A new study shows that just 10 days on a McDonald’s diet could kill off the good bacteria in your gut. Tim Spector, a genetics professor at King’s College London, wanted to see how much junkfood someone could handle, so he volunteered his son Tom as a test subject in his newest experiment: to eat at the Maccy D’s for 10 straight days.

According to Medical Daily “Tom lost half of his bifidobacteria (healthy bacteria designed to suppress inflammation in the digestive tract) after eating McDonald’s. His firmicutes, whose job is to extract energy from food, also became replaced with obesity-linked bacteroidetes. He lost nearly 40 percent of his total bacteria variety” – Two weeks later, after returning to a healthy diet regimen, his microbes still were unable to recover.

“We rely on our bacteria to produce much of our essential nutrients and vitamins while they rely on us eating plants and fruits to provide them with energy and to produce healthy chemicals which keep our immune system working normally,” Spector wrote. “We are unlikely to stop people eating fast food, but the devastating effects on our microbes and our long-term health could possibly be mitigated if we also eat foods which our microbes love like probiotics (yogurts), root vegetables, nuts, olives and high-fiber foods.”