Ask a Dietitian: Can you actually lose weight by eating only McDonald's?

Welcome to Ask A Dietitian, a new series where Yahoo Canada digs into food trends and popular nutrition questions with registered dietitian Abbey Sharp.

This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis or treatment. Contact a qualified medical professional before engaging in any physical activity, or making any changes to your diet, medication or lifestyle.

You might not think losing weight and eating only fast food go hand-in-hand, but for one U.S. man — it worked.

Kevin Maginnis, known on TikTok as Big Mac Coach, embarked on a challenge to eat nothing but McDonald’s for 100 days to try and lose weight.

He began his journey in February, eating nothing but half portions of the fast food for every single meal. On day 100, Maginnis stood 58.5 pounds lighter.

Can you really lose weight eating only McDonald's?

People reported the man from Tennessee also saw other significant changes to his health. After doing bloodwork, his triglycerides were down 205 points and his cholesterol 65 points. He said he was "pre-diabetic before, down to healthy ranges now."

How is this possible? According to a registered dietitian, it's not that surprising.

RD Abbey Sharp told Yahoo Canada even though it worked for him, there are many reasons why she wouldn't recommend it.

Read on for everything you need to know.

What to expect with this diet: Expert

When thinking about eating just McDonald's for 100 days, expert Sharp had many concerns.

"The first thing I thought about was how constipated I would be."

Sharp explained men need around 38 grams of fibre a day. She added that is "honestly hard for the average person to do on a normal balanced diet, that's not made up exclusively of fries and Big Macs."

One Big Mac combo, including a Big Mac burger and an order of fries, amounts to 820 calories and just six grams of fibre.

According to Sharp, that's not enough. Especially if you're only eating half-portions.

"If we're not doing snacks, and we're just having McDonald's, the iceberg lettuce on the burger is just not going to cut it."

Besides expect a lot of constipation in the short-term, Sharp said she would also be concerned about the person’s microbiome with this diet, which "is not something we can analyze in a standard bloodwork set."

The dietitian also added antioxidants and micronutrients could be lacking with this meal plan. Sharp said having potatoes for nearly every meal would result in "a lot" of vitamin C. But, you'd be missing other unique antioxidants "that we know have specialized disease fighting properties."

Found in "a variety of colourful vegetables and fruits," these include: lycopene, beta carotene, anthocyanins and sulfur compounds.

Weight loss and bloodwork

Despite the lack of key nutrients, Maginnis's bloodwork showed improvements.

"I wasn't surprised at all when I heard that," Sharp admitted.

"The reason he saw improvements in those standard bloodwork measurements is because of the weight loss."

She explained losing just 10 per cent of your body weight can improve cholesterol levels and blood sugar levels, which reduces the risk of type two diabetes.

"Sugar is not the cause of the disease itself," she claimed.

"If you're able to maintain a healthy body weight eating McDonald's, you definitely could look quite healthy on paper."

As for weight loss, Sharp said she wasn't surprised by that either.

"You can lose weight eating anything. You can lose weight eating just straight-up butter and candy if you wanted to," she claimed, "as long as you were able to successfully create a calorie deficit."

Though she said eating half-portions of McDonald's "absolutely can and does work" in social experiments with finite dates — it's not a longterm weight loss plan.

"In real life, frequently eating fast food is more likely to cause weight gain, and the reason for this is that meals like McDonald's have a low satiety to high calorie ratio."

Simply put, the food doesn't make you feel full for a normal period of time.

"If we were to prepare a 1000-calorie meal made up of a big massive salad with vegetables, and lean protein and whole grains, and healthy fats like nuts and seeds or avocado, you'd probably feel very full before you even made your way through."

Another concern Sharp has with a McDonald's-only diet is simply losing interest in food — another potential cause for weight loss.

The expert said the novelty of Big Macs and fries would eventually wear off. "The food would just stop tasting so amazing where you feel like you don't even want to finish it anymore."

Sharp explained this is an evidence-based phenomenon called sensory specific satiety.

"You're forced eat the same thing every day. Over time, you would likely just want to eat the bare minimum to kind of suppress hunger."

She added this would also "make that calorie deficit easier" as "you just stop eating for any kind of pleasure."

'No such thing as a bad food'

Despite societal negative connotations with fast food, Sharp says "there is no such thing as a bad food."

Instead, it's all about balance.

"That's not how nutrition works, right? We have to look at the overall diet and lifestyle in order to determine whether not that food or meal fits into that."

Even with McDonald's and other fast food, "there are absolutely redeeming qualities."

However, Sharp said "this would definitely never be a diet that I recommend for weight loss, nor would it be a diet that I recommend for health."

Just because it worked for one man in the short-term, doesn't mean it'll work for everyone, she explained.

"Everybody has to find a way of eating that feels good to them, that is sustainable to them."

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