I ate nothing but vegetarian fast food for a week — and discovered a huge problem with the industry

As someone who loves both meat and chain restaurants, I was only half prepared when I adopted a 100% fast-food diet with no meat allowed.

After spending a week trying to eat healthy while consuming nothing but fast food, I figured that going vegetarian for five days would be a breeze. I already knew fast-food menus like the back of my hand, and I had plenty of vegetarian friends I could consult.

Plus, a growing percentage of the American population has already gone vegetarian. While about 3% of the US identifies as vegetarian or vegan, an increasing number of people are cutting meat from their diet; 26% to 41% of Americans report that they cut down on the amount of meat they ate in the past year.

If fast-food chains want to compete with the new wave of trendy fast-casual restaurants, they need to appeal to the vegetarian market — including people who want to decrease the amount of meat they eat without cutting it from their diet.

So, I became a short-term vegetarian to see which fast-food chains were evolving to meet the meatless needs of consumers.

My first and most important rule was that I could eat only at fast-food chains. (I would consume at least three meals a day.)

That means no notoriously veggie-friendly fast casuals like Sweetgreen or Chipotle.

My orders would be 100% vegetarian, so no burgers or chicken fingers. I would not, however, be going vegan, so I could still eat things like eggs and cheese.

The challenge would last for one work week, from Monday to Friday.

I kicked off the week with the biggest name in fast food: McDonald's.

McDonald's has an extensive breakfast menu with a lot of range when it comes to health and meatiness. I ordered an ice coffee and a fruit-and-maple oatmeal. 

The McDonald's oatmeal was certainly more flavorful than my typical morning bowl, packed with apples, cranberries, and (according to McDonald's) two types of raisins. It was also sweeter than I'm used to, with 32 grams of sugar — more than half of the daily recommended dosage. Clearly, vegetarian does not necessarily mean nutritious. 

For lunch I visited a chain highly recommended by vegetarian friends: Taco Bell.

Taco Bell is the only national fast-food chain to have a menu certified by the American Vegetarian Association. Even before the chain rolled out the certified menu last October, Taco Bell had plenty of vegetarian cred thanks to the ease with which customers could substitute beans and rice for meat in most menu offerings. 

"Vegetarian has been really big for us recently," because of its relevance to millennials, Taco Bell's dietitian and product developer Missy Nelson told Business Insider. 

Looking at Taco Bell's menu, it's clear there is plenty to choose from. I decided on a bean burrito and a spicy potato soft taco. The burrito was straightforward but tasty, while the soft taco packed a bit of heat into the potatoes — two solid options I'd order again.