Food-focused watchdog group Center for Science in the Public Interest has its hooks in Long John Silver's, accusing the seafood chain of serving the least healthful meal in the country.
The advocacy group said it ran laboratory tests on the chain's Big Catch meal, including batter-fried haddock and side dishes such as hush puppies and onion rings.
The results showed 33 grams of trans fat, according to the group -- or more than two weeks' worth of the two-grams-a-day allotment recommended by the American Heart Assn.
The heart organization said that trans fats, also known as partially hydrogenated oils, are created in an industrial process that adds hydrogen to liquid vegetable oils to make them more solid.
High levels of trans fats increase consumers' risk of developing heart disease and stroke and are also linked to type 2 diabetes, according to the AMA. Trans fats occur naturally in small amounts in some meat and dairy products.
Long John Silver's, based in Louisville, Ky., did not immediately respond to requests for comment. Some other fast food chains, such as KFC and Taco Bell, have not served trans fat for years.
[Updated, July 2, 1:50 p.m.: In a statement Tuesday afternoon, Long John Silver's said the Big Catch is a limited-time special that can be paired with several different sides, including corn, green beans, rice, cole slaw and fries. The whitefish used is three times the weight of the company's usual uncooked whitefish, it said.
"Long John Silver's offers a variety of meal choices including baked fish and shrimp that can satisfy almost every diner's dietary choices," the company said. "We stand behind our published food data and will review any requests from CSPI that raise questions about our data."]
The Big Catch meal also includes 19 grams of saturated fat, 1,320 calories and nearly 3,700 milligrams of sodium, according to the CSPI.
The advocacy group, which is often dubbed the "Food Police" by the restaurant industry, also accused Long John Silver's of overstating the amount of fish and under-reporting the amount of trans fat in the meal.
The fast food chain says the main part of the meal features 7 or 8 ounces of haddock; the CSPI said its tests showed 4½ ounces of fish and 3 ounces of batter.
The watchdog organization also said that the onion rings it tested had 19.5 grams of trans fat, compared with the 7 grams noted by Long John Silver's.
In California, however, the chain's locations use canola oil, according to the CSPI. California is the first state to ban trans fats in restaurants.
The CSPI said it threatened Long John Silver's with a lawsuit if the chain continues to use partially hydrogenated oil and doesn't change its meal disclosures to accurately reflect nutritional content.