mcdonald's diet study

McDonald’s Diet Study: Fast Food Chain Menu Items Decrease Stomach Bacteria

Thryve Inside recently conducted an internal study about the changes in gut bacteria when eating a diet solely comprised of McDonald’s menu items. Results found that following a three-day McDonald’s diet may decrease biodiversity in your gut. In turn, opting for Super Size and regularly have a Big Mac for a meal may impact your overall health. Let’s take a closer look at what happened when our volunteer went all Morgan Spurlock and tested out a diet strictly from McDonald’s fast food restaurants.


McDonald’s Diet Study Methodology

mcdonald's diet ronald mcdonald
Our Thryve Inside McDonald’s diet study was conducted in three phases over an eight-day period. Phase A saw our volunteer eat their everyday food items. They followed this protocol for two days.
In Phase B, the participant consumed McDonald’s meals for their sustenance [1]. Mornings saw an Egg McMuffin or yogurt parfait. Afternoons ranged from chicken nuggets to a side salad. Dinner ran the gamut of having a Big Mac to yet another Egg McMuffin. Hey, McDonald’s restaurants through the United States serve breakfast all day now!
During Phase C, it was back to normal kitchen operations in the home of the volunteer. Every day of this McDonald’s diet study, we sampled the gut biome of our participant. Let’s take a look at the differences a McDonald’s diet has on the GI tract, as compared to regular eating habits.


McDonald’s Diet Study Results

McDonald’s diets are hard to study. As the world saw with Super Size Me, Morgan Spurlock followed a strict diet of McDonald’s foods and soft drinks.
mcdonald's diet health
During the 30-day period, Morgan Spurlock gained 24.5 pounds, and his cholesterol shot up 65 points [2]!
On the other hand, high school science teacher John Cisna lost 37 pounds by eating a McDonald’s diet for 90 days [3]. Comparatively, our study is only eight days, with just three of these days eating food solely off the Golden Arches menu.
We chose three days because research shows that the gut biome can alter within three days of environmental changes [4]. Going HAM on an Egg McMuffin multiple times a week is definitely a change for our volunteer. So, here’s what we found.


Lowers Stomach Bacteria Diversity

The most glaring result of consuming just McDonald’s meals is that eating food products only from this fast food chain can have an adverse impact on gut diversity. Our volunteer had a 6.7% decrease in species within their gut biome.


Use of Vegetable Oil Blend and Overall Health

We believe the reason for this decrease is that many of McDonald’s food products are fried with a vegetable oil blend.
mcdonald's menu items
Preparation techniques for McDonald’s food sees the food products introduced to cooking oil twice. That’s double the saturated fat in our diet!
Their vegetable oil blend is used in the facility where food is prepped and frozen prior to shipment. Then, the fast food products are fried in oil again at McDonald’s restaurants [5].
While this blend is free of trans fat, it’s made up of many other fatty acids that aren’t very efficient for our bodies to burn off.


Unhealthy Fats
According to current ingredient information, McDonald’s vegetable oil blend consists of:
mcdonald's diet large fries
• Canola oil
• Corn Oil
Soybean Oil
• Hydrogenated Soybean Oil with Tert-butylhydroquinone (TBHQ)
• Citric acid
• Dimethylpolysiloxane
For one, corn oil is made with crops using with genetically-modified organisms (GMOs). Plus, soybean oil is a common trigger of food allergies. They are rounded out with a healthy fat in canola oil. However, many commercial vegetable oil blends are extracted using solvents. So, that right there negates any potential health benefits.


Along with citric acid, THBQ act as a preservative. Not much research has been done on this synthetic molecule. However, some studies suggest that it may exhibit some antioxidant activities [6].
With that said, other studies point to potential dangers involving THBQ. One study involving rats noted that excess THBQ promoted liver and kidney problems [7].
Notably, the Food and Drug Administration (FDA) suggests that a vegetable oil blend doesn’t contain more than 0.2g of THBQ [8].
According to the FDA,

“The total antioxidant content of a food containing the additive will not exceed 0.02 percent of the oil or fat content of the food, including the essential (volatile) oil content of the food. “

Food and Drug Administration (FDA)
So, if you’re eating a diet of only McDonald’s food, then there’s a good chance you are far exceeding the recommended caloric intake of this additive.
While there are negative connotations around the vegetable oil blend used to cook McDonald’s food, this fast-food joint is making changes to improve the quality. According to the corporate website of the fast-food joint, their long-term plan is for McDonald’s restaurants to cook food products with 100% palm oil [9].


Increases Bacteria Associated with Weight Gain

While some stomach bacteria became compromised by consuming McDonald’s breakfast, lunch, and dinnertime, others flourished. Consuming a McDonald’s diet tipped the gut biome balance in favor of Firmicutes intestinal flora.
Research indicates that about 90% of your gut bacteria consists of either Firmicutes andBacteroidetes [10]. Ideally, you should have these two strains in balance, with the remaining ten per cent teeming with a wide variety of life.
An analysis looking at obesity and Firmicutes concluded,

“The content of Firmicutes was gradually increased while the content of Bacteroidetes was decreased with increasing body mass index (BMI). The F/B ratio also raised with increasing BMI. In an unadjusted logistic regression model, F/B ratio was significantly associated with BMI (OR = 1.23, 95% CI 1,09–1,38). This association continued to be significant after adjusting for confounders such as age, sex, tobacco smoking and physical activity [11].”

BMC Microbiol
Unfortunately, our volunteer who followed a McDonald’s diet saw their Bacteroidetes to Firmicutes ratio jump from 1:1 during Phase A to 1:4 in Phase B. After returning to a typical day of eating in Phase C, the levels of Bacteroidetes to Firmicutes returned to normal.


Might Decrease Bacteria That Repairs Gut Lining

Bacteroidetes do more than just keep Firmicutes in check. They play a large role in repairing the gut lining. They achieve this by producing many metabolites that aid in cell proliferation around the gut barrier.
A meta-analysis of the benefits of Bacteroidetes noted,

“Carbohydrate fermentation by Bacteroides and other intestinal bacteria results in the production of a pool of volatile fatty acids that are reabsorbed through the large intestine and utilized by the host as an energy source, providing a significant proportion of the host’s daily energy requirement [12].”

American Society for Microbiology
When we eat foods that promote food allergies, it creates inflammation in the stomach. Chowing down all day on foods prepped in a hydrogenated vegetable oil blend might cause the immune cells to work all day.


Lowers Faecalibacterium

Another bacteria associated with repairing the gut lining is Faecalibacterium. This genus of bacteria is actually under the Firmicutes phylum. However, it does more to help with weight loss than hinder it.
mcdonald's diet
These intestinal florae are known to consume acetate. As a result, they produce butyrate.


Butyrate is one of the most important short-chain fatty acids. It is the most significant source of energy for our colon cells.


Based on the snapshot of our test subject’s gut biome, following a McDonald’s diet every day could’ve led to chronic inflammation for our volunteer. Long-term inflammation may eventually result in a litany of health problems, including Leaky Gut Syndrome or Small Intestinal Bacteria Overgrowth (SIBO).


Might Negatively Impact Mental Health

mcdonald's diet mood
One of the first side effects of a McDonald’s diet that Morgan Spurlock noted in Super Size Me was that he was developing mood swings. Throwing off the Bacteroidetes to Firmicutes can do that.
Many studies show that Bacteroidetes are deeply ingrained in the grey matter of the brain [13]. They hang out in the frontal cortices and insula. These areas are instrumental in processing information.
Furthermore, Bacteroidetes are also known to cluster in the hippocampus. This region of the brain is essential for memory and emotion. Therefore, having lower levels of this stomach bacteria may impair your cognitive function, as well as produce mood swings.


Boosts Candida Levels

Perhaps the most negative effect of following a McDonald’s diet is the impact it has on fungal growth in the gut biome. In our series of diet studies, the McDonald’s diet protocol was the only one where we tested for Candida overgrowth.
mcdonald's diet and candida
Candida levels seemed to peak after the second day of Phase B. As the volunteer transitioned back to their normal kitchen habits, the levels of Candida dropped off. However, there was an unexpected rise once again before tapering off. This difference further illustrates the three-day passing-of-the-torch that happens among microbes when you change your diet. One can hypothesize there was a battle for supremacy from Phase B to C.
This tug-of-war was between Candida that feasted on McDonald’s food and healthy bacteria that were flourishing during the host’s return to a regular diet. When the special dietary needs of Candida were no longer met, Candida overgrowth was no longer a concern for our test subject.


Can You Lose Weight By Eating McDonald’s?

If John Cisna followed a McDonald’s diet and saw weight loss benefits, you probably could, too. However, you must take great care to ensure you are receiving adequate nutrition. Our test subject did a typical McDonald’s diet of a Big Mac and side salad and consumed egg sandwiches every morning.
John Cisna followed a recommended caloric intake of 2,000 calories. Yet, he still had his fill of ice cream sundaes and sugary drinks.
As John Cisna told Huffington Post, typical McDonald’s meals looked like
• Minute Maid OJ
• Sausage Burrito
• Fruit and Maple Oatmeal


• Fruit and Yogurt Parfait
• Premium Southwest Salad
• Apple Slices


• Large Diet Coke
• Premium Grilled Chicken Classic Sandwich
• Hot Fudge Sundae
• Small Fries
On days he opted for a Big Mac, he would adjust the rest of his caloric intake for the day [14]. Those are the times where the side salad and Egg White Delight come in handy. Miraculously, he saw a weight loss of 37 pounds and a 79-point drop in cholesterol.
Nobody knows how John Cisna achieved this. Morgan Spurlock and Thryve Inside studies don’t have the data to back up these benefits. So, if you are to follow a McDonald’s diet, please check with your physician.
Get blood levels done regularly to ensure you aren’t developing high cholesterol. Also, be sure to get your gut tested to make sure that pathogenic bacteria aren’t taking over the system.
With that said, McDonald’s is always okay as an occasional treat. Heck, even Kim Kardashian West gets ice cream as her McDonald’s breakfast [15]. McDonald’s is an institution in the United States. Just like alcohol, be sure to enjoy your McDonald’s meals responsibly!


Click Here To View Resources



[1] “McDonald’s Menu: Our Full McDonald’s Food Menu: McDonald’s.” McDonald’s Menu: Our Full McDonald’s Food Menu | McDonald’s, 2020,
[2] Stossel, John. “’Super Size Me’ Carries Weight With Critics.” ABC News, ABC News Network, 6 Jan. 2006,
[3] Kaufman, Gil. “You Could Totally Lose Weight On The McDonald’s Diet, But It Wouldn’t Be Pretty.” MTV News, 23 Oct. 2015,
[4] Feltman, Rachel. “The Gut’s Microbiome Changes Rapidly with Diet.” Scientific American, Scientific American, 14 Dec. 2013,
[5] Pepys, Sammy. Fat Is My Friend . 2QT Publishing, 2016, Google Books,
[6] Gharavi, Negar, et al. “Chemoprotective and Carcinogenic Effects of Tert-Butylhydroquinone and Its Metabolites.” Current Drug Metabolism, U.S. National Library of Medicine, Jan. 2007,
[7] Peters, Melanie M.C.G., and Maria I. Rivera. “Glutathione Conjugates of Tert-.Butyl-Hydroquinone, a Metabolite of the Urinary Tract Tumor Promoter 3-Tert-.Butyl-Hydroxyanisole, Are Toxic to Kidney and Bladder.” Cancer Research, 56, 1006-101 1, March 1, 1996, 1 Mar. 1996,
[8] “CFR – Code of Federal Regulations Title 21.”, 1 Apr. 2019,
[9] “Palm Oil.” McDonald’s, 31 Mar. 2020,
[10] Castaner, O., Goday, A., Park, Y. M., Lee, S. H., Magkos, F., Shiow, S., & Schröder, H. (2018). The Gut Microbiome Profile in Obesity: A Systematic Review. International journal of endocrinology, 2018, 4095789.
[11] Koliada, A., Syzenko, G., Moseiko, V., Budovska, L., Puchkov, K., Perederiy, V., Gavalko, Y., Dorofeyev, A., Romanenko, M., Tkach, S., Sineok, L., Lushchak, O., & Vaiserman, A. (2017). Association between body mass index and Firmicutes/Bacteroidetes ratio in an adult Ukrainian population. BMC microbiology, 17(1), 120.
[12] Wexler, Hannah M. “Bacteroides: the Good, the Bad, and the Nitty-Gritty.” Clinical Microbiology Reviews, American Society for Microbiology Journals, 1 Oct. 2007,
[13] Tillisch, Kirsten, et al. “Brain Structure and Response to Emotional Stimuli as Related to Gut Microbial Profiles in Healthy Women.” Psychosomatic Medicine, U.S. National Library of Medicine, Oct. 2017,
[14] Almendrala, Anna. “The ‘McDiet’ Helped One Science Teacher Lose Weight — But Is It Actually Healthy?” HuffPost, HuffPost, 7 Dec. 2017,
[15] Williams, Bre. “Kim Kardashian West’s McDonald’s Order Has the Internet Divided-Here’s Why.” Showbiz Cheat Sheet, 4 Feb. 2020,

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