It's common knowledge that fast food isn't exactly healthy, but just how unhealthy is it? According to Tim Spector, a professor of genetic epidemiology at King's College London, it is devastating to a human's gut bacteria. He writes in The Conversation, that eating that type of highly processed food can wreak total havoc on the healthy bacteria in a person's digestive system. Spector wanted to find out just how bad McDonald's is for gut bacteria for his upcoming book The Diet Myth.

His gut bacteria were "devastated."

Spector had his son, a student at the University of Aberystwyth, eat only McDonald's for 10 days straight. Tom was allowed to eat Big Macs, chicken nuggets, fries, and Coca-Cola. Throughout the experiment, Tom sent his stool samples out to be analyzed by different labs. Tom notes that he felt fine for the first three days, but he started to slowly go "downhill." He became lethargic and after a week, his friends said he had taken on a "strange grey" color.

As for the lab results, they were shocking. Spector writes that Tom's gut bacteria were "devastated." Over the 10 days, Tom lost nearly 1,400 types of bacteria species, or nearly 40 percent of his total variety. Even after two weeks of returning to a normal diet, the microbes in his gut did not recover. Spector notes that "the loss of [gut bacteria] diversity is a universal signal of ill health." He adds that humans rely on the bacteria "to produce much of our essential nutrients and vitamins."