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Eating McDonald’s for 10 days straight can wreak havoc on the healthy bacteria in your body that makes up your microbiome, according to one man who tried it so the rest of us don’t have to.

Tim Spector, professor of genetic epidemiology at King’s College London, enlisted his son Tom, a genetics student at the University of Aberystwyth, to undergo an experiment where he subsisted on McDonalds food for ten days. Tom could eat a Big Mac or Chicken nuggets as well as fries and Coke — and Dad footed the bill.

Tom said that for three days he felt ok, but then started to become more lethargic and turned a slight gray color according to his friends. He reported feeling bad the last few days and says he also experienced some withdrawal symptoms. Tom sent stool samples to different labs throughout his trial, and the final results came from Cornell University and the British Gut Project.

The results revealed that Tom’s gut microbes were “devastated,” Spector reports in the The Conversation. Tom had lost around 40% of his bacteria species, about 1,400 types. That loss of microbiome diversity is of concern since that can be a red flag for health issues such as obesity and diabetes.

A sample size of one is by no means up to scientific rigor, but the findings suggest a possible other side effect of a diet that’s low in fruits and veggies and high in junk food. A healthy microbiome can support a healthy immune system, and eating unhealthy food for days on end may interfere with that relationship.

Of course, eating the same thing every day can also cause a shift; it’s not clear that a diet of only pasta or only salad leaves would be much better for your gut diversity. A healthy, diverse diet with more healthy food than not is always a safe bet.

Read next: Why McDonald’s Big Turnaround Plans Mean Nothing to Consumers

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