It's been more than a decade since Morgan Spurlock made headlines with his 2004 documentary "Super Size Me," in which the filmmaker found out what being on an all-McDonald's diet for a month does to your body. (Spoiler alert: Not great things. Not great at all.)

Now, a former Iowa science teacher is telling kids you can super-size it and lose weight. John Cisna is touring the nation's high schools and colleges preaching the benefits of his McDonald's diet and how he lost close to 40 pounds eating nothing but items off the Golden Arches' menu for three months.

Maybe you attend one of the 90 schools Cisna has visited to discuss his 2014 book, "My McDonald's Diet: How I Lost 37 Pounds in 90 Days and Became a Viral Media Sensation."

Or perhaps you've heard him explain how you can "lose weight while still eating the foods you love, like Big Macs and hot fudge sundaes," and how a "sensible calorie plan and moderate exercise can help you melt away the pounds, lower your cholesterol and energize your life." Sounds awesome, right?

Cisna has even been hired by the burger chain as a "brand ambassador." A spokesperson told MTV News that the company actually reworked some of the science behind Cisna's originally self-funded diet research on his Ronald McDiet.

"As part of his relationship with McDonald’s, the original documentary John Cisna created to track his experiment was updated to better reflect the importance of food choice and balance together with moderate physical activity," the spokesperson said.

That got us wondering: What might a McDonald's-only diet actually do to your body? And is it really possible to lose weight chowing down on Big Macs and fries?

It Can Be Done, But It's Not A Great Idea

As hard as it may be in practice, the formula for losing weight is actually pretty simple: calories in versus calories out. "In theory, you could subsist on [a McDonald's] diet, but it's not recommended," said Katherine Brooking, registered dietician, nutritionist and author of "The Real Skinny: Appetite for Health's 101 Fat Habits & Slim Solutions."

On any "clinically controlled extreme diet" involving severe caloric restriction, "the average adult or child is almost guaranteed to lose weight," Brooking told MTV News. "It's the simple law of thermodynamics -- energy in and energy out." That means even 800 calories of cheese doodles a day, or fries, will get the lbs. off.

But when you're talking about a balanced, nutritional diet, it's not just about the amount of calories -- it's about the quality. To illustrate this, Brooking played around with the McDonald's menu calculator to create a basic meal. A regular hamburger has 470 calories (under 500 is reasonable for a meal), 4.5 grams of saturated fat and over 600 milligrams of sodium -- which is decent, but a bit high for anyone with hypertension.

"The real point, though, is that you are missing out on a lot of whole grains, there's nothing coming from fresh fruits or vegetables and you're missing out on a lot of nutrition from plant-based foods," she said, adding that "a plant-based diet is optimal for long-term health."

Also, Cisna says he weighed 280 pounds when the experiment started, meaning he would probably need more than 3,150 calories a day just to maintain his weight, so reducing his intake to 2,000 a day, even if it consisted of a burger, fries and a shake, would help explain his quick weight loss. Those numbers would not hold true for a middle- or high-schooler starting out at a much lower weight.

When Brooking checked out some of the "healthier" options, such as the Premium Bacon Ranch Salad with Buttermilk Crispy Chicken, she found that even with the low fat dressing you're looking at almost half a day's worth of sodium.

What Happens To Your Body After Eating A Big Mac?

Beyond not offering a balanced meal, certain menu items can do a number on your digestive system. The site Fast Food Menu Price recently posted a graphic illustrating what happens an hour after you eat a Big Mac (which clocks in at 540 calories). According to their data, in the first 10 minutes your blood sugar spikes to "abnormal levels" thanks to the release of dopamine in the brain, a reaction similar to the one you get from drugs such as cocaine or heroin.

Within 20 minutes, the sodium and high-fructose corn syrup in the bun have a similar effect due to their addictive nature, which makes your body crave another hit. Just 10 minutes later, the 970 milligrams of sodium might start causing symptoms of dehydration, which are kind of similar to hunger pangs. The combination could trick your brain into thinking you're still hungry because your body's insulin response brings down your levels of glucose, making you want to eat more and starting the cycle over again.

In addition, a fast-food diet high in sodium, processed sugar and unhealthy fats and low in fresh fruits and vegetables can lead to a variety of potentially deadly ailments, from diabetes and heart disease to high cholesterol and hypertension.

But You Don't Have To Starve Or Only Eat Salad To Stay Healthy And Lose Weight

 You can lose weight just from cutting calories, even if those calories aren't particularly nutritious. But, according to Dr. Lisa Moskovitz, registered dietician CEO of the New York Nutrition Group, a lot of people who resort to extreme diets are resigned to starving all the time, being deprived and eating bland food.

"When people hear about diets that incorporate foods they like it gains a lot of attention, so if you can lose weight eating McDonald's, great. But it's not the healthiest way," she said. "What it comes down to is that once the weight is gone, unless you have some sound behavioral changes and changed your habits that you had before nothing will be any different."

With the recent news that the chain will now offer its very popular breakfast menu items all day, there are even more options to have McD's breakfast for breakfast, lunch or dinner. But, Brooking argues, the premise of the McDonald's diet "is shameful because you should be telling young people that if once in a while you have fast food, here's some ideas on how to do it and stay in a reasonable calorie range, but really you should be saving money by cooking at home or in your dorm room."

Moskovitz agreed that sending a message about fast food dieting to high school and college-age people -- who are most vulnerable in those years to bad eating habits that could lead to eating disorders or obesity later in life -- is "absurd."

"Many people falsely assume that when you're eating healthy or to lose weight you can't have anything you want and you have to feel deprived," she said. "That's not true. Our message is that everything can fit in and nothing is off limits. ... There's a time and a place for everything as long as you're eating balanced meals. Telling them that you could eat McDonald's is the opposite of what you want to speak about."

While the McDonald's spokesperson said Cisna was not available for comment, she told MTV News in a statement that the author's original experiment was "conducted independent of McDonald’s, and McDonald’s Corp. did not learn about John nor his experiment until it gained national media attention in January 2014." The chain named him an official brand ambassador later that year.

Spokesperson Lisa McComb added that Cisna's story is "not a weight loss plan, and we do not recommend that anyone eat every meal at one restaurant every day for an extended period. Rather, John’s story is about making informed and balanced choices no matter where you choose to eat and incorporating exercise into your daily routine."

A Change.org petition was started this week to keep Cisna's documentary out of schools, noting that Cisna didn't count sodium and that many items on the chain's menu are so high in calories they might account for a student's entire caloric quota for a whole day.