What Junk Food Does to Your Gut Health

Digital Vision/Getty Images
Digital Vision/Getty Images

Binging on fast food could be one of the most effective ways to kill off the gut bacteria that keep you fit, healthy, and happy: Epidemiologist Tim Spector wrote about the devastating effects of processed food on the microbiome in The Conversation after conducting a home experiment, in which his son, a college student at The University of Aberystwyth, ate nothing but McDonald’s for breakfast, lunch, and dinner for over a week (with beer and chips as an occasional night snack). After just ten days of packing in Big Macs, Chicken nuggets, fries, and Cokes, the result was exactly what you’d expect: Spector’s son felt lethargic, sick, and his skin took on a gray shade. But the most surprising change of all: Lab results showed he had already killed off nearly 1,400 species of gut bacteria in his stomach, 40 percent of the total variety he’d had before.

Keeping a diverse community of microbes in our guts is a key way our bodies fend off illnesses like the common cold, diabetes, obesity, and also low energy or mood swings. Some experts say Spector’s study is yet another block of evidence showing what you eat is just as important as how much you eat. “Junk food, including most fast food and processed food, is bad for us. We should all know that by now,” says Justin Sonnenburg, a microbiology professor at Stanford University and co-author of The Good Gut. “This study further highlights what other scientists have shown in more rigorous studies, that eating a diet low in dietary fiber will have a large negative impact on the microbiota. This case study also highlights how rapidly the gut microbiota responds to diet.”

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But other experts say we can’t make any conclusions from Spector’s experiment. “The single-person (his son) dietary trial is a bit over the top, and is, it goes without saying, unscientific,” says Dr. Eugene Chang, a professor of medicine at the University of Chicago. “It is unclear, for instance, what the changes found in his son’s gut microbiome actually mean. These measurements only tell you what microbes are present and nothing about what they are doing.”

Nevertheless, Chang agrees with Spector that such a diet does promote a loss of diversity that could affect long-term health. In Sonnenburg’s new book, he describes the mass extinction of good bacteria, which occurs from the overuse of antibiotics and a diet heavy on processed food, as partly the reason why conditions like depression, allergies, cancer, and more are on the rise. In April, University of Pittsburgh researchers found that swapping the Western high-fat diets of a group of African-Americans with the vegetable-heavy traditional meals of South Africans lowered the Westerner’s risk for colon cancer in just two weeks, and worsened the South African’s metabolic profile. Like Spector’s son, there was a big shift in the diversity of their gut bacteria.

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Realistically, you won’t be binging on ten days of McDonald’s like Spector’s son any time soon, but it won’t hurt in the meantime to watch what you eat. Eating a diet rich in plants including vegetables, legumes, whole grains, and nuts, is key, says Sonnenburg. “Fruits will help nurture a healthy microbiota,” he adds. And fermented foods, like sauerkraut or miso, which contain living microbes, “also appear to be an important part of a diet that supports the microbiota.”

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