The hidden ingredients in your favorite chain pizzas

About 1 in 8 Americans consumes pizza on any given day.

Vani Hari, the author and blogger who goes by Food Babe, conducted an investigation to see what ingredients were in pizza from popular chain restaurants like Domino's, Pizza Hut, and Papa John's.

Hari looked for ingredient lists online. When she couldn't find them, she followed up with the companies.

While most of the chains complied, Little Caesars, California Pizza Kitchen (CPK), and Mellow Mushroom all refused to give Hari more details, saying that revealing ingredients could give competitors an advantage.

Here's the chart showing additives in chain pizza.

Food Babe

Hari writes her most disturbing finding was what she calls "hidden MSG."

Monosodium glutamate, or MSG, is a controversial flavor enhancer. Critics of the ingredient say it is linked to headaches, obesity, fatigue, and Alzheimer's disease. However, there's no conclusive scientific evidence that it's dangerous.

"Instead of letting you know that they are putting MSG in your food, they are using an FDA loophole to sneak processed free glutamic acid into your food, which has the same effect as MSG - all without warning you," Hari writes.

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Another writer who tried to identify the "better ingredients" in Papa John's pizza says she was shut down by the company.

Melanie Warner from US News And World Report was curious about the ingredients Papa John's has advertised for so long.

While packaged product companies have to disclose ingredients, restaurants don't, Warner writes.

Still, chains like Taco Bell, McDonald's, and Subway voluntarily provide the information.

Warner went to her local Boulder Papa John's and asked an employee for more information about the ingredients. He directed her to the company's website, but ingredients aren't listed online either.

Finally, Warner left messages with various company representatives but says no one got back to her.

She did gain some insight from a worker named Charlie, however, who told her that the company's "never frozen" mantra is bogus:

"We get deliveries in every three days, so nothing that's in the fridge is more than a few days old. And we form the dough here. It doesn't come ready to go, though it is made in a central facility and then frozen," he told Warner.