Diet Dr Pepper might be healthier than regular Dr Pepper, but does its name imply it's good for the waistline?

The U.S. Court of Appeals for the Ninth Circuit grappled with that question in a California class action alleging that Diet Dr Pepper deceives consumers into believing they can lose weight, or at least not gain weight, when drinking the soda. It's the latest in a series of cases, all brought by San Diego plaintiffs attorney Jack Fitzgerald, aimed at shaking up the health claims of diet sodas such as Diet Coke and Diet Pepsi.

"Common sense and common experience tells us that people drink diet soda to avoid the negative health consequences that come along with regular soda, including weight gain, because of its calories," Fitzgerald told the panel on Wednesday in San Francisco.

"Is that true? I mean, I'm not sure that it's common sense," Ninth Circuit Judge Ryan Nelson interjected. "I know a lot people that would argue just the opposite—that actually, regular soda is more healthy, and that seems to be the common assumption."

But the panel also had questions for Evan Young, a partner at Baker Botts in Austin, Texas, representing Dr Pepper/Seven Up Inc., who argued that the term "diet" in reference to soda was understood to mean fewer calories.

"Doesn't that beg the question about, why don't you just say Dr Pepper Zero?" Nelson asked him.

"It's historical," Young replied. "This is a phrase that was used long ago."

The Ninth Circuit, which also is set to decide a related case over Diet Coke, submitted on the briefs, is the second appeals court to weigh in on diet soda's labeling. On June 27, the U.S. Court of Appeal for the Second Circuit found in a published opinion that "diet," in the context of Diet Coke, did not mean weight loss but, instead, fewer calories. In previous summary orders, the Second Circuit upheld dismissal of two other New York cases this year involving Diet Dr Pepper and Diet Pepsi.

In California, judges have dismissed two of the three cases that Fitzgerald has brought. William Orrick, of the U.S. District Court for the Northern District of California, tossed the Diet Dr Pepper case last year and found that reasonable consumers would not believe the plaintiff's claims and, even if they did, there was no scientific basis to allege that Diet Dr Pepper's artificial sweetener, aspartame, caused weight gain.

Fitzgerald continued to insist that scientific studies backed his claims.

But the panel mostly focused on what a reasonable consumer would believe about diet soda, at times throwing in their own experience. Judge Jay Bybee said his wife drinks Diet Coke because of the taste.

Nelson posited, would the same consumer claims apply to Dr Pepper Ten, which advertises that it contains 10 calories per serving?

"Diet, we submit, goes above and beyond merely indicating zero calories," Fitzgerald replied.

Diet soda, he wrote in court filings, is so ubiquitous that it shows up in the dictionary under the word "diet." Dr Pepper, in a response brief, noted that Fitzgerald's dictionary definitions referenced the word "diet" as a noun or verb, not as an adjective, as used in its product's name.

Bybee called Dr Pepper's grammar argument initially "well taken" but, on second thought, considered the name Diet Dr Pepper to be a "single entity."

Whether it's one word or not, Young argued, the focus of the case is on the word "diet." In court filings, Dr Pepper noted that Congress approved the continued use of "diet" on soda labels when it passed the Nutrition Labeling and Education Act in 1990.

And, Young said on Wednesday, the Second Circuit had "covered the waterfront" on these cases, most notably that diet soda, compared to the regular version of the same drink, has fewer calories, but it is not healthier than water.

The plaintiff's case cited numerous Diet Dr Pepper advertisements over the decades, prompting Nelson to repeatedly ask Young whether the plaintiff's claims could survive if Diet Dr Pepper implied in advertisements that it would help consumers lose weight.

Young turned again to the Second Circuit.

In all three cases, he said, the Second Circuit found there "was nothing in those ads that made any promise."