I consumed nothing but fast food for a week, and it was the easiest and cheapest diet plan I've ever tried

When I told friends I was eating nothing but fast food for a week, most immediately thought of the (in)famous Morgan Spurlock documentary "Super Size Me."

No, I would respond. I'm eating healthy.

As fast-food chains increasingly try to appeal to health-conscious Americans, supposedly nutritious items are popping up on menus more and more.

From KFC's grilled chicken to McDonald's (shockingly caloric) kale salad, massive restaurant chains want to signal to customers that nutrition and fast food can go hand in hand.

If these chains want to compete with the new wave of healthier fast-casual alternatives, they're going to have to prove they can become daily go-tos for nutrition-savvy millennials.

So I decided to see for myself how the chains, and my stomach, held up after a week of eating only at fast-food restaurants.

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 health-food-obsessed fast casuals like Sweetgreen or Chipotle. It's all about chains best known for burgers, fries, and fried chicken 24/7. (The one exception: I could drink alcohol.)

My secondary rule: I must try and eat as healthy as possible at these fast-food restaurants. My definition of health was pretty general. My meals should to add up to less than the FDA's daily recommended limit in calories, fat, and sodium, while providing me with enough protein to fill me up.

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

I started off the week at a familiar breakfast destination: Dunkin' Donuts.

There's one obvious "healthy" option on the menu: the Egg White Flatbread, which 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 actually tasty and pretty hearty. However, the longer you wait, the more disturbing the congealed egg white and cheese appear. I pair the flatbread with a coffee and head to work.

Lunch at Wendy's revealed just how gross grilled chicken could get.

After finding a reasonably healthy breakfast at Dunkin' Donuts, I was optimistic about what Wendy's would serve up for lunch. I'm generally a fan of the chain, and the Asian Cashew Chicken Salad fit all my qualifications (again, high sodium but low calorie and otherwise healthy), so I ordered it with high hopes.

These hopes were not met.

First, let me say the salad was not without its merits. The fire-roasted edamame was delicious. The dressing was nice. The cashews were spicy. But the grilled chicken was irredeemable.

At first, I ignored the floppy texture of the chicken. However, the more of the meat I ate, the more alien it seemed. It wasn't the taste — it was the strange consistency that seems unique to some fast-food chicken used to top salads.

I didn't finish the dish.