I spent a week eating vegetarian fast-food from all the major chains - and it was absolutely terrible

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.

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.

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.

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 can substitute beans and rice for meat in most menu offerings.

"Vegetarian has been really big for us recently,” due to 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.

To celebrate Leap Day, Arby's debuted a "vegetarian" menu for one day, and one day only.

Of course, the menu was a joke. A chain with the slogan "we have the meats" wasn't going to actually offer a vegetarian menu. Instead, Arby's simply sold their typical, meat-heavy sandwiches — minus the meat.

However, when I tentatively asked the cashier if this location was doing the "meatless, Leap Day deal," he didn't bat an eyelash.

I decided to try the Loaded Italian, which typically comes with ham, salami, and pepperoni. Sans meat, I had a sandwich with Provolone cheese, banana peppers, lettuce, tomato, and red onion, plus red wine vinaigrette and garlic aioli.

The banana peppers packed a punch, and, in general, the sandwich tasted like a salad between two piece of bread. After eating half, I added in some curly fries as the "meat," which further improved the sandwich.

My only true complaint was the aioli, which really just tasted like plain old mayonnaise and should not be slathered on top of an all-vegetable sandwich.

Day 1 total:

Calories: 1,520

Protein: 38 g (out of the ideal minimum of 46)

Carbs: 207 g

Sodium: 5,490 mg (more than twice the recommended 2,400 g)

Price: $16.74

The Egg White Flatbread has been recommended as one of the 11 healthiest fast-food breakfast items around. It's a little high in sodium (610 grams) but also high in protein (15 grams).

If you eat the flatbread while it's hot, it's a great breakfast option. However, the longer you wait, the more disturbing the congealed egg white and cheese appear.

While many burger joints have the same lackluster options for vegetarians (weak salads, salty fries, and milkshakes), Wendy's also serves up loaded baked potatoes.

The broccoli and cheese baked potato was hearty and authentic in a way that few fast-food offerings manage to be. Let's be real, it's hard to mess up a baked potato.

However, like Dunkin's breakfast flatbread, if you wait too long and allow the cheese to congeal, the dish goes from enjoyable to nausea-inducing.

Seeking vegetables — which were ironically difficult to procure on my vegetarian diet — I paired the dish with a forgettable Caesar salad.

Chick-fil-A was my savior during my healthy fast-food diet. However, the chain did not offer the same relief as I attempted to eat vegetarian.

While you can get one of Chick-fil-A's salads without chicken, I decided to opt for two of the few intentionally vegetarian options: the waffle fries and the Superfood Side. Despite ordering the "large" kale-based salad, the result wasn't nearly as big as I had hoped — despite costing $5.

While both options were delicious, if you're trying to eat meatless at Chick-fil-A, I would recommend shelling out at least $10 on Superfood Sides to accompany your waffle fries.

Day 2 total:

Calories: 1,500

Protein: 46 (right on the nose!)

Carbs: 180

Sodium: 1,970

Price: $17.94

Once again, the vegetarian-friendly fast-food joint didn't disappoint.

I ordered ice coffee and a breakfast burrito, swapping nacho cheese for pico de gallo to cut calories. However, I could have ordered any number of items from the chain's breakfast menu, and simply held the meat.

The breakfast burrito was solid as expected, though probably would have been better if I hadn't held the nacho cheese.

I've heard from vegetarian friends that grilled cheeses are on a number of fast-food chains' "secret menus," since pretty much any restaurant that serves burgers has the necessary ingredients. However, Five Guys is one of the few to actually feature grilled cheese on the menu.

The burger chain's grilled cheese, which I ordered topped with grilled onions and mushrooms, is pure, greasy goodness. Five Guys prepares its food fresh, so the cheese is perfectly gooey and delicious.

If you're at Five Guys and you're a vegetarian (or simply aren't craving a burger for some reason), ordering this sandwich will give you the same joy and nutritionally-based guilt as getting a burger.

Adding to the meat-free gluttony, I also ordered a side of fries and grabbed some free peanuts. Be warned — even a small side of fries from Five Guys is huge. After finishing my grilled cheese, I could only finish about half of the fries... which was probably for the best.

I realized while wallowing in a food coma after finishing my Five Guys meal that I hadn't eaten any fruits or vegetables all day — if you don't count ketchup, fries, or salsa.

With that in mind, I headed to Subway for a dinner of a Veggie Delite sandwich and some apple slices.

The Veggie Delite provided a nice contrast to my uber-greasy lunch. While the packaged apple slices tasted slightly off in a way that fresh apples rarely do, it was good to have my first fruit of the day at 8 p.m.

Day 3 total:

Calories: 1,465

Protein: 43 g

Carbs: 149 g

Sodium: 2,113 g

Price: $17.36

I returned to the chicken-loving chain for a Greek yogurt parfait and an iced coffee.

Unlike McDonald's yogurt parfait, Chick-fil-A avoided falling into the trap of over-sweetening its breakfast, instead providing a healthy, filling breakfast option. However, if you haven't had the chain's iced coffee, be warned — it comes sweetened with "pure cane sugar." If you prefer your coffee unsweetened, be sure to specify that while ordering.

McDonald's Southwest Salad is delicious — and that isn't just my fast-food addled brain talking.

I expected the Southwest Salad to be an anemic, iceberg lettuce fail that I would forget within the hour. Instead, McDonald's actually managed to craft a delicious combination of romaine, baby spinach, and baby kale, topped with corn, beans, peppers, and cheese. This was as tasty and nutritious as many of the salads I've purchased for twice the price at local cafes and fast-casual chains.

Unfortunately, McDonald's missed the mark on the sides. Since I didn't expect much from the salad, I decided to take advantage of McDonald's all-day breakfast and also order a hash brown and an egg McMuffin. While the hashbrown was as tasty as always, I was given an eggless sausage McMuffin — which was basically just two cheesy halves of an English muffin after I tossed the meat in the trash.

By Day 4, it had become clear that breakfast is often vegetarian's best bet for getting protein by way of eggs at fast-food chains.

So many of the vegetarian options at fast-food chains are empty calories. At this point in the week, I'd eaten potatoes prepared five different ways — baked, on a breakfast taco, and as curly, waffle, and traditional fries.

Dunkin's new veggie "GranDDe Burrito" provided a welcome respite from potatoes, even if the spicy, southwestern burrito is essentially a mediocre omelet wrapped in a tortilla. The cheese of my burrito had migrated to the bottom, making for a meal that was tolerable in the first half and quite tasty at the end.

I paired the burrito with an enormous banana, my first fruit of the day.

Day 4 total:

Calories: 1,610

Protein: 55 g

Carbs: 211 g

Sodium: 2,417 g

Price: $17.80

This time, McDonald's listened to my request to hold the meat.

However, as a McMuffin mega-fan, I have to say that this was one of the saddest McMuffins I have ever eaten. The egg was cold and dry, and the cheese was barely melted.

I am not a pizza snob — I love pizza, and will happily eat whatever brand is available. So, I decided that I should hit up Pizza Hut.

When I began eating my personal pan cheese pizza, which I ordered as part of a meal with three breadsticks, all was well. The pizza was warm, the cheese was melty, and the crust was impressive. The breadsticks were also good: salty and addictive.

However, as I ate more and the meal cooled down, a sense of disgust set in. The cheese was too greasy and cold, while the breadsticks hardened. I eventually dumped the last fourth of the pizza and one of the breadsticks in the trash — but it was too late.

All afternoon I felt queasy and ill. I struggled to focus, as my stomach rebelled against the pizza. By dinner time — and after a visit to the gym — I finally felt a bit better, but I will not be stopping by this Pizza Hut again any time soon.

In my defense, it was only a small transgression. Instead of sticking to fast-food chains, I decided it was time to visit a fast-casual that's all about vegan food for a bit of contrast.

By Chloe is a fast-casual concept that opened its first location in New York City in 2015. After a burger-free week of fast food, I ordered sweet potato fries and the guac burger (made with a black bean, quinoa, sweet potato patty).

The meal was delicious, especially after a week of scraping the bottom of the vegetarian barrel. The standout was my cocktail, an iced Irish coffee topped with coconut whipped cream richer than 99% of the traditional cream I've tasted. However, visiting the location, it was clearly nowhere close to fast-food — it took me 10 minutes to order and another 15 to get my food.

Day 5 total (approximations, as By Chloe does not provide nutritional information):

Calories: 1,640

Protein: 71 g

Carbs: 196 g

Sodium: 3,330 g

Price: $35.08 (including a $10 cocktail)

Going vegetarian was way easier than eating healthy at fast-food chains. I didn't have to break down every little aspect of nutrition — I just needed to figure out whether there was meat in each dish.

It ended up costing about the same amount of money as well, as I spent a total of $104.92 over the course of the week.

At the same time, I actually felt a lot grosser while eating vegetarian.

Eating "healthy" at fast-food chains forced me to think creatively and pick items that had nutritious value, even if they didn't taste great. Several times during my week of vegetarian living, I found myself feeling bloated and queasy, because the best vegetarian options were some variation on fried potatoes and straight-up carbs.

First, look for menus that allow for substitutions. Taco Bell is the best at this, but some of my favorite meals of the week, like McDonald's Southwest salad, were vegetarian takes on traditionally non-veggie items. Even unexpected options like an Arby's sandwich minus the meat aren't all bad!

Second, remember that vegetarian doesn't always mean healthy. Most vegetarians and vegans already know this, but pseudo-vegetarians like me may assume that cutting meat automatically equals a healthier meal. While swapping out meat can be an easy way to cut calories, my experiences at Five Guys and Pizza Hut taught me the obvious lesson that vegetarian options can make you feel just as terrible as an overstuffed burger.

Finally, check out all-day breakfast. After endless variations on the potato, breakfast options like a burrito or McMuffin can provide a little protein and break up the routine. Plus, all-day breakfast items can serve as the main course, a side, or a snack — the recipe for vegetarian success.