A food watchdog group has sued General Mills Inc. for allegedly misleading customers with its Cheerios Protein cereal, though the packaged giant says it’s fully in compliance with federal labeling rules.

Cheerios Protein does not contain much more protein than regular Cheerios even though it’s marketed that way, according to the suit filed by the Center for Science in the Public Interest (CSPI) in U.S. District Court in Northern California.

“Rather than protein, the principal ingredient that distinguishes Cheerios Protein from Cheerios is sugar,” CSPI said in the suit.

Golden Valley-based General Mills countered in a press statement that Cheerios Protein contains 18 percent more protein by weight than original Cheerios.

“We don’t normally respond to these publicity-seeking lawsuits from CSPI — but we do reject their comparisons,” General Mills said. “Cheerios Protein is accurately labeled — and provides a good source of protein in every labeled serving.”

Cheerios Protein meets the federal Food and Drug Administration standard “as a good source of high quality protein,” the company said. Conventional Cheerios does not meet the FDA requirement as a good source of protein.

Cheerios Protein was launched in 2014 to capitalize on the protein trend sweeping the food industry. The cereal has 7 grams of protein per serving while regular Cheerios has 3 grams of protein per serving, according to the “Nutrition Facts” panel printed on each box.

CSPI attributes most of the difference to a larger serving size listed on the box for Cheerios Protein: 55 grams vs. 28 grams for regular Cheerios.

General Mills is saying that, even accounting for the serving size difference, Cheerios Protein still has 18 percent more protein than Cheerios.

The difference in serving sizes between Cheerios Protein and regular Cheerios is set by FDA regulations, according to General Mills. The FDA spells out serving sizes by weight, which will differ based on the density of a cereal. Cheerios Protein is a denser cereal, so to get a full serving, it will weigh 55 grams, the company said. Original Cheerios is less dense, so a full serving is 28 grams.

As for sugar, regular Cheerios has 1 gram per serving, according to the Nutrition Facts box. Cheerios Protein has 16 to 17 grams, depending on the variety, the most of any iteration of Cheerios.