The firm was paid $3.5 million for its work, industry executives now acknowledge.

“As you know, our sponsorship of this campaign remains confidential,” said an email sent in 2009 by a staff member at the trade association to executives at Cargill, in advance of the release of advertisements by Berman & Company defending the industry. “We are funding Berman & Company directly, not the Center for Consumer Freedom, which is running the ads. If asked, please feel free to state the following: ‘The Corn Refiners Association is not funding the Center for Consumer Freedom.’ ”

John W. Bode, president of the Corn Refiners Association, said the organization had no choice but to defend its product, and he blamed the sugar industry for funding research that he said falsely suggested his products presented a health risk. But he conceded the organization could have been more forthright about how the various efforts were being funded.

“In retrospect,” he said, “we could have been more clear and timely in the disclosure.”

The lawsuit, filed in federal court in California in April 2011, centers on a claim by the sugar industry that television and print advertisements funded by the corn refiners were false and misleading in asserting that high-fructose corn syrup was a “natural” product and nutritionally the same as sugar.

But court documents suggest that the Sugar Association, a Washington-based trade group controlled by some of the nation’s biggest sugar companies, started the fight several years earlier.

Back in 2003 it formally began a secret effort it called “food and beverage industry replacement of H.F.C.S. with sucrose,” meaning replacing corn-based sweeteners with sugar, the court documents show, reproducing a Sugar Association memo. The association, in a letter to its members marked “highly confidential,” celebrated in 2004 how it had “fed the media with the science to help fuel the public concern and debate on H.F.C.S.,” as well as meeting “face-to-face with Coke and Pepsi representatives promoting all-natural sugar.”

But that was only the start of its efforts.

Documents released as part of the lawsuit show that starting in 2011, it sent a total of $500,000 to a Washington nonprofit group, Citizens for Health, which calls itself the “consumer voice of the natural health community” on its website. The money was intended to help the small group, run by a lawyer named James S. Turner, to oppose the corn refiners’ effort to get permission from the Food and Drug Administration to rename their product as sugar — a petition the F.D.A. ultimately rejected.

Citizens for Health says on its website that it has “over 100,000 supporters.” But the bulk of its support comes from industry sources rather than consumers: its 2011 federal tax returns show that the $200,000 contribution from the sugar industry that year was more than half its annual budget.