"We've actually never made this designation before," Jeff Cronin, CSPI's director of communications, told The Salt. "No exaggeration. No one here has ever seen this much trans fat in a meal."

And if that weren't enough, the Big Catch also contains 19 grams of saturated fat, 1,320 calories and almost 3,700 milligrams of sodium, CSPI found.

At almost all of its locations nationwide, Long John Silver's fish is fried in partially hydrogenated oil. These oils have fallen out of favor with the food industry because they contain trans fats, which have been implicated in heart disease. In 2006, KFC switched to trans-fat-free oil after CSPI sued the group over its use of partially hydrogenated oils.

To find what is swimming around in the Big Catch meal, CSPI visited several Long John Silver's locations around Washington, D.C., and commissioned tests from the laboratory Eurofins. The results were startling enough for CSPI to to dub the meal "heart attack on a hook."

CSPI also found something off about the weight of the fish, too. When the researchers brought their Big Catch meals back to CSPI headquarters, they found that, once its breading was removed, a naked filet of fish weighed in at about 4 to 5 ounces.

In other words, customers aren't "getting 7 to 8 ounces of pure fish. They're getting fish plus several ounces of greasy batter," says CSPI Executive Director Michael Jacobson. "In our mind, this is the worst. It's the Mount Everest of trans fat."

In a statement, Long John Silver's told The Salt that "we stand behind our published food data and will review any requests from CSPI that raise questions about our data." The Big Catch, the company added, is a limited-time special "that delivers tremendous value to value hungry consumers."

Despite the "worst meal" designation, Long John Silver's Big Catch has plenty of fans, judging from the company's Facebook page, which features dozens of photos from customers posing with empty white to-go boxes. One post boasts of a customer who finished the Big Catch platter "in under 7 min!"

Interestingly, in another post on its Facebook page from May, Long John Silver's notes that "many doctors recommend eating seafood twice a week. We recommend getting it with Hushpuppies and fries!"

Copyright 2013 NPR.

Long John Silver's Big Catch platterLong John Silver's Big Catch platter

'Heart Attack On A Hook': Meet America's 'Worst Restaurant Meal'