We know by now that gut bacteria — the rich microbiomes of “good” bacteria that thrive in our gastrointestinal systems — are important for both our physical and mental health. But our modern, Western diets may be gradually depleting these good bacteria over time, which could result in deficiencies passed down over generations, a new Stanford University School of Medicine study finds.

America is notorious as a fast food nation, with obesity and diabetes rates soaring higher every year. Poor diets, low physical activity, and a switch to sedentary work styles have all contributed to the epidemic — imbalanced gut bacteria have also been linked to obesity. Just 10 days of eating a McDonald’s diet results in a completely destroyed gut microbiome. The new study builds upon this notion by finding that our poor diets, which occur over a widespread population, may have serious consequences in our future, and in the futures of later generations.

The missing key may be fiber; it’s essential to maintaining good gut bacteria. The researchers examined mice that had been raised in aseptic environments — meaning they didn’t have access to normal nutrient-rich foods, and their gut microbiomes were depleted entirely. First, they inserted human gut bacteria into the mice to recreate their microbiomes equally, then divided them into two groups: one would eat a fiber-rich diet, and the other would eat a nearly no-fiber diet.

At first, the gut bacteria remained the same, but after a few weeks they began to change significantly; the number of bacterial species in the stomachs of no-fiber diet mice had dropped by 75 percent. “Within a couple of weeks, we saw a massive change,” said Justin Sonnenburg, associate professor of microbiology and immunology and a senior author of the study, in the press release. “The low-fiber-intake mice harbored fewer bacterial species in their gut.”

When all of the mice were switched back to a high-fiber, nutrient-rich diet, the previously deprived ones recovered slightly — but could never reach the same levels of gut bacteria diversity as before. In addition, when generations of mice were fed low-fiber diets, babies were born from depleted mothers with imbalanced microbiomes. This suggests that consistently restricting ourselves of gut bacteria could result in a permanently changed microbiome that’s passed down generation to generation.

People living in modern, industrialized societies sport more low-fiber diets than traditional hunter-gatherer and rural societies, and this results in a less diverse microbiome among people living in industrialized countries, the researchers noted. In short, the diversity of our gut microbiomes are dwindling when compared to how they were in our past, when we ate more fresh fruit, vegetables, grains, corn, and bran.

“Numerous factors including widespread antibiotic use, more frequent cesarean sections and less frequent breastfeeding have been proposed for why we see this depletion in industrialized populations,” said Erica Sonnenburg, a senior research scientist at Stanford and an author of the study, in the press release.

If we continue eating the low-fiber, high-fat diets we’re consuming now, imbalanced gut microbiomes can lead to a slew of health problems — from an increased risk for anxiety and depression to a weakened immune system. All of these problems could, in theory, be passed down to later generations if we keep up our poor diets and obesity epidemic.

“We would have difficulty living without [gut bacteria],” Justin Sonnenburg said in the press release. “They fend off pathogens, train our immune systems and even guide the development of our tissues.”

Source: Sonnenburg E, Smits S, Tikhonov M, Higginbottom S, Wingreen N, Sonnenburg J. Diet-induced extinctions in the gut microbiota compound over generations. Nature, 2016.