A batter-laden fried fish dish packs two weeks worth of harmful trans fat in a single serving and was named worst restaurant meal in America Tuesday by a US consumer advocacy group.

The Big Catch meal, sold at the fast food chain Long John Silver's, contains 33 grams of trans fat and 3,700 milligrams of sodium, said the Center for Science in the Public Interest.

People should limit themselves to two grams of trans fat daily, according to the American Heart Association, and most people should eat 1,500 milligrams of sodium per day, the Institute of Medicine says.

"Long John Silver's Big Catch meal deserves to be buried 20,000 leagues under the sea," CSPI executive director Michael Jacobson said in a statement announcing the group's pick of worst restaurant meal in America.

"This company is taking perfectly healthy fish — and entombing it in a thick crust of batter and partially hydrogenated oil. The result? A heart attack on a hook."

The fish is battered and fried in partially hydrogenated soybean oil, and sold with onion rings and hush puppies, which are fried balls made of leftover batter drippings, cornmeal and onion.

Its total calorie count is low for a fast food meal — just 1,320, CSPI said.

But its artery-clogging trans fat is twice the level of the worst KFC dish, which had 15 grams of trans fat before a 2006 CSPI lawsuit led the chicken chain to stop using partially hydrogenated oil.

"Trans fat from partially hydrogenated oil is a uniquely damaging substance that raises your bad cholesterol, lowers your good cholesterol, and harms the cells that line your blood vessels," said Walter Willett, nutrition department chair at the Harvard School of Public Health.

"It might have been defensible to use hydrogenated oil in the 1980s, before trans fat's harmfulness was discovered, but no longer."

Long John Silver's introduced the "Big Catch" in May, describing it as "the largest fish we have ever offered, weighing in at 7-8 ounces of 100 percent premium Haddock caught in the icy waters of the North Atlantic."

But that claim did not stand up to the scrutiny of CSPI inspectors, who picked apart the breading from the fish and said they found "an average of about four and a half ounces of actual fish and almost three ounces of oil-soaked batter."

Long John Silver's, which calls itself the "largest quick service seafood restaurant in the world," responded to an AFP request for comment by describing the dish as "a limited time only special that delivers tremendous value to value-hungry consumers."

"The Big Catch can be paired with a variety of side items including corn, green beans, rice, cole slaw, fries, onion rings and hush puppies," it said.

"We stand behind our published food data and will review any requests from CSPI that raise questions about our data."

CSPI said it plans to sue the chain if it continues to use partially hydrogenated oil in its deep-fryers and if it continues to misrepresent the amount of fish in the meal and the nutrition information for the side items.

The group's researchers found that the meal's onion rings were advertised to contain seven grams of trans fat but actually contained 19.5 grams.

The threat to sue was described as "outrageous" by Baylen Linnekin, executive director of a Washington nonprofit called Keep Food Legal.

"I am very much in favor of groups like CSPI," said Linnekin, a lawyer whose membership group says it is devoted to "food freedom" and opposes bans.

"Where I tend to depart from their actions is when they threaten to sue," he told AFP.

"It is one thing to put out information and to rail against too many calories or trans fats or what-not. It is an entirely different thing to sue a company."