The F.T.C., meanwhile, said the company employed misleading marketing, contending that Pom had exaggerated or overstated research results in advertising, on the Web and in press communications. The prostate cancer study cited in the ad, for example, concluded that further research was needed to determine whether the improved blood scores would result in a concrete medical benefit, according to the agency’s complaint.

Pom denies that it is misleading anyone. The First Amendment protects commercial speech that is truthful and not misleading, Mrs. Resnick says. Companies have a constitutional right to publicly discuss their research results. A second study, she says, also suggested that the juice holds promise for people with prostate cancer.

“We believe the manner in which we have communicated the results of our science research is truthful and appropriate,” Mrs. Resnick says.

Besides, she says, Pom should not be considered a functional food. “It’s a natural food,” she said.

Pom Wonderful has filed a federal law suit contending that the F.T.C. is overstepping its authority and setting new standards for advertising of foods and dietary supplements.

The agency has filed its own claims, which an administrative law judge in Washington is scheduled to hear on May 24.

SO what’s a shopper to do?

“This is very confusing to consumers. It’s confusing to a lot of health professionals,” says Wahida Karmally, the director of nutrition at the Irving Institute for Clinical and Translational Research at Columbia University Medical Center. “Just because they call it functional, it doesn’t mean it’s going to be good for you.”

Regulators agree.

Jennifer Thomas, director of the division of enforcement at the F.D.A.’s Center for Food Safety and Applied Nutrition, recommends that consumers take a close look at nutrition panels and ingredient lists, and not just read marketing claims. People may also want to peruse the F.D.A.’s Web site.