Chick-fil-A's popular chicken sandwich contains a controversial ingredient

What makes Chick-fil-A's classic chicken sandwiches so delicious?

Some may swear by the perfectly crispy coating. Others may point to the tangy pickles. And plenty of people have alluded to the fact that the chain uses peanut oil to fry up its filets.

The truth is that all of these sandwich ingredients (and yes, there are a lot) likely play into why it's been so popular for years, but there's one ingredient in particular Chick-fil-A uses in its sandwiches and nuggets that is not found in comparable items at McDonald's, Wendy's and Burger King.

And that ingredient is monosodium glutamate, more commonly known as MSG.

While many people think MSG is only found in Chinese food, it's prevalent in dozens of everyday food items, including snacks like Doritos and mixes like Hidden Valley's ranch dressing powder.

But what is it anyway?

It's the sodium salt derived from the amino acid glutamic acid, which is naturally present in our bodies and is also found in foods like Parmesan cheese, tomatoes and mushrooms. Chefs and food companies love it because MSG isn't just a salt but it's also a flavor enhancer that provides an umami quality (also known as the fifth taste beyond sweet, sour, salty and bitter) to dishes, giving them a heartier, richer kick.

Is MSG dangerous?

Over the years, MSG has gotten a bad rap, with many claiming it causes symptoms ranging from headaches to nausea after consumption. The backlash it received in the late 1980s and early 1990s has been hard to shake — several authors have attributed this in part to racist, anti-Asian sentiment — and many people continue avoid MSG today in the same way that people assume all preservatives are bad.

But, according to the Food and Drug Administration (FDA), MSG is "generally recognized as safe," and there are no reputable scientific studies that have been able to conclusively prove that MSG caused those reported symptoms. A spokesperson for the FDA told TODAY Food via email that "MSG is not a true allergen in that there has been no evidence that substances in MSG cause immune mediated food allergic reactions."

Think of MSG like peanuts: It will be dangerous if you are allergic or have a natural intolerance to it. And, just like any other type salt, MSG shouldn't be consumed in massive quantities.

Nutrition and wellness expert Frances Largeman-Roth, RDN, told TODAY that it can be difficult to pinpoint the origin of some food sensitivities, and that's especially true when you're eating a dish that has a lot of ingredients. "I think that people can legitimately feel bad after eating a meal that is very high in sodium, whether that sodium comes from salt or MSG," Largeman-Roth said. "That doesn't mean that MSG is harmful. Also, they may have a sensitivity to gluten or another food additive that could be in their meal."

Though MSG occurs naturally, to make the flavoring agent, it was first extracted and crystallized from seaweed broth. Today, it's created during a fermentation process similar to making yogurt or soy bean paste.

MSG is just one of many FDA-approved food additives, like monocalcium phosphate, sodium bicarbonate and sodium acid pyrophospate, which are used for different purposes, such as leavening. Many of these are also found in fast-food items, too.

But MSG shouldn't just be considered as a cheap way to enhance flavor. Award-winning chefs like David Chang of Momofuku use MSG in their dishes. The enhancer is also available online and at grocery stores. It's also found in foods meant to further enhance the flavor of any dish, like McCormick's Culinary Flavor Enhancer.

In the same way that Chipotle tried to appeal to more health-conscious eaters by promoting its all-natural food (though few nutritionists would ever consider a 1,000-calorie burrito to be a health food), many restaurants today still proudly advertise that their food is free of MSG — even if menu items are still filled with lots of other additives.

When reached via email about whether its breaded chicken contains any MSG, a Burger King spokesperson told TODAY, "Burger King has made substantial changes to its menu to embrace and promote 100% real food — fresh produce, beef and vegetables — and this will increasingly become part of our core marketing for our guests. Part of this important commitment is fully removing MSG from our menu and we're proud to say that our plan is to have this complete by the end of this year."

Burger King does use MSG in the current recipe for its Chicken Fries.

As for the beloved chicken-first chain?

A spokesperson for Chick-fil-A Inc. told TODAY that the company is aware that some customers may be sensitive to MSG and while their classic chicken sandwich, spicy chicken sandwich and chicken nuggets all contain MSG, the chain is "researching the removal of MSG from these menu items." The spokesperson did not provide a timeline for this potential transition but noted that many items on the menu, including the chain's grilled items, do not contain any MSG.

If Chick-fil-A does end up removing MSG from its menu altogether, there may actually be an unintended consequence. Along with its ability to make food taste more potent, MSG is also considered as a tool to help lower the total sodium count of processed foods or recipes. Said Largeman-Roth, "You can lower the sodium in a dish by [up to] 25% using MSG because it boosts flavor so well."