If calories in, calories out is true, you'd lose weight on fast food as long as you stay in a calorie deficit. But ... does CICO really work? Let's find out.

Can you lose weight on fast food? Or, more specifically, lose fat eating only McDonald’s?

It sounds ridiculous, but we decided to put it to the test. For the next 30 days, I’ve convinced Brendan, my videographer, to eat nothing but McDonald’s to see if fat loss really is as simple as calories in, calories out.

Or, if all that junk food has consequences on his body and health that I never could’ve expected.

Day 1

So Brendan is someone who tries to eat clean every day, but over the last few months, he’s hit a fat loss plateau.

I’ve been telling him the problem is less about what he eats and more about how many calories he’s actually consuming.

So when I said he could technically lose weight on fast food, he was skeptical. 

Jeremy:

"Okay, how about this: I’ll not only pay for all your meals, but if it doesn't work, I’ll give you a $1000 dollars?"

Brendan:

"But what if it does work?"

Jeremy:

"Then you'll look ripped. It's win-win for you."

So now that I’ve got my money and reputation on the line, it’s time to create a McDonald’s meal plan that'll help him lose weight on fast food.

Based on Brendan’s current weight (158 lbs), to lose weight on fast food, he’ll need to eat a daily target of 2000 calories.

But to avoid losing muscle and ending up weaker after the 30 days, he’ll also need to hit a bare minimum of 120 grams of protein.

Today’s breakfast (Egg Mcmuffin) is not a bad start, coming in at what McDonald’s claims is 290 calories and 16 grams of protein. But because these foods are so calorie-dense, it’s taken Brendan less than 3 minutes to finish eating — leaving him hungry.

To see how all this highly processed food is affecting Brendan's body, he’ll not only be doing a DEXA scan to measure his body fat and muscle mass, but we’re also conducting pre and post-blood tests.

We’ll also be testing Brendan’s strength before and after the experiment to give us another way to assess if he’s lost any of his hard-earned muscle and strength.

But for now, here's what I had Brendan eat for lunch. A chicken BLT and 2 packets of apple slices.

But even with what seems to be “real” chicken and apple slices to balance out his lunch, Brendan’s already struggling. And to help me truly understand what he’s truly up against, I sat down with PhD researcher and published fitness author Dr. Layne Norton.

Dr. Layne Norton:

"Even though some people might think it sounds fun to lose weight on fast food, it's going to get pretty monotonous over time. If you don't have enough novelty, then you can get bored of it, and you don't have satisfaction from your diet. Your adherence can wane from that aspect as well."

So, using Layne’s advice, the meal plan I made for Brendan consists of 3 meals and 2 snacks per day. 

But rather than locking him into a strict meal plan that'll get him to lose weight on fast food, I gave him a few different options for each meal.

He’ll be logging everything into our new BuiltWithScience+ app.

So, as long as he stays within his daily target of 2000 calories and at least 120 grams of protein, he can modify the meals however he likes.

Layne, however, did raise some concerns.

Dr. Layne Norton:

"I don't think I would recommend this diet.

Probably about 15 years ago, a guy named Mark Haub, who was a professor at K State, did what he originally called the convenience store diet or the 7/11 diet, but it got changed to the Twinkie diet.

He was eating 1800 calories a day from junk food, and he said that he was pretty hungry for quite a bit.

He said at first, it was kind of cool to eat junk food, but after a while, he just kind of wanted a really big salad.

And so I think that from the satiety perspective, it's not a diet I'd recommend people stick to."

Day 2

However, as it turns out, not every healthy meal option I selected was available in Canada. 

And suddenly, the slate of food meal options I had tried to put there for Brendan to lose weight on fast food was almost cut in half.

So, he had to start getting creative — like making his own salad out of his wrap.

Brendan also started complaining about his racing heart.

Honestly, though, I don’t think Brendan’s racing heart has much to do with his new diet aiming to help him lose weight on fast food. That's because he’s not only been drinking extra coffee to help with his hunger, but he’s also pretty nervous about what this diet might do to his health. 

And I don’t blame him.

Almost every item on the McDonald’s item is full of saturated fat, highly refined, and stripped of pretty much everything that makes them good. It’s also very limited. Apple slices are the only fruit Brendan can eat over the next 30 days. And even their vegetables are questionable. 

As a result, aside from sodium, he’s not even close to getting even just 50% of most of his recommended nutrients every day.

Which honestly has me concerned about what this will do to his blood work after 30 days of trying to lose weight on fast food.

Day 5

On day 5, Brendan shared 2 common problems he ran into while trying to lose weight on fast food:

1. He doesn't always get the order he requests, which means more calories.
2. He's having some ... well, difficulties in the toilet.

Dr. Layne Norton explains:

"I think dietary fiber absolutely helps with stool elimination frequency and ease of elimination.

So I think he'll still poop, but it might be just a little bit less frequent and a little bit more difficult. But he's also eating in a calorie deficit, and as your overall food volume goes down, your amount of faeces that you produce is going to go down."

The recommended fiber intake for Brendan is about 35 grams a day.

On his McDonald’s meal plan, that aims to help him lose weight on fast food, however, he’s getting less than a third of this, barely reaching 10 grams daily.

The “blockage” he’s experiencing has definitely made accurately tracking his weight loss a lot more difficult than I anticipated. 

While Brendan was blaming his lack of progress on his low fiber intake, I wasn’t fully convinced.

Dr. Layne Norton:

"If he is in a calorie deficit, he will lose weight.

If he doesn't lose weight, it will be because he was either sneaking food outside of it or because the calorie deficit amount was just a miscalculation based on the estimation."

So I did some deeper investigation into McDonald’s’s claims about its calories and found what might just be the true culprit behind why Brendan hasn't managed to lose weight on fast food despite supposedly being in a calorie deficit.

Day 15

Jeremy:

"I think McDonald's is hiding, is hiding some stuff. I think they are heavily under reporting the calorie content of their meals."

Brendan:

"I think that's a 100% correct."

Jeremy:

"And we saw it with the egg McMuffin, right? I mean, that was like what they listed it as 280 calories."

Brendan:

"Yes. If it's just got the little, the tiny ham slice. Yes."

Jeremy:

"Yeah. So they're saying that an egg plus an English muffin bun plus ham, plus cheese, American cheese, plus what we didn't know was spread butter on the bun. There's no way that's 280. That is at least 400 plus."

But there’s another thing I completely overlooked that’s making this calorie discrepancy even worse.

A 2023 study compared two groups of people eating the same number of calories, around 2,100 per day.

• Group 1: Ate a highly processed diet with only 6 grams of fiber
• Group 2: Ate a less processed diet with 26 grams of fiber

Here’s what the researchers found:

• With the highly processed diet, about 95% of the calories were absorbed by the body.
• But with the less processed diet, only 89% of the calories were absorbed. The rest? They were excreted.

So even though both groups ate the same number of calories on paper, the less processed diet led to an extra 116 calories lost in feces — calories that never actually made it into the body.

And here’s the kicker: nutrition labels don’t account for this.

So, with Brendan’s heavily processed diet, he may not be in the calorie deficit we initially thought he was, especially with the “hidden calories” that McDonald’s doesn’t seem to take into account. No wonder he hasn't managed to lose weight on fast food despite his "calorie deficit"!

Which means… no more butter on his breakfast sandwiches and no sauces on his wraps and burgers.

But to make this easier for Brendan, I wanted to give him a special meal that should tide him over for his final 2 weeks. 

A cheat meal.

In the research world, cheat meals are referred to as “planned hedonic deviation”, and they’ve been shown to be an effective tool to help people stay on track with their diet.

In Brendan’s case, however, his cheat meal looks a little different.

His body was craving nutrient-, fiber-rich foods, so we went to Whole Foods.

And when he scanned his meal using our Built With Science+ app, he also realized he was getting way more food for a fraction of the calories than his existing meal plan that aimed to get him to lose weight on fast food.

Day 21

Brenda:

"21 Days in, and I just hit my lower weight yet.

It still feels like there are crazy spikes up and down, but every week my lowest weight is a little bit lower than the week before so that is definitely a good sign.

And also really trying to push myself during each workout because I have this constant fear that any weight loss on McDonalds alone is gonna include muscle, but I will say I have gotten a lot better at customizing my meals to pack in more fiber and a lot more protein.

But, honestly, I think the biggest adjustment now that things have settled in, is that I’m just snacking less through the day.

Besides having a bag of apple slices in the afternoon, I am really just eating breakfast, lunch, and dinner, and while there definitely is some hunger as a result of that, I feel like it mostly feels alright."

Dr. Layne Norton:

"When people aren't snacking, they tend to do actually better with adherence and whatnot. It's kind of the mindless eating that really becomes problematic."

Day 30

Brendan:

"So it is day 30 of trying to lose weight on fast food, and I am going to get my final Blood results and my Dexa Scan.

Over the past few days, I have also managed to have been able to swap a few different meal items that I was able to fit into my diet, including the sauce-free fish fillet, which I wouldn’t recommend, and the McDouble Cheeseburger, which honestly might have been my favorite.

I even managed to get a breakfast muffin and still hit my target for the day.

But honestly, I’m just looking forward to this "lose weight on fast food" experiment being over so I can finally eat a “real” salad again."

Strength Update

How did Brendan's strength change during his time trying to lose weight on fast food? Well, surprisingly:

• Bench press: Improved 7 to 9 reps
• Pull-ups: Improved 10-11 reps
• Deadlifts: Stayed the same, but Brendan forgot to wear wrist straps, which was why he failed at the same place.

So, overall, Brendan managed to gain 2 extra reps on his bench and pull-ups, and matched his deadlift strength although he forgot to wear wrist straps, limiting his strength. 

Blood Work Update

But now it’s time to see how his blood work had been affected by the "lose weight on fast food" experiment.

The main things we’re looking for are his cholesterol levels, his HbA1c, which is the best blood marker for diabetes, his inflammation levels, and his liver health.

Normally, all of these would be negatively impacted by a junk food diet.

But what actually happened?

Nothing.

All his blood work stayed pretty much the same, which is surprising. 

But remember how Layne Mentioned Professor Mark Haub, who ate nothing but Twinkies and protein shakes for 12 weeks?

He not only lost 27 lbs, but all of his blood markers improved in the process. 

It turns out weight loss alone is a major health booster and can “balance out” the negative effects you’d expect from a nutrient-deprived, junk food diet.

Body Composition Update

Which now begs the question. What actually happened to Brendan’s weight, fat, and muscle mass? 

After only eating McDonald’s for 30 days, Brendan dropped 5lbs on the scale, lost three cm off his waistline and according to the results of his DEXA scan ... dropped from 23% body fat percentage to 22% BF.

TL;DR

• To find out if CICO really works, I had Brendan, my videographer, attempt to lose weight on fast food — eating nothing but McDonald’s for 30 days.
• A few problems Brendan ran into during the 30-day “lose weight on fast food” experiment included not getting the order he requested (which meant more calories) and bowel movement problems due to a lack of dietary fiber.
• Brendan also wasn’t losing weight as expected, which got me investigating. Turns out, there were 2 culprits for this: 1) McDonald’s under-reports the calorie contents of their meals, and 2) the body absorbs more calories from a highly processed diet.
• After making a few minor adjustments to his meal plans, Brendan finally hit his lowest weight after 21 days of the “lose weight on fast food” experiment.
• At the end of 30 days, Brendan got stronger on the bench press and pull-ups, dropped 5 lbs, lost 3 cm off his waistline, dropped from 23% to 22% BF, and his blood work remained the same. 

Now, while Brendan got some pretty impressive results from this "lose weight on fast food" 30-day experiment, it was the most difficult 5 lbs to lose in his life.

And, if Brendan ended up gaining weight on this diet, it’s likely his blood work results would have turned out much differently. 

But this experiment does show you how fat loss really is a numbers game.

So, if having your favorite burger or ice cream every now and then helps keep you on track, go for it! As long as you’re aware of how quickly those calories add up.

But if you’ve tried counting calories in the past and found it too tedious, or it just didn’t work, then I’ve got some good news for you.

With my brand new BuiltWithScience+ app, you can just snap a picture of your meal, and it’ll instantly estimate the calories and portions and log it directly into the app. Plus, you’ll be able to track your workouts, body weight, and progress all in one place.

I guarantee it’s the only fitness app you need, and as if I was personally coaching you every single week.

And you can try it completely free for 2 weeks by clicking on the link below, and see the difference yourself:

Click the button below to try the BWS+ app for 2 weeks, for free, no strings attached:

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And then you can also check this article out next for a tasty high protein fat loss meal plan to get started with right away. 

Thanks for sticking to the end, and I’ll see ya next time!