In January, the Center for Science in the Public Interest, a consumer advocacy group in Washington, sent a letter to the F.D.A. identifying marketing claims on 19 widely sold sugary beverages from five brands that it said were misleading. The group highlighted several flavors of Honest iced tea, which carry the “just a tad sweet” claim, as well as brands like Steaz, which markets several flavors of organic iced tea as “lightly sweetened” despite containing 20 grams of sugar per serving, or roughly five teaspoons.

“If you look at the amounts of sugar in these things that say they are lightly sweetened — I mean, really?” said Marsha Cohen, a law professor and expert on food law at the University of California, Hastings College of the Law. “What’s heavily sweetened to them? It’s a little crazy.”

Sarah Sorscher, the deputy director of regulatory affairs at C.S.P.I., said that such claims give consumers the wrong impression about what it means to have a healthy level of sweetness in their beverages.

“People are drinking these thinking they’re less sweet options,” she said. “But they’re still enormously high in added sugars, and so they’re miseducating consumers about what it means to have a healthy diet.”

At the core of the issue are so-called nutrient content claims. In the early 1990s, the F.D.A. ruled that companies could advertise their products as low in fat, cholesterol, calories and sodium if the amount of those nutrients in their products met certain thresholds. But at the time, health authorities were less concerned about sugar, and the F.D.A. chose not to set a threshold for low-sugar claims because there was no scientific consensus on a healthy level of daily sugar intake. The agency states in its food-labeling guide for industry that the low-sugar claim has not been defined and “may not be used” in marketing.

C.S.P.I. argues that companies flout the rule by using synonyms for low sugar that are known under F.D.A. regulations as “implied” nutrient content claims, such as slightly sweet and lightly sweetened. It said that the agency should take “immediate enforcement action” against companies and define a low-sugar product as one that contains less than three grams of added sugar per serving. That is similar to requirements for other low-nutrient claims, and it is equivalent to roughly 5 percent of the F.D.A’.s daily value, or recommended limit, for added sugar intake.

In a statement, the F.D.A. said that it was reviewing the letter from C.S.P.I. and planned to respond. Steaz did not respond to a request for comment. Honest Tea said that part of the company’s founding mission was to meet consumer demands for beverages that tasted “less sweet than others in the market.”