10 days of McDonalds

In response to a post on the challenges of research on food, James responded with an email recommending I read “The Diet Myth” by Prof Tim Spector. I’ve been working my way through this fascinating book in the past few weeks and one of the anecdotes in the book that’s stuck with me is an experiment he ran with Tom, his son, as a test subject. His son had a simple task – stick to a diet of McDonalds for 10 straight days so we can see its effects on your gut bacteria.

Through the course of the book, Prof Spector explains the key role gut bacteria plays in our health. There’s been a growing body of research on the subject too.

This post, however, isn’t about that (more on that later). This just looks at the effect the McDonalds diet on his son’s health and gut bacteria. Below are a couple of excerpts from Tom’s article in The Telegraph.


At first, my friends were quite supportive. They thought the idea was interesting and far-fetched, a mini Super Size Me diet. But by the eight or ninth day they started saying things like: “Are you sure your liver functions are intact? You look really quite jaundiced. You should really consider stopping.”

Straight after the experiment, I drove to the supermarket and got two big bags of salad. I ate them all. I was over the moon. And the test results were fascinating. I’d lost 1,400 bacterial species in my gut in just 10 days, which was extraordinary. After a week back on my normal student diet I’d recovered a bit but not completely. I still don’t know if I’ve completely restored the diversity of species to my gut.

I was the first person to look at the gut microbiome change on a McDonald’s diet, but I’m just one person. In terms of scientific reliability, that doesn’t score well. But we sent samples to three different laboratories, who tested in different ways, and my results did show really interesting trends across all three.

The experiment has definitely made me think about the bad food that I am eating. I used to work as a commis chef, so I can cook. I’m eating a lot more fruit and vegetables. 


By the end of the 10 day experiment, Tom had lost a devastating loss of microbiome diversity (1,400 less species in the gut), a loss of appetite, lethargy, and constipation.

The fascinating part to me is that all it took was 10 days..