The results were revealed on the television program The Doctors, where it was found that even just a month of eating fast food can cause significant changes to the liver.
French fries in particular were dangerous because of the extra ingredients added.
"We know that they are adding salt, and cooking it in fat, but they're also putting sugar on them too. Why sugar? Because it helps get them golden crispy," said Dr. Drew Ordon, who appears on the show and is the author of the book Better in 7.
Foods like fried chicken and onion rings were especially bad for the liver.
"The amount of fat and saturated fats creates a condition called fatty liver," Ordon said.
He said the changes in liver enzymes are in line with the effects of hepatitis. That disease can ultimately cause liver failure.
The U.S. has 160,000 fast food restaurants serving an estimated 50 million customers every day.
"We're all guilty, and every now and then you have to splurge, but the problem is that so many people are getting into eating fast food, especially kids, as their staple, and I think that's the point," Ordon said.
Just ordering a salad won't help as Ordon warned that any item marked healthy or fresh at a fast food restaurant likely has added chemicals, as there are not clear regulations for those foods.
"Some places actually put propylene glycol on the salads, which is anti-freeze, the reason behind that is that it prevents wilting," said Ordon. "And although they say a little anti-freeze isn't going to hurt you, obviously given a choice you don't want to be eating anti-freeze."