I went on the Subway Diet for a week - and it's clear why the chain is no longer America's destination for healthy fast food

The Subway Diet was extremely successful when it came on the scene in the early 2000s.

Student turned former Subway spokesperson Jared Fogle famously lost more than 200 pounds eating the sandwiches; Subway hired him in 2000.

Much of the chain's growth in the early 2000s was tied to Fogle's weight-loss success story, with Subway's chief marketing officer once crediting Fogle for as much as half of the chain's growth. Sales tripled to $11.5 billion in 2011, from around $3 billion in 1998, according to Nation’s Restaurant News.

However, what's considered healthy has changed in the past five years, and Fogle is currently serving a nearly 16-year prison sentence, after being charged for having sex with minors and possessing child pornography. Subway cut ties with him last summer.

If Subway wants to stage a comeback, it needs to find a way to appeal to discerning, health-conscious consumers and have a diet that stands out, jailed ex-spokesperson aside.

While Fogle stuck to the limited menu of one 6-inch turkey sub for lunch and a footlong veggie sub for dinner, pairing both meals with a bag of baked chips and a diet soda, I was not going to maintain such a strict diet.

My rules: I would only order from Subway's Fit Fresh menu. I planned to use advice from Subway's corporate dietitian, Lanette Kovachi, on how to best maximize nutrition: Order nine-grain wheat bread, fresh veggies, and cut mayo and creamy dressings.

Since Fogle allowed himself chips, I decided I could have sides (and occasional snacks) of my choosing. In general, I would try and maintain a healthy diet. I wasn't trying to lose 200 pounds — I was trying to see if twice-daily Subway was a feasible option for someone trying to eat healthily today.

"We don’t expect people to eat all Subway," Kovachi told me when we met at a Subway event in February. "We want people to have a balanced diet."

But Kovachi also told me she believed that the "Subway Diet" is embedded in the DNA of the chain. If Subway wants to regain its former glory, it needs to reclaim the Subway diet and revamp it for 2016.

The company is making some strides. Kovachi said that the chain's menu had great variety in grains and vegetables, and that it had solid options for those looking to exercise portion control — no 6-inch sub is much more than 600 calories, and the Fresh Fit sandwiches are all less than 400 calories. Recently, she said, the chain has been trying to appeal to modern customers by emphasizing health positives, as opposed to the dated no-fat trend.

My plan for Day 1 was a cost-efficient one. I would get two Subway sandwiches for the price of one, thanks to Subway's breakfast deal that is offering free 6-inch breakfast sandwiches for the entire month of May with the purchase of a 6-inch sandwich before 9 a.m.

Unfortunately, I forgot one thing — breakfast is Subway's worst meal.

I managed to finish about 75% of my egg-and-cheese breakfast flatbread, which tasted bland and only slightly better than it looked.

My lunchtime Veggie Delite with vinegar was slightly better. The Veggie Delite is basically salad on a sandwich, something that I was able to enjoy at this point. (Spoiler: this enjoyment would not last long.) The predominant flavor was the tangy banana peppers, the crown jewel in Subway's topping selection, which abated the forgettable bread and mediocre produce.

After feeling slightly queasy all day after my breakfast sandwich, I decided that my two Subway sandwiches of the day meant I was entitled to a burger, fries, and margarita for dinner. I can try to justify this decision. (The burger place had margarita deals. I wanted to see friends. It was Cinco de Mayo). But at the end of the day only one thing remains true — I ended Day 1 with a stomachache.

Worse, I couldn't even blame Subway for my discomfort. I wasn't trying to be healthy, even if I had technically followed my Subway diet "rules," proving that there's a lot of room for grave slipups on the diet.

After a healthy breakfast of oatmeal, I purchased a carved-turkey 6-inch sandwich. The carved turkey is a relatively new addition to the menu, and it's much better than its slimy take on the classic lunch meat.

I didn't want any more Subway, and I also had the strange feeling that I had just eaten the equivalent of a block of Styrofoam. The carved-turkey 6-inch has 330 calories, according to Subway, including 25 grams of protein. Despite this, at 4 p.m. I felt dead — though the jury is still out on if this is simply because it was Friday at 4 p.m.

So I ate a half of a grilled cheese as a "snack."

Have you tried to eat a Veggie Delite footlong? I have! It is terrible.

My Day 2 (minus the grilled cheese) was all about sticking to the original Subway diet. For lunch: a turkey 6-inch. For dinner: a Veggie Delite footlong with vinegar and cheddar cheese.

I made it about one-third of the way through the sandwich before I became convinced I could not eat any more. The combination of bread and vegetables was somehow both brain-numbingly bland and increasingly repulsive with every bite. While the Veggie Delite had tasted fine the previous day, the idea of eating the entire boring sandwich now seemed impossible.

I managed to finish half of the footlong after taking an hour-long break to drink wine, and convinced a friend to eat the other half. His review: "Felt like I was eating flavored bread."

I was going to watch the America's Cup in the Financial District in the early afternoon, so decided to seek out Subway shops in the area.

There are seemingly endless Subway locations in New York City — including several in the area where I was headed. So I was able to easily stop by a Subway on the way there, without wasting any time.

When I ate nothing but healthy fast food for a week, one of the positives I noticed was that I could find a fast-food restaurant pretty easily at any time of the day in New York, instead of restricting myself to eating at home or searching for restaurants that served a specific type of food. The Subway diet is similar: The chain has more than 26,000 locations across the US, so you're always pretty close to a Subway.

I had gone for a run in the morning, and I felt reasonably nourished by my roast-beef sandwich and Sunchips. The meat, vegetables, and bread all formed together in a mostly tasteless, inoffensive sandwich. I didn't hate it, but I certainly didn't love it. It was sustenance in its most rudimentary form — though I did attempt to liven it up with this Instagram-friendly photo.

When my friends kindly invited me over for dinner, I told them I would attend, but I would be bringing my own food — Subway. They seemed concerned about my diet decision, but they agreed to allow me to eat my sandwich in peace.

As I ate my 6-inch rotisserie chicken sandwich with avocado and vegetables, I was struck by how glum and boring my tasteless meal felt in comparison to the scent of their vegetarian carbonara and radicchio salad. I found myself wondering if this was how people who went on Soylent diets felt?

Part of this was my fault — in an effort to eat healthy, I skipped out on mayonnaise and dressings that could pack a punch. As a result, banana peppers were the only element bring flavor to my meals.

In the end, I ended up nibbling on some carbonara and radicchio, and enjoying it infinitely more than my sandwich.

Burnt out after my depressing dinner, I went for a run and put off breakfast on Sunday morning, mostly because I didn't want to think about my Subway lunch. I didn't end up eating bagels until 1:30 p.m. — making it far too late to cram two sandwiches into my Day 4 routine.

On the way home from my very late breakfast, I picked up a Subway sandwich (carved turkey with guacamole). After a few hours in the fridge, I pulled it out to eat as dinner, only to discover the bread had partially disintegrated on top. I understand this is a risk one takes when eating a sandwich with avocado, but, with a heartier bread, this should not be a concern.

I ate the sandwich as quickly as possible, gagging a little.

I scarfed down a grilled-chicken sandwich with spinach for lunch, barely tasting anything. (I had at this point developed an aversion to iceberg lettuce.)

The smell of Shake Shack washed over me as I sat in Madison Square Park. I longed for a delicious burger. Instead, I angrily chomped on some Sun Chips.

While talking with a friend about my annoyance with Subway, she revealed she was a huge fan of the chain. Intrigued, I asked her for her sandwich of choice.

Following her instructions, I ordered carved turkey, provolone, lettuce, pickles, banana peppers, and cucumbers on nine-grain wheat bread. I skipped her suggestion of jalapenos, but added her favorite dressing, sweet onion, which she told me was they key to a successful sandwich.

She was 100% correct.

The sweet-onion dressing finally added some much-needed flavor to the sandwich, after a couple days of mounting blandness. Further, I was running late for an event, so I scarfed down the sandwich while speed walking the streets of Brooklyn. This may be the perfect way to eat a Subway sandwich — you're not too focused on your food, and need something portable.

This was the best sandwich I had all week.

However, the Subway magic I had unlocked on Day 5 did not continue into Day 6.

I got the exact same sandwich for lunch, substituting carved turkey for rotisserie chicken. But it wouldn't hold together — as I picked it up, it immediately fell apart, scattering chicken and veggies. The bread was crumbly and maybe even a bit stale. It was, once again, a fine sandwich, but not something I enjoyed eating.

At this point, I had visited Subway eight times, so the smell of Subway locations was starting to infect my brain. I sat and ate my sandwich inside this location, which I normally don't do, so I got a good whiff of the bready aroma. From the very first day, it has been a smell that makes me lose my appetite — something that only grew worse with time.

My salad was ... wet. And onion-y.

Still on my sweet-onion kick, I got spinach with onions, tomatoes, cucumber, carved turkey, croutons, and sweet-onion dressing. What you see is what you get with this one — the ingredients didn't mix together particularly well, and everything just tasted kind of wet. Sweetgreen it was not.

I ate the salad as quickly as possible, but the overwhelming taste of onion remained. The salad was about 20% onion. Even after eating a piece of toast, an orange, a scoop of chocolate ice cream, and brushing my teeth twice, the onion flavor lingered.

For lunch, I ordered the Sweet Onion Chicken Teriyaki sandwich, which had been recommended to me by Subway lovers. It was passable — the teriyaki did add a little flavor to the sandwich. However, at this point I was so burnt out by Subway, there was little the chain could do to redeem itself.

My totals:

Sandwiches eaten: 13

Money spent: $83.30, including tax

Most inexpensive sandwich: 6-inch Veggie Delite, at $4.64, with a free breakfast sandwich on Day 1

Best overall sandwich: carved turkey, provolone, lettuce, pickles, banana peppers, and cucumbers on nine-grain wheat bread, with sweet-onion dressing

True, many of Subway's sandwiches are low-calorie options, and customers can pack in protein and vegetables. But calories are just one part of nutrition. Subway's sandwiches are high in sodium. My favorite, the carved-turkey sandwich, reportedly has 890 mg, a little over one-third of the recommended 2,300 mg daily.

The real problem, though, is that I never felt healthy after finishing a meal. Despite the company's "Eat Fresh" motto, consuming a sandwich made with ingredients like floppy eggs and mostly tasteless veggies never feels half as fresh as something like a salad from Sweetgreen, where flavorful and often local ingredients are front and center.

The chain is making some genuinely positive changes, such as introducing carved turkey, which debuted in April and quickly became my favorite protein at the chain.

"The outlook for 2016 is more optimistic based on the terrific feedback we received from our guests about our culinary improvements and exciting new menu offerings," the company said in a statement to Business Insider earlier in May.

Even with new options, however, the chain didn't have enough variety or flavor to keep me interested all week. By Day 2, I was exhausted by the sameness of every seemingly interchangeable sandwich.

Apparently, I'm not alone.

"While Subway remains a top choice for many consumers who love its range and competitive pricing, other consumers have become bored with the offering," Neil Saunders, CEO of retail consulting firm Conlumino, told Business Insider's Hayley Peterson earlier in May. "This is especially true of younger millennials who prefer a more diverse range of options, including salads and healthier choices."

After a week of eating Subway twice a day, I agree. Ultimately, the sandwich chain has failed to stay fresh — something it needs to fix if it ever wants to truly be the leading destination for healthy fast food.