soft drinks and food products help people lose weight.

The California-based group, U.S. Right to Know, filed citizen petitions in April asking the agency and the U.S. Food and Drug Administration to stop companies from branding products with the word “diet” if they contain an artificial sugar substitute, including those containing aspartame, the most widely used sweetener.

In a Sept. 18 letter to Gary Ruskin, executive director of U.S. Right to Know, a senior Federal Trade Commission official said the petition was “carefully considered,” including scientific studies and literature reviews it cited as demonstrating low-calorie drinks actually can contribute to weight gain.

“Upon review, we have determined not to take additional action at this time,” wrote Mary Engle, the agency’s associate director for advertising practices. “We considered a number of factors related to resource allocation and enforcement priorities, as well as the nature of any FTC Act violation and the type and severity of any consumer injury.”

In a separate response, a Food and Drug Administration official advised the consumer group on Sept. 22 that it would need more time to consider its complaint.

The trade commission’s decision was a triumph for the soft drink industry, which has been beset with flagging sales, even for its diet drinks.

Aspartame, sold mainly under the brand name NutraSweet, has been used for decades in diet soft drinks such as Diet Coke and, until last spring, Diet Pepsi. Two weeks after the Right to Know group filed its petitions, Pepsico announced it was abandoning use of NutraSweet in Diet Pepsi, Caffeine Free Diet Pepsi and Wild Cherry Diet Pepsi at the requests of consumers and would instead sweeten them with Sucralose, sold as Splenda, and acesulfame potassium, also known as Ace K.

In its petition, the Right to Know group had cited scientific studies concluding that artificial sweeteners not only don’t help people lose weight, but also that they could contribute to weight gain. The American Beverage Association and other industry groups strongly disputed that assertion and present dueling studies.

Ruskin called it “regrettable that the FTC won’t act to halt the deceptions of the ‘diet’ soda industry.”